Production of super absorbent polymers on a continuous belt reactor

0001The present invention relates to a process for production of superabsorbent polymers on a continuous belt reactor, wherein a continuous polymerization belt rests at least partly upon the upper surface of at least one continuous support belt.

0002Superabsorbent polymers are in particular polymers of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, crosslinked ethers of cellulose or of starch, crosslinked carboxymethylcellulose, partially crosslinked polyalkylene oxide or natural products swellable in aqueous fluids, such as guar derivatives for example. Such polymers are used as products capable of absorbing aqueous solutions to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening.

0003Superabsorbent polymers typically have a Centrifuge Retention Capacity in the range from 25 to 60 g/g, preferably of at least 30 g/g, more preferably of at least 32 g/g, even more preferably of at least 34 g/g and most preferably of at least 35 g/g. Centrifuge Retention Capacity (CRC) is determined by EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 241.2-05 "Centrifuge retention capacity".

0004To improve their performance characteristics, for example permeability, superabsorbent polymeric particles are generally postcrosslinked. This postcrosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, ground and screened particles of the base polymer are surface coated with a postcrosslinker, dried and thermally postcrosslinked. Useful crosslinkers for this purpose include compounds comprising at least two groups capable of forming covalent bonds with the carboxylate groups of the superabsorbent polymer particles or capable of crosslinking together carboxyl groups or other functional groups of at least two different polymeric chains of the base polymer.

0005The production of superabsorbent polymers is described for example in the monograph "<nplcit id="ncit0001" npl-type="b"><text>Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pages 69 to 117</text></nplcit>.

0006Kneading reactors or belt reactors are suitable reactors. In a kneader, the polymer gel which is produced in the course of the polymerization of an aqueous monomer solution is for example continuously comminuted by contrarotatory stirring shafts, as described in <patcit id="pcit0001" dnum="WO200138402A1"><text>WO 2001/38402 A1</text></patcit>. The polymerization on a belt is described for example in <patcit id="pcit0002" dnum="DE3825366A1"><text>DE 38 25 366 A1</text></patcit> and <patcit id="pcit0003" dnum="US6241928B"><text>US 6,241,928</text></patcit>. The polymerization in a belt reactor produces a polymer gel which has to be comminuted in a further process step, for example in a meat grinder, extruder or kneader.

0007<patcit id="pcit0004" dnum="DE3544770A1"><text>DE 35 44 770 A1</text></patcit> describes a continuous belt reactor for the production of superabsorbent polymers.

0008<patcit id="pcit0005" dnum="EP955086A1"><text>EP 955 086 A1</text></patcit> discloses a process for polymerization of an aqueous monomer solution on a continuous belt. The change of thickness in the direction of width of the formed polymer gel is maintained to not more than 20%.

0009<patcit id="pcit0006" dnum="EP1683813A1"><text>EP 1 683 813 A1</text></patcit> describes continuous belt reactors having a fluororesin layer with a low melt viscosity. The fluororesin layer can be laminated on the continuous belt or be fixed on a continuous chain conveyor.

0010It is an object of the present invention to provide an improved process for production of superabsorbent polymers on a continuous belt reactor.

0011We have found that this object is achieved by a process for production of superabsorbent polymers on a continuous belt reactor, comprising <ol id="ol0001" ol-style=""><li>i) a continuous polymerization belt and</li><li>ii) at least one continuous support belt,</li></ol> wherein the continuous polymerization belt i) rests at least partly upon the upper surface of the at least one continuous support belt ii) and the continuous polymerization belt i) comprises a carcass and a cover, wherein the first section of the continuous polymerization belt i) forms a trough and the lateral edges of the continuous polymerization belt i) are curved upwardly from the horizontal plane by at least one fixed support means.

0012The continuous polymerization belt i) can be made of various materials, although these must meet the requirements of good tensile strength and flexibility, good fatigue strength under repeating bending stresses, good deformability and chemical resistance to the individual reaction components under the conditions of the polymerization. These demands cannot be met by a single material. Therefore, a multi-layer material must be used as continuous polymerization belt i) of the present invention.

0013The mechanical requirements can be satisfied by a carcass of, for example, fabric inserts of natural and/or synthetic fibers or glass fibers or steel cords.

0014The chemical resistance can be achieved by a cover of, for example, polyethylene, polypropylene, polyisobutylene, halogenated polyolefines such as polyvinyl chloride or polytetrafluorethylene, polyamides, natural or synthetic rubbers, polyester resins or epoxy resins. The preferred cover material is silicone rubber.

0015The continuous support belt ii) used in the invention is preferably a metallic belt, more preferably a stainless steel belt. Especially metallic continuous support belts ii) can be used under high tensile stress that prevents sagging of the continuous polymerization belt i). Thus, the supported continuous polymerization belt i) shows a reduced sagging compared to the prior art continuous belt reactors using idlers as support means.

0016Another effect of the inventive process is that the necessary tension of the continuous polymerization belt i) can be reduced. Thus, the continuous polymerization belt i) of the inventive process has a highly improved serviceable life. <ul id="ul0001" list-style="none"><li><figref idref="f0001">Fig. 1</figref> is a schematic side view of a preferred embodiment of the process of the present invention.</li><li><figref idref="f0001">Fig. 2</figref> is a cross-sectional view of a preferred embodiment of the process of the present invention.</li></ul>

0017Preferably, the continuous polymerization belt i) (2) rests upon at least two continuous support belts ii) (3). The usage of more than one continuous support belt ii) reduces the necessary tension of each continuous support belt ii).

0018The lateral edges of the continuous polymerization belt i) are curved upwardly from the horizontal plane by at least one fixed support means. A fixed support means is a support means that does not move with the continuous polymerization belt i). A fixed support means can be, for example, slide bars that curve the lateral edges of polymerization belt i) upwardly. It is also possible that these slide bars are part of the support construction (8).

0019Preferably, the first section of the continuous polymerization belt i) (2) is not supported by a continuous support belt ii) (3). Without support of a continuous support belt ii), the first section forms a trough by the weight of the added monomer solution (5).

0020Additionally, the continuous support belts ii) can be stiffed in transverse direction.

0021In another preferred embodiment of the present invention at least one continuous support belt ii) forms at the end in downward direction a barrier for liquids. The barrier can be formed by an additional roll (4) that lifts the continuous support belt ii) (3). The barrier prevents that monomer solution can flow into the next production steps on breakdown of the polymerization.

0022The polymerization is an exothermic reaction. The formed polymer gel must be cooled to prevent overheating.

0023The at least one continuous support belt ii) (3) cools the formed polymer gel (1) by its heat capacity. Therefore, the cooling of the formed hydrogel can be improved by the at least one continuous support belt ii).

0024Thus, in a preferred embodiment of the present invention the at least one continuous support belt ii) is a metallic belt with a basis weight of at least 1 kg/m². Metallic materials have a high heat transfer rate. The basis weight of the at least one continuous support belt ii) is preferably from 3 to 25 kg/m², more preferably from 5 to 20 kg/m², most preferably from 7 to 15 kg/m². The cooling effect of the at least one continuous support belt ii) increases with the basis weight. Too high basis weights of the at least one continuous support belt ii) causes mechanical problems.

0025Preferably, the at least one continuous support belt ii) have meshes with a mesh size from 6 to 35 mm, more preferably from 9 to 30 mm, most preferably from 12 to 25 mm.

0026Preferably, the at least one continuous support belt ii) have a thickness from 4 to 35 mm, more preferably from 8 to 30 mm, most preferably from 12 to 25 mm.

0027In a preferred embodiment of the present invention the at least one continuous support belt ii) is a flat wire belt. <patcit id="pcit0007" dnum="US6530469B"><text>US 6,530,469</text></patcit> describes useful flat wire belts. A preferred flat wire belt is PACTITE® Flatwire Belting (Maryland Wire Belts Inc., US).

0028The strip thickness of the flat wire belt is preferably from 0.6 to 2 mm, more preferably from 0.8 to 1.8 mm, most preferably from 1 to 1.6 mm.

0029The preferred flat wire belts have a high basis weight and a high surface area. Thus, the flat wire belts have a high heat capacity and a high heat transfer rate to the surrounding airspace.

0030In another preferred embodiment of the present invention the at least one continuous support belt ii) slides at least partly on a fixed surface. A fixed surface is a surface that does not move with the at least one continuous support belt ii).

0031Preferably less than 20 %, more preferably less than 15 %, most preferably less than 10 %, of the width of the at least one continuous support belt ii) slides on the fixed surface.

0032Preferably, at least one continuous support belt ii) (3) slides on at least one slide bar (7).

0033The at least one continuous support belt ii) slides preferably on at least two slide bars, more preferably on at least three slide bars, most preferably on at least four slide bars. The slide bars stabilizes the at least one continuous support belt ii) in longitudinal direction. Thus, the tension of the at least one continuous support belt ii) can be reduced.

0034The upper surface of the slide bars has a friction coefficient of preferably less than 0.5, more preferably less than 0.4, most preferably less than 0.3. It is possible to use slide bars wherein only the upper surface of the slide bars is coated with a material (6) having a low friction coefficient, i.e. polytetrafluoroethylene.

0035The monomer solutions or monomer suspensions usable in the process of the present invention comprises <ol id="ol0002" compact="compact" ol-style=""><li>a) at least one ethylenically unsaturated acid-functional monomer,</li><li>b) at least one crosslinker,</li><li>c) if appropriate one or more ethylenically and/or allylically unsaturated monomers copolymerizable with a), and</li><li>d) if appropriate one or more water-soluble polymers onto which the monomers a), b) and if appropriate c) can be at least partly grafted.</li></ol>

0036Suitable monomers a) are for example ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid and/or salts of these acids. Acrylic acid and methacrylic acid are particularly preferred monomers. Acrylic acid is most preferable.

0037Useful monomers a) are further styrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid and 2-hydroxyethylacrylate.

0038The proportion of the total amount of monomers a) which is attributable to acrylic acid and/or its salts is preferably at least 50 mol%, more preferably at least 90 mol% and most preferably at least 95 mol%.

0039The monomers a) and especially acrylic acid comprise preferably up to 0.025 % by weight of a hydroquinone half ether. Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

0040Tocopherol refers to compounds of the following formula: <chemistry id="chem0001" num="0001"><img file="EP2107939B2_D0001.tif" /></chemistry> where R¹ is hydrogen or methyl, R² is hydrogen or methyl, R³ is hydrogen or methyl and R⁴ is hydrogen or an acid radical of 1 to 20 carbon atoms.

0041Preferred R⁴ radicals are acetyl, ascorbyl, succinyl, nicotinyl and other physiologically tolerable carboxylic acids. The carboxylic acids can be mono-, di- or tricarboxylic acids.

0042Preference is given to alpha-tocopherol where R¹ = R² = R³ = methyl, especially racemic alpha-tocopherol. R⁴ is more preferably hydrogen or acetyl. RRR-alpha-Tocopherol is preferred in particular.

0043The monomer solution comprises preferably not more than 130 weight ppm, more preferably not more than 70 weight ppm, preferably not less than 10 weight ppm, more preferably not less than 30 weight ppm and especially about 50 weight ppm of hydroquinone half ether, all based on acrylic acid, with acrylic acid salts being arithmetically counted as acrylic acid. For example, the monomer solution can be produced using an acrylic acid having an appropriate hydroquinone half ether content.

0044The superabsorbent polymers are in a crosslinked state, i.e., the polymerization is carried out in the presence of compounds having two or more polymerizable groups which can be free-radically interpolymerized into the polymer network. Useful crosslinkers b) include for example ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane as described in <patcit id="pcit0008" dnum="EP530438A1"><text>EP 530 438 A1</text></patcit>, di- and triacrylates as described in <patcit id="pcit0009" dnum="EP547847A1"><text>EP 547 847 A1</text></patcit>, <patcit id="pcit0010" dnum="EP559476A1"><text>EP 559 476 A1</text></patcit>, <patcit id="pcit0011" dnum="EP632068A1"><text>EP 632 068 A1</text></patcit>, <patcit id="pcit0012" dnum="WO9321237A1"><text>WO 93/21237 A1</text></patcit>, <patcit id="pcit0013" dnum="WO2003104299A1"><text>WO 2003/104299 A1</text></patcit>, <patcit id="pcit0014" dnum="WO2003104300A1"><text>WO 2003/104300 A1</text></patcit>, <patcit id="pcit0015" dnum="WO2003104301A1"><text>WO 2003/104301 A1</text></patcit> and <patcit id="pcit0016" dnum="DE10331450A1"><text>DE 103 31 450 A1</text></patcit>, mixed acrylates which, as well as acrylate groups, comprise further ethylenically unsaturated groups, as described in <patcit id="pcit0017" dnum="DE10331456A1"><text>DE 103 31 456 A1</text></patcit> and <patcit id="pcit0018" dnum="DE10355401A1"><text>DE 103 55 401 A1</text></patcit>, or crosslinker mixtures as described for example in <patcit id="pcit0019" dnum="DE19543368A1"><text>DE 195 43 368 A1</text></patcit>, <patcit id="pcit0020" dnum="DE19646484A1"><text>DE 196 46 484 A1</text></patcit>, <patcit id="pcit0021" dnum="WO9015830A1"><text>WO 90/15830 A1</text></patcit> and <patcit id="pcit0022" dnum="WO200232962A2"><text>WO 2002/32962 A2</text></patcit>.

0045Useful crosslinkers b) include 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 diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate and also trimethylolpropane triacrylate and allyl compounds, such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid and also vinylphosphonic acid derivatives as described for example in <patcit id="pcit0023" dnum="EP343427A2"><text>EP 343 427 A2</text></patcit>. Useful crosslinkers b) further include pentaerythritol diallyl ether, pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol diallyl ether, glycerol triallyl ether, polyallyl ethers based on sorbitol, and also ethoxylated variants thereof. The process of the present invention utilizes di(meth)acrylates of polyethylene glycols, the polyethylene glycol used having a molecular weight between 300 and 1000.

0046However, particularly advantageous crosslinkers b) are di- and triacrylates of 3- to 20-tuply ethoxylated glycerol, of 3- to 20-tuply ethoxylated trimethylolpropane, of 3- to 20-tuply ethoxylated trimethylolethane, especially di- and triacrylates of 2- to 6-tuply ethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane, of 3-tuply propoxylated glycerol, of 3-tuply propoxylated trimethylolpropane, and also of 3-tuply mixedly ethoxylated or propoxylated glycerol, of 3-tuply mixedly ethoxylated or propoxylated trimethylolpropane, of 15-tuply ethoxylated glycerol, of 15-tuply ethoxylated trimethylolpropane, of at least 40-tuply ethoxylated glycerol, of at least 40-tuply ethoxylated trimethylolethane and also of at least 40-tuply ethoxylated trimethylolpropane.

0047Very particularly preferred for use as crosslinkers b) are diacrylated, dimethacrylated, triacrylated or trimethacrylated multiply ethoxylated and/or propoxylated glycerols as described for example in <patcit id="pcit0024" dnum="WO2003104301A1"><text>WO 2003/104301 A1</text></patcit>. 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. These are notable for particularly low residual levels (typically below 10 weight ppm) in the water-absorbing polymer and the aqueous extracts of water-absorbing polymers produced therewith have an almost unchanged surface tension compared with water at the same temperature (typically not less than 0.068 N/m).

0048The amount of crosslinker b) is preferably from 0.001 to 10 wt.%, more preferably from 0.01 to 5 wt.% and most preferably from 0.1 to 2 wt.%, all based on monomer a).

0049Examples of ethylenically unsaturated monomers c) which are copolymerizable with the monomers a) are acrylamide, methacrylamide, crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate and dimethylaminoneopentyl methacrylate.

0050Useful water-soluble polymers d) include polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, polyglycols or polyacrylic acids, preferably polyvinyl alcohol and starch.

0051The solids content of the monomer solution is preferably at least 30 wt.%, more preferably at least 35 wt.%, most preferably at least 40 wt.%. The solids content is the sum of monomer a), crosslinker b), monomer c) and polymer d). The usage of aqueous monomer suspensions with high solids contents is also possible.

0052The monomer solution or the monomer suspensions is polymerized on continuous polymerization belt i) forming a polymer gel.

0053The width of the continuous polymerization belt i) is preferably from 1 to 10 m, more preferably from 2 to 8 m, most preferably from 3 to 6 m. The length of the continuous polymerization belt i) is preferably from 3 to 50 m, more preferably from 5 to 40 m, most preferably from 10 to 30 m. The residence time on the continuous polymerization belt i) is preferably from 5 to 120 minutes, more preferably from 10 to 60 minutes, most preferably from 12 to 40 minutes.

0054The thickness of the formed polymer gel layer is preferably from 1 to 20 cm, more preferable from 2 to 15 cm, most preferable from 5 to 10 cm. Next, the polymer gel is comminuted in a further process step, for example in a meat grinder, extruder or kneader.

0055The acid groups of the hydrogels obtained are typically in a partially neutralized state, the extent of neutralization preferably being in the range from 25 to 95 mol%, more preferably in the range from 50 to 80 mol% and even more preferably in the range from 60 to 75 mol%, for which the customary neutralizing agents can be used, for example alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and also mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred as alkali metals, but most preference is given to sodium hydroxide, sodium carbonate or sodium bicarbonate and also mixtures thereof.

0056Neutralization is preferably carried out at the monomer stage. This is customarily accomplished by admixing the neutralizing agent as an aqueous solution, as a melt or else preferably as a solid material. For example, sodium hydroxide having a water fraction of distinctly below 50 % by weight can be present as a waxy mass having a melting point above 23 °C. In this case, metering as piece goods or melt at elevated temperature is possible.

0057Neutralization can also be carried out after polymerization, at the hydrogel stage. But it is also possible to neutralize up to 40 mol-%, preferably from 10 to 30 mol-% and more preferably from 15 to 25 mol-% of the acid groups before polymerization by adding a portion of the neutralizing agent to the monomer solution and setting the desired final degree of neutralization only after polymerization, at the hydrogel stage. When the hydrogel is neutralized at least partly after polymerization, the hydrogel is preferably mechanically comminuted, for example by means of a meat grinder, in which case the neutralizing agent can be sprayed, sprinkled or poured on and then carefully mixed in. To this end, the gel mass obtained can be repeatedly grindered for homogenization.

0058The hydrogel is then preferably dried with a belt dryer until the residual moisture content is preferably below 15 % by weight and especially below 10 % by weight, the water content being determined by EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 230.2-05 "Moisture content". Selectively, drying can also be carried out using a fluidized bed dryer or a heated plowshare mixer. To obtain particularly white products, it is advantageous to dry this gel by ensuring rapid removal of the evaporating water. To this end, the dryer temperature must be optimized, the air feed and removal has to be policed, and at all times sufficient venting must be ensured. Drying is naturally all the more simple - and the product all the more white - when the solids content of the gel is as high as possible. The solids content of the gel prior to drying is therefore preferably between 30 % and 80 % by weight. It is particularly advantageous to vent the dryer with nitrogen or some other non-oxidizing inert gas. Selectively, however, simply just the partial pressure of the oxygen can be lowered during drying to prevent oxidative yellowing processes. But in general adequate venting and removal of the water vapor will likewise still lead to an acceptable product. A very short drying time is generally advantageous with regard to color and product quality.

0059A further important function of drying the gel is the ongoing reduction in the residual monomer content of the superabsorbent. This is because any residual initiator will decompose during drying, leading to any residual monomers becoming interpolymerized. In addition, the evaporating amounts of water will entrain any free water-vapor-volatile monomers still present, such as acrylic acid for example, and thus likewise lower the residual monomer content of the superabsorbent.

0060The dried hydrogel is then ground and classified, useful grinding apparatus typically including single or multiple stage roll mills, preferably two or three stage roll mills, pin mills, hammer mills or swing mills.

0061The polymer obtained may subsequently be postcrosslinked. Useful postcrosslinkers are compounds comprising two or more groups capable of forming covalent bonds with the carboxylate groups of the polymers. Useful compounds are for example alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyglycidyl compounds as described in <patcit id="pcit0025" dnum="EP83022A2"><text>EP 83 022 A2</text></patcit>, <patcit id="pcit0026" dnum="EP543303A1"><text>EP 543 303 A1</text></patcit> and <patcit id="pcit0027" dnum="EP937736A2"><text>EP 937 736 A2</text></patcit>, polyhydric alcohols as described in <patcit id="pcit0028" dnum="DE3314019A1"><text>DE 33 14 019 A1</text></patcit>, <patcit id="pcit0029" dnum="DE3523617A1"><text>DE 35 23 617 A1</text></patcit> and <patcit id="pcit0030" dnum="EP450922A2"><text>EP 450 922 A2</text></patcit>, or β-hydroxyalkylamides as described in <patcit id="pcit0031" dnum="DE10204938A1"><text>DE 102 04 938 A1</text></patcit> and <patcit id="pcit0032" dnum="US6239230B"><text>US 6,239,230</text></patcit>. It is also possible to use compounds of mixed functionality, such as glycidol, 3-ethyl-3-oxetanemethanol (trimethylolpropaneoxetane), as described in <patcit id="pcit0033" dnum="EP1199327A2"><text>EP 1 199 327 A2</text></patcit>, aminoethanol, diethanolamine, triethanolamine or compounds which develop a further functionality after the first reaction, such as ethylene oxide, propylene oxide, isobutylene oxide, aziridine, azetidine or oxetane.

0062Useful postcrosslinkers are further said to inclule by <patcit id="pcit0034" dnum="DE4020780C1"><text>DE 40 20 780 C1</text></patcit> cyclic carbonates, by <patcit id="pcit0035" dnum="DE19807502A1"><text>DE 198 07 502 A1</text></patcit> 2-oxazolidone and its derivatives, such as N-(2-hydroxyethyl)-2-oxazolidone, by <patcit id="pcit0036" dnum="DE19807992A1"><text>DE 198 07 992 A1</text></patcit> bis- and poly-2-oxazolidinones, by <patcit id="pcit0037" dnum="DE19854573A2"><text>DE 198 54 573 A2</text></patcit> 2-oxotetrahydro-1,3-oxazine and its derivatives, by <patcit id="pcit0038" dnum="DE19854574A1"><text>DE 198 54 574 A1</text></patcit> N-acyl-2-oxazolidones, by <patcit id="pcit0039" dnum="DE10204937A1"><text>DE 102 04 937 A1</text></patcit> cyclic ureas, by <patcit id="pcit0040" dnum="DE10334584A1"><text>DE 103 34 584 A1</text></patcit> bicyclic amide acetals, by <patcit id="pcit0041" dnum="EP1199327A2"><text>EP 1 199 327 A2</text></patcit> oxetanes and cyclic ureas and by <patcit id="pcit0042" dnum="WO200331482A1"><text>WO 2003/31482 A1</text></patcit> morpholine-2,3-dione and its derivatives.

0063Preferred postcrosslinkers are oxazolidone and its derivatives, in particular N-(2-hydroxyethyl)-2-oxazolidone, glycidyl compounds, in particular ethylene glycol diglycidyl ether, polyols, in particular glycerol, and ethylene carbonate.

0064The amount of postcrosslinker is preferably in the range from 0.001 % to 5 % by weight, more preferably in the range from 0.01 % to 2.5 % by weight and most preferably in the range from 0.1 % to 1 % by weight, all based on the polymer.

0065Postcrosslinking is customarily carried out by spraying the hydrogel or the dry polymeric particles with a solution; preferably an aqueous solution, of the postcrosslinker. Spraying is followed by thermal drying, and the postcrosslinking reaction can take place not only before but also during drying.

0066The postcrosslinker is advantageously mixed with the polymer by the process of the present invention and subsequently thermally dried.

0067Contact dryers are preferable, shovel dryers more preferable and disk dryers most preferable as apparatus in which thermal drying is carried out. Suitable dryers include for example Bepex^® dryers and Nara^® dryers. Fluidized bed dryers can be used as well.

0068Drying can take place in the mixer itself, by heating the shell or blowing warm air into it. It is similarly possible to use a downstream dryer, for example a tray dryer, a rotary tube oven or a heatable screw. But it is also possible for example to utilize an azeotropic distillation as a drying process.

0069Preferred drying temperatures range from 50 to 250 °C, preferably from 50 to 200 °C, and more preferably from 50 to 150 °C. The preferred residence time at this temperature in the reaction mixer or dryer is below 30 minutes and more preferably below 10 minutes.

0070The present invention provides an improved process for production of superabsorbent polymers of uniform quality and low maintenance costs in a large scale on a continuous belt reactor.