CA2319457A1

CA2319457A1 - Cross-linking of hydrogels with phosphoric acid esters

Info

Publication number: CA2319457A1
Application number: CA002319457A
Authority: CA
Inventors: Rudiger Funk; Volker Frenz; Uwe Stuven; Friedrich Engelhardt; Thomas Daniel
Original assignee: Individual
Current assignee: BASF SE
Priority date: 1998-02-21
Filing date: 1999-02-19
Publication date: 1999-08-26
Also published as: EP1056788A1; DE19807500C1; JP2002504567A; WO1999042495A1

Abstract

The invention relates to a method for the surface secondary cross-linking of water-absorbent polymers, according to which the polymer is sprayed with a solution for surface secondary cross-linking. As cross-linking agent said solution contains phosphoric acid esters of the formula (1) or (2), where X is OH or NH2 and R is C1-C12 alkylene, or a mixture of such esters dissolved in an inert solvent. The moist product during or after spraying is subjected to secondary cross-linking and dried by raising the temperature to 50-250 ~C. The invention also relates to water-absorbent polymers obtainable in accordance with the above method and to their use in hygiene articles, packing materials and non-woven materials.

Description

Crosslinking of hydrogels using phosphoric esters Description The present invention relates to a process for the gel or surface postcrosslinking of water-absorbing hydrogels using phosphoric esters as crosslinkers, to polymers obtainable in this way and to their use in hygiene articles and as packaging materials.
Hydrophilic highly swellable hydrogels are, in particular, polymers composed of (co)polymerized hydrophilic monomers, or are graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or crosslinked starch ethers, crosslinked carboxymethylcellulose, partially crosslinked polyalkylene oxide, or natural products that are swellable in aqueous liquids: guar derivatives, for example.
Hydrogels of this kind are used as products for absorbing aqueous solutions in the production of diapers, tampons, sanitary towels and other hygiene articles, and as water retainers in market gardening.
To improve service properties such as diaper rewet and AUL
(absorbency under load), for example, hydrophilic highly swellable hydrogels are generally subjected to surface or gel postcrosslinking. This postcrosslinking is known to the person skilled in the art and is preferably carried out in the aqueous gel phase or as surface postcrosslinking of the milled and sieved polymer particles.
Crosslinkers suitable for this purpose are compounds comprising at least two groups which are able to form covalent bonds with the carboxyl groups of the hydrophilic polymer. Examples of suitable crosslinkers are diglycidyl or polyglycidyl compounds, such as diglycidyl phosphonate, alkoxysilyl compounds, polyaziridines, polyamines and polyamidoamines, and these compounds can also be used in mixtures with one another (see for example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438).
Polyamidoamines which are suitable as crosslinkers are described in particular in EP-A-0 349 935.
A major disadvantage of these crosslinkers is their high reactivity. Although this is desirable in terms of chemical reaction, it carries with. it a relatively high toxicological potential. In production operations, the processing of such crosslinkers necessitates special protective measures in order to meet the requirements of the governing safety provisions and ~

workplace hygiene. Furthermore, the use of polymers modified in this way in hygiene articles appears to be dubious.
Polyfunctional alcohols are also known as crosslinkers. For example, EP-A-0 372 981, US-A-4 666 983 and US-A-5 385 983 teach the use of hydrophilic polyalcohols and the use of polyhydroxy surfactants. According to these documents the reaction is carried out at temperatures of 120 - 250°C. The process has the disadvantage that the esterification reaction which leads to crosslinking is relatively slow even at such temperatures.
It is an object of the present invention to provide gel or surface postcrosslinking equivalent to or superior to the prior art by using relatively inert compounds capable of reacting with carboxyl groups. This object is to be achieved with a very short reaction time and a very low reaction temperature. Ideally, the prevailing reaction conditions shall be the same as those obtaining when highly reactive epoxides are used.
We have found that this object is achieved, surprisingly, by esters of phosphoric acid with di- or polyols and amino alcohols.
The present invention accordingly provides a process for the surface postcrosslinking of water-absorbing polymers, which comprises (i) treating the polymer with a surface postcrosslinking solution which includes a crosslinker comprising one or more esters of phosphoric acid of the formula OH OH
X-R-0-P=O (1) or O-P=O (2), OH R-O
where X is OH or NH2 and R is C1-C12-alkylene, dissolved in an inert solvent and (ii) postcrosslinking and drying the moist product during or after the treatment by raising the temperature.
The postcrosslinking temperature is preferably 50-250°C, in particular between 50-200°C, specifically between 100-180°C.
In order to accelerate the reaction of the surface postcrosslinking solution, an acidic catalyst can be added.
Catalysts which can be used in the process of the invention are all inorganic acids, their corresponding anhydrides, and organic acids and their corresponding anhydrides. Examples are boric acid, sulfuric acid, hydroiodic acid, phosphoric acid, tartaric acid, acetic acid, and toluenesulfonic acid. Also suitable in particular are their polymeric forms, anhydrides, and the acid salts of the polybasic acids. Examples of these are boron oxide, sulfur trioxide, diphosphorus pentoxide, and ammonium dihydrogen phosphate.
The process of the invention is preferably carried out by spraying a solution of the surface postcrosslinker onto the dry base polymer powder. Following spray application, the polymer powder is dried thermally, it being possible for the crosslinking reaction to take place either before or during drying. Preference is given to the spray application of a solution of the crosslinker in reaction mixers and spray mixers or in mixing and drying systems such as, for example, Lodige mixers, ~BEPEX
mixers, ~NAUTA mixers, ~SHUGGI mixers or ~PROCESSALL. It is, moreover, also possible to use fluidized-bed dryers. Drying can take place in the mixer itself, by heating the outer casing, or by blowing hot air in. Likewise suitable is a downstream dryer, such as a shelf dryer, a rotary dryer or a heatable screw, for example. Alternatively, azeotropic distillation, for example, can be utilized as a drying technique. The residence time at the preferred temperature in the reaction mixer or dryer is from 5 to 90 minutes, preferably less than 30 minutes and, with very particular preference, less than 10 minutes.
As the inert solvent, preference is given to the use of water and of mixtures of water with monohydric or polyhydric alcohols. It is, however, also possible to use any organic solvent of unlimited miscibility with water, such as certain esters and ketones, for example, which are not themselves reactive under the process conditions. Where an alcohol/water mixture is used, the alcohol content of this solution is, for example, 10-90% by weight, preferably 30-70% by weight, in particular 40-60% by weight. Any alcohol of unlimited miscibility with water can be used, as can mixtures of two or more alcohols (e.g., methanol +
glycerol + water). Particular preference is given to the use of the following alcohols in aqueous solution: methanol, ethanol, isopropanol, ethylene glycol and, with particular preference, 1,2-propanediol and also 1,3-propanediol. The surface postcrosslinking solution is used in a ratio of 1-20% by weight, based on the polymer mass. Particular preference is given to a solution quantity of 2.5-15% by weight with respect to polymer.
The crosslinker itself is used in an amount of 0.01-1.0% by weight, based on the polymer used.

The water-absorbing polymer is preferably a polymeric acrylic acid or a polyacrylate. This water-absorbing polymer can be prepared in accordance with a method known from the literature.
Preference is given to polymers containing crosslinking comonomers (0.001-10 mol%); very particular preference is given, however, to polymers obtained by free-radical addition polymerization using a polyfunctional ethylenically unsaturated free-radical crosslinker which additionally carries at least one free hydroxyl group (such as, for example, pentaerythritol triallyl ether or trimethylolpropane diallyl ether).
The hydrophilic highly swellable hydrogels to be employed in the process of the invention are, in particular, polymers composed of (co)polymerized hydrophilic monomers, or are graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or crosslinked starch ethers, or natural products which are swellable in aqueous liquids: guar derivatives, for example. These hydrogels are known to the person skilled in the art and are described, for example, in US-A-4 286 082, DE-C-27 06 135, US-A-4 340 706, DE-C-37 13 601, DE-C-28 40 010, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-35 3 3337, DE-A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261, DE-A-35 11 086, DE-A-31 18 172, DE-A-30 28 043, DE-A-44 18 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A-0 312 952, EP-A-0 205 874, EP-A-0 499 774, DE-A-26 12 846, DE-A-40 20 780, EP-A-0 205 674, US-5 145 906, EP-A-0 530 438, EP-A-0 670 073, US-Pr4 057 521, US-A-4 062 817, US-A-4 525 527, US-A-4 295 987, US-A-5 011 892, US-A-4 076 663 or US-A-4 931 497. The content of the abovementioned patent documents is expressly incorporated into the present disclosure by reference.
Examples of hydrophilic monomers suitable for preparing these hydrophilic highly swellable hydrogels are polymerizable acids, such as acrylic acid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, malefic acid including its anhydride, fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-methylpropanephosphonic acid, and also their amides, hydroxyalkyl esters and amino- or ammonium-containing esters and amides. Also suitable, furthermore, are water-soluble N-vinyl amides such as N-vinylformamide or else diallyldimethylammonium chloride. Preferred hydrophilic monomers are compounds of the formula \ /
C=C (3) / \

in which R1 hydrogen, methyl or ethyl, R2 is -COOR4, a sulfonyl group, a phosphonyl group, a (C1-C4)-alkanol-esterified phosphonyl group, or a group of the formula .

s ~C~ ~C\ ,R 4 N ~ CHZ
H

in which R3 is hydrogen, methyl, ethyl or a carboxyl group, R4 is hydrogen, amino-(C1-C4)-alkyl, hydroxy-(C1-C4)-alkyl, alkali metal or ammonium ion and R5 is a sulfonyl group, a phosphonyl group, a carboxyl group or the alkali metal or ammonium salts of these groups.
Examples of (C1-C4)-alkanols are methanol, ethanol, n-propanol, isopropanol and n-butanol. Particularly preferred hydrophilic monomers are acrylic and methacrylic acid and the alkali metal ammonium salts of these acids, for example, sodium acrylate, potassium acrylate or ammonium acrylate.
Suitable graft bases for hydrophilic hydrogels obtainable by graft copolymerization of olefinically unsaturated acids or of their alkali metal or ammonium salts maybe natural or synthetic in origin. Examples are starch, cellulose and cellulose derivatives, and also other polysaccharides and oligosaccharides, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, and hydrophilic polyesters.

Suitable polyalkylene oxides have, for example, the formula X
R6 - O - (CHy- CH - O)n - R7 (5) in which R6 and R7 independently of one another are hydrogen, alkyl, alkenyl or aryl, X is hydrogen or methyl, and n is an integer from 1 to 10,000.
R6 and R7 are preferably hydrogen, (C1-C4)-alkyl, (C2-C6)-alkenyl or phenyl. Particularly preferred hydrogels are polyacrylates, polymethacrylates, and the graft copolymers described in US-A-4 931 497, US-A-5 Oll 892 and US-A-5 041 496.
The hydrophilic highly swellable hydrogels are preferably in crosslinked form; that is, they include compounds having at least two double bonds which have been copolymerized into the polymer network. Particularly suitable crosslinkers are methylenebisacrylamide and methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids with polyols, such as diacrylate or triacrylate, examples being the diacrylates and dimethacrylates of butanediol and of ethylene glycol, and trimethylolpropane triacrylate, and also allyl compounds such as~~
allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid, and vinylphosphonic acid derivatives as described, for example, in EP-A-0 343 427. In the process of the invention, however, particular preference is given to hydrogels prepared using polyallyl ethers as crosslinkers and by acidic homopolymerization of acrylic acid. Suitable crosslinkers are pentaerythritol tri-and tetraallyl ether, polyethylene glycol diallyl ether, monoethylene glycol diallyl ether, glycerol di- and triallyl ether, polyallyl ethers based on sorbitol, and alkoxylated variants thereof.
The hydrophilic highly swellable hydrogels can be prepared by conventional polymerization processes. Preference is given to addition polymerization in aqueous solution by the process known as gel polymerization. In this process from 15 to 50% strength by weight aqueous solutions of one or more hydrophilic monomers, and, if desired, of a suitable graft base, are polymerized in the presence of a free-radical initiator, preferably without mechanical mixing, utilizing the Trommsdorff-Norrish effect (Makromol. Chem. 1 (1947) 169).
The polymerization reaction can be conducted in the temperature range between 0°C and 150°C, preferably between 10°C and 100°C, either at atmospheric pressure or under an increased or reduced pressure. The polymerization may also be performed in an inert gas atmosphere, preferably under nitrogen.
The polymerization can be initiated using high-energy electromagnetic radiation or by the customary chemical polymerization initiators. Examples of these are organic peroxides, such as benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide and cumene hydroperoxide, azo compounds, such as azodiisobutyronitrile, and inorganic peroxo compounds, such as (NH4)2Sy08, KZS208 or H202. They can if desired be used in combination with reducing agents such as sodium hydrogen sulfite or iron(II) sulfate, or redox systems. Redox systems include a reducing component, which is generally an aliphatic or aromatic sulfinic acid, such as benzenesulfinic acid or toluenesulfinic acid or derivatives of these acids, such as Mannich adducts of sulfinic acid, aldehydes and amino compounds, as described in DE-C-13 O1 566.
The qualities of the polymers can be improved further by continuing to heat the polymer gels for a number of hours within the temperature range from 50 to 130°C, preferably from 70 to 100°C .
The resultant gels are neutralized to the extent of 0-100 mol%
based on monomer employed, preferably 25-100 mol% and with particular preference 50-85 mol%, it being possible to use the customary neutralizing agents, preferably alkali metal hydroxides or alkali metal oxides, and with particular preference sodium hydroxide, sodium carbonate or sodium bicarbonate. Neutralization is usually effected by mixing in the neutralizing agent as an aqueous solution or else, preferably, as a solid. For this purpose the gel is mechanically comminuted, by means of a miNCer for example, and the neutralizing agent is sprayed on, scattered over or poured on, and then carefully mixed in. To effect homogenization the resultant gel mass may be passed through the mincer again a number of times.

The neutralized gel mass is then dried with a belt dryer or roll dryer until the residual moisture content is less than 10% by weight, preferably below 5% by weight. The dried hydrogel is then ground and sieved, the usual grinding apparatus being roll mills, pin mills or vibrator mills. The preferred particle size of the sieved hydrogel lies in the range 45-1000 Eun, with particular preference 45-850 Eun and with very particular preference 200-850 Eun.
According to the invention, acrylate polymers are crosslinked using esters of di- or polyols and aminoalcohols. The esters of the polyols are described by the formula OH
HO-R-O-P=O (6) OH
and the esterification of aminoalcohols results in compounds of the formula OH
HyN - R - O - P = O (7).
OH
The use of polyols may also lead to the formation of cyclic esters of the general formula OH
O-P-O (2), R - O
R3 Ri \ /
C=C (2) / \

In the formulae (2), (6) and (7), R is C1- to C12-alkylene, preferably.C2- to C6-alkylene, especially -CH2-CHZ- or - CH -CHZ -These compounds may be obtained not only by esterification of the free phosphoric acid with the alcohols, by transesterification reactions or by the reaction of the diols, polyols or aminoalcohols with phosphorus pentoxide or phosphorus oxychloride.
In order to ascertain the quality of surface postcrosslinking the dried hydrogel is then tested using the test methods known from the prior art and described below:
Methods:
1) Centrifuge retention capacity (CRC):
This method measures the free swellability of the hydrogel in a teabag. Approximately 0.200 g of dry hydrogel is sealed into a teabag (format: 60 mm x 60 mm, Dexter 1234T paper) and soaked for minutes in 0.9% strength by weight sodium chloride solution.
The teabag is then spun for 3 minutes in a customary commercial 25 spindryer (Bauknecht WS 130, 1400 rpm, basket diameter 230 mm).
The amount of liquid absorbed is determined by weighing the centrifuged teabag. The absorption capacity of the teabag itself is taken into account by determination of a blank value (teabag without hydrogel), which is deducted from the weighing result 30 (teabag with swollen hydrogel).
Retention CRC [g/g] _ (weighing result teabag - blank value -initial weight of hydrogel) . initial weight of hydrogel 2) Absorbency under load (0.3 / 0.5 / 0.7 psi):
For the absorbency under load, 0.900 g of dry hydrogel is distributed uniformly on the screen base of a measuring cell. The measuring cell consists of a Plexiglas cylinder (height = 50 mm, diameter = 60 mm) whose base is formed by sticking on a screen of steel mesh (mesh size 36 microns, or 400 mesh).
A cover plate is placed over the uniformly distributed hydrogel and loaded with an appropriate weight. The cell is then placed on a filter paper (S&S 589 black band, diameter = 90 mm) lying on a porous glass filter plate, this filter plate itself lying in a Petri dish (height = 30 mm, diameter = 200 mm) which contains 0.9% strength by weight sodium chloride solution so that the liquid level at the beginning of the experiment is level with the top edge of the glass frit. The hydrogel is then left to absorb the salt solution for 60 minutes. Subsequently, the complete cell 5 with the swollen gel is removed from the filter plate and the apparatus is reweighed following removal of the weight.
The absorbency under load (AUL) is calculated as follows:
10 AUL (g/gj = ( Wb - Wa ) / Ws where Wb is the mass of the apparatus + gel after swelling, Wa is the mass of the apparatus + initial weight of gel before swelling, and Ws is the initial weight of dry hydrogel.
The apparatus is measuring cylinder + cover plate.
Examples Preparation of crosslinkers 1 to 3 Crosslinker 1:
2-Aminoethyl dihydrogenphosphate In a three-neck flask with stirrer, internal thermometer, reflux condenser and gas inlet tube, 1 mol of aminoethanol is dissolved in 2 mol of water and POC13 is gradually added dropwise with cooling: The resulting hydrochloric acid is neutralized with ammonia and subsequently stirred at room temperature for 24 hours. The water is then distilled off and the ester prepared is recrystallized from water / alcohol at low temperature to provide a brownish compound having a melting point of 240°C (with decomposition). The compound is moderately soluble in water and water/alcohol mixtures and is slow to hydrolyze.
Crosslinker 2:
2-Hydroxypropyl or 1-hydroxy-2-propyl dihydrogenphosphate or mixture of the two compounds Propanediol is reacted with POC13 in chloroform in a similar manner to the preparation of crosslinker 1 to provide a hygroscopic brownish compound which is but slow to hydrolyze, yet is readily soluble in water and water/alcohol mixtures.

IZ
Crosslinker 3:
Mono- and diphosphoric esters or mixture of the two compounds.
A three-neck flask with stirrer, internal thermometer and condenser is charged with 3 mol of 2-hydroxyethyl methacrylate and 1 mol of phosphorus pentoxide is added in such a way with cooling that 40°C is not exceeded. This is followed by stirring at room temperature for 1 hour to provide a brownish substance which is paraffinic below 20°C, yet is readily soluble in water and water/alcohol mixtures. The clear viscous compound is dissolved in water and free-radically polymerized at elevated temperature under nitrogen to form a low molecular weight polymer.
Inventive examples:
The inventive examples illustrate the effect of the surface postcrosslinking on the superabsorbent polymers. As will be known to those skilled in the art, this postcrosslinking can be determined by measuring the centrifuge retention capacity (CRC) and the absorbency under load (AUL). This surface crosslinking causes the CRC to decrease typically by 5-10 g/g, while the AUL
0.7 psi increases by about 10 and the AUL 0.3 psi by more than 20 g/g.
Example 1 Base polymer In a 40 1 plastic bucket, 6.9 kg of pure acrylic acid are diluted with 23 kg of water. 45 g of pentaerythritol triallyl ether are added with stirring to this solution, and the sealed bucket is rendered inert by passing nitrogen through it. The polymerization is then initiated by adding about 400 mg of hydrogen peroxide and 200 mg of ascorbic acid. After the end of the reaction the gel is mechanically comminuted and sodium hydroxide solution is added in an amount sufficient to achieve a degree of neutralization of 75 mol%, based on the acrylic acid employed. The neutralized gel is then dried on a roll dryer, ground with a pin mill and, finally, isolated by sieving. This is the base polymer used in the subsequent examples. CRC, AUL 20 and AUL 40 are reported in the table.

Example 2 The base polymer prepared in Example 1 is sprayed with crosslinker soluton in a blaring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer used, is obtained:
0.40% by weight of crosslinker 1, 5% by weight of propylene glycol and 5% by weight of water as solvent. The moist polymer is then divided into two batches (a) and (b), which are each dried at 175°C. The drying time is 60 min for batch (a) and 90 min for batch (b).
The crosslinking effect is not attributable to the propylene glycol, since surface postcrosslinking is obtained even in purely aqueous systems if the use level is increased. CRC and AUL are reported in the table.
Example 3:
The base polymer prepared in Example 1 is sprayed with crosslinker soluton in a blaring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer used, is obtained:
3.0% by weight of crosslinker 1 and 10% by weight of water. The moist polymer is then dried at 175°C
(a) for 60 min and (b) for 90 min.
The use of catalysts shortens the reaction time for the surface postcrosslinking and hence the residence time in the reactor.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 4 The base polymer prepared in Example 1 is sprayed with crosslinker soluton in a blaring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer used, is obtained:
1.5% by weight of crosslinker 2, 10% by weight of water and 0.2%
of boric acid as catalyst. The moist polymer is then dried at 175°C

' . CA 02319457 2000-08-02 (a) for 30 min and (b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 5 The base polymer prepared in Example 1 is sprayed with crosslinker soluton in a blaring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer used, is obtained:
5.0% by weight of crosslinker 3, 12% by weight of water and 0.2%
by weight of ammonium dihydrogenphosphate. The moist polymer is then dried at 175°C
(a) for 30 min and (b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 6 The base polymer prepared in Example 1 is sprayed with crosslinker soluton in a blaring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer used, is obtained:
0.20% by weight of crosslinker 2, 5% by weight of methanol, 5% by weight of water and 0.2% of ammonium dihydrogenphosphate. The moist polymer is then dried at 175°C
(a) for 30 min and (b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 7 The base polymer prepared in Example 1 is sprayed with crosslinker soluton in a blaring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer used, is obtained:

0.50% by weight of crosslinker 2, 5% by weight of 1,2-propanediol and 5% by weight of water. The moist polymer is then dried at 175°C
(a) for 30 min and (b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 8 The base polymer prepared in Example 1 is sprayed with crosslinker soluton in a blaring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer used, is obtained:
0.20% by weight of crosslinker 3, 6% by weight of 1,2-propanediol and 6% by weight of water. The moist polymer is then dried at 175°C
(a) for 30 min and (b) for 60 min.
(c) For comparison, the base polymer is sprayed in similar fashion with 6% by weight of 1,2-propanediol and 6% by weight of water. The moist polymer is then dried at 175°C for 60 min. This comparative example thus does not employ a crosslinker. CRC, AUL 20 and AUL 40 are reported in the table.

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Claims

We claim:

1. A process for the surface postcrosslinking of water-absorbing polymers, which comprises (i) treating the polymer with a surface postcrosslinking solution which includes a crosslinker comprising one or more esters of phosphoric acid of the formula where X is OH or NH2 and R is C1-C12-alkylene, dissolved in an inert solvent and (ii) postcrosslinking and drying the moist product during or after the treatment by raising the temperature.

2. The process as claimed in claim 1, wherein the water-absorbing polymer is a polymeric acrylic acid or a polyacrylate, especially a polymeric acrylic acid or polyacrylate obtained by free-radical addition polymerization in the presence of a polyfunctional ethylenically unsaturated free-radical crosslinker which may additionally carry one or more free hydroxyl groups.

3. The process as claimed in claim 1 and/or 2, wherein the crosslinking is effected using a catalyst comprising an inorganic acid, its corresponding anhydride or an organic acid or its corresponding anhydride.

4. The process as claimed in claim 3, wherein the acid is boric, sulfuric, hydroiodic, phosphoric, tartaric, acetic or toluenesulfonic acid or the polymeric forms, anhydrides or acid salts thereof.

5. The process as claimed in one or more of claims 1 to 4, wherein the inert solvent is water, a mixture of water with organic solvents of unlimited solubility in water, or a mixture of water with monohydric or polyhydric alcohols.

6. The process as claimed in claim 5, wherein if an alcohol/water mixture is used the alcohol content of this solution is 10-90% by weight, preferably 30-70% by weight.

7. The process as claimed in claim 5 or 6, wherein the alcohol is methanol, ethanol, isopropanol, ethylene glycol, 1,2-propanediol or 1,3-propanediol.

8. The process as claimed in one or more of claims 1 to 4, wherein the surface postcrosslinking solution is employed in a proportion of 1-20% by weight, in particular 2.5-15% by weight, based on the mass of the polymer.

9. A water-absorbing polymer prepared by the process as claimed in one or more of claims 1 to 8.

10. The use of a polymer prepared by the process as claimed in one or more of claims 1 to 8 in a hygiene article, packaging material or nonwoven.