Water-absorbent resin

Abstract

A water-absorbent resin comprises a network of partially neutralized, chain-extended or crosslinked polyacrylic acid polymers having a weight-average molecular weight of from 1000 to 100000 g / mol. The polyacrylic acid polymers are chain-extended or crosslinked via ester bonds between pendant carboxylic groups of the polyacrylic acid polymers and polyol molecules. The water-absorbent resin has an improved biodegradability.

Description

240163W001 1Water-absorbent resinThe present invention relates to a water-absorbent resin comprising a network of chain-extended or crosslinked polyacrylic acid polymers.Superabsorbent polymers (SAPs) are primarily used in personal care products which absorb body fluids, for example, baby diapers, adult incontinence products and feminine hygiene products. In these applications, SAP particles are incorporated into absorbent structures which contain, for example, synthetic and natural fibers which may or may not be in the form of non-woven structures, and toughened masses of fibers, such as fluff pads.For sustainability reasons, consumers have expressed an increased desire for biodegradability and / or biocompostability in absorbent sanitary articles. As such, biopolymers and bio-derived polymers for integration in the absorbent articles have been examined for their absorbency properties, including naturally occurring materials and their derivatives, such as starches, celluloses, and polysaccharides. However, these biopolymers are often more expensive to make, and have inferior absorption properties relative to traditional, non-biocompostable materials, such as polyacrylates.Traditionally, superabsorbent polymers are synthetically derived polyacrylic acids that are partially neutralized. A preferred molecular structure of SAP is a network of high molecular weight main chains of polyacrylic acid into which crosslinks are introduced. In order to render the polymer insoluble in water and exhibit a high degree of swelling, the distance between the crosslinking points must be sufficiently long and there must be no soluble portions. Water absorption capacity is controlled by crosslink density.Polyacrylic acids are generally considered non-biodegradable, and non-biocompostable. The mechanism of degradation of high molecular weight polyacrylate is not fully elucidated but the available evidence suggests that the carbon chain of the polymeric backbone is shortened in a step-wise fashion from the terminals. Hence, the degradability of the carbon chain of polyacrylates is most likely related to the molecular weight. This implies that smaller polymers are easier to degrade than larger ones.There is a need for polyacrylic acid-based water-absorbent resins having improved biodegradability.The invention relates to a water-absorbent resin comprising a network of partially neutralized, chain-extended or crosslinked polyacrylic acid polymers having a weight-average molecular weight of from 1000 to 100000 g / mol, wherein the polyacrylic acid polymers are chain-extended or crosslinked via ester bonds between pendant carboxylic groups of the polyacrylic acid polymers and polyol molecules.240163W001 2Without wishing to be bound by theory, it is believed that the ester bonds between pendant carboxylic groups of the polyacrylic acid polymers and the polyol molecules are readily hydrolyzed in the presence of moisture in a biological environment e.g. by esterases or in the presence of acidic compounds. The carbon chains of the polymeric backbone of polyacrylic acid polymers are shortened in a step-wise fashion from the terminals. Hence, the relatively short polyacrylic acid chains of the inventive water-absorbent resin network having a relatively low molecular weight enable an improved biodegradability.The terms "chain-extended” and "crosslinked” are not sharply delineated from one another. A "chain-extended” polyacrylic acid polymer is generally considered to be the reaction product of a polyol and a carboxylic acid group at or close to the terminal of a first polyacrylic acid polymer and said polyol and a carboxylic acid group at or close to the terminal of a second polyacrylic acid polymer.A "crosslinked” polyacrylic acid polymer is the reaction product of a polyol with carboxylic acid groups of two polyacrylic acid polymers, wherein at least one carboxylic acid group is located away from the end of the polyacrylic acid polymer so that branching or crosslinking is introduced into the molecule.The term "polyacrylic acid” or "acrylic acid polymer” is used to encompass a variety of polymers with high percentages of, e.g., at least 50 or at least 75 mol-% of the repeating units being derived from acrylic acid. The polyacrylic acid polymers may be homopolymers of acrylic acid or copolymers of acrylic acid with other unsaturated, polymerizable carboxylic monomers such as methacrylic acid, maleic acid, itaconic acid, maleic anhydride, and the like. The polyacrylic acid polymers, encompass copolymers of acrylic acid and optionally other polymerizable carboxylic monomers with acrylate esters, acrylamides, olefins, vinyl esters, vinyl ethers, or styrenics. Acrylic acid homopolymers are generally preferred.The polyacrylic acid polymers have weight-average molecular weights in the range of from 1000 to 5000 g / mol, preferably 1000 to 2000 g / mol, more preferably 1000 to 1500 g / mol. Examples of suitable commercially available polyacrylic acids include, but are not limited to, Sokalan PA 25 S (available from BASF SE), Sokalan PA 25 PN-55 (available from BASF SE), Acusol 445N (available from Dow Chemical Company, Midland, Mich.) and Acusol 445ND (available from Dow Chemical Company, Midland, Mich.). Acusol 445N and Acusol 445ND both have a molecular weight of about 4,500 g / mol. Sokalan PA 25 S and Sokalan PA 25 PN-55 both have a molecular weight of about 4,000 g / mol.The molecular weight of the poly acrylic acids constituting an inventive network can be ascertained by subjecting the network to a hydrolysis test under conditions under which the ester bonds are hydrolyzed and the poly acrylic acid backbone is left intact. The hydrolysis test can be carried out as described in EP 2013251 B1,

[0021] : 20 mg of the water-absorbent resin is left in 10 g of a 0.1 mol / L aqueous solution of sodium hydroxide at 80 °C for 3 weeks.240163W001 3The molecular weight of the polyacrylic acids can then be determined via Gel-Permeation Chromatography (GPC) (also referred to as "Size-Exclusion Chromatography (SEC)”) following a procedure as described in EP 2013251 B1 ,

[0158] -

[0161] , Equivalent analytical methods can also be used.The polyacrylic acid polymers of the invention are chain-extended or crosslinked via polyol molecules.The nature of the polyol is not particularly restricted. Examples for suitable polyols include alkanepolyols and intramolecular or intermolecular dehydration products thereof, sugar polyols, dextrins, polyvinyl alcohols, acrylic polyols, and mixtures thereof.Alkanepolyols and intramolecular or intermolecular dehydration products thereof, preferably Ca-Ca-Alkanepolyols and intramolecular or intermolecular dehydration products thereof, include glycerin, 1,2,6-tri hydroxy hexane, 1 ,2,3-butanetriol, 1 ,2,3-hexanetriol, trimethylolethane, trimethylolpropane, pentaerythritol, sorbitol, sorbitan, 1,2,3-cyclohexanetriol and polyglycerin.The sugar polyol may be selected from glucose, fructose, sucrose, lactose, maltose, erythritol, threitol, arabitol, xylitol, ribitol, mannitol, sorbitol, galactitol, fucitol, iditol, inositol, volemitol, quinic acid, shikimic acid, isomalt, maltitol, lactitol, maltotriitol, maltotetraitol, polyglycitol, and mixtures thereof.Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch and glycogen. Dextrins are mixtures of polymers of D-glucose units linked by a-(1-4) or a-(1-6) glycosidic bonds. Dextrins suffice the following general formula (I)For the purpose herein, suitable dextrins are those of formula (I) with n being in the range of from 1 to 18.Polyvinyl alcohols suffice the following general formula (II)240163W001 4For the purpose herein, suitable polyvinyl alcohols are those of formula (II) with n being in the range of from 500 to 3000, preferably 900 to 2250.Acrylic polyols include copolymers of hydroxyethyl (meth)acrylate and other vinyl monomers.Examples of the polyol include diols and polyols having at least three hydroxyl groups per molecule of polyol. The polyol may have at least three hydroxyl groups per molecule of polyol, e.g. 3 to 50 or 3 to 8, hydroxyl groups per molecule of polyol.Alkanepolyols are generally preferred, preferably C2-C8-alkanepolyols, more preferably Ca-Cs-alkanepolyols, in particular Ca-Cs-alkanepolyols having two or three hydroxyl groups.The water-absorbent resin is generally obtainable by chain-extending and crosslinking the polyacrylic acid polymers with 1 to 25 wt.-%, preferably 2 to 10 wt.-% of the polyol, based on the weight of the polyacrylic acid polymers.The chain-extended or crosslinked polyacrylic acid polymers of the invention are partially neutralized. This means that the pendant carboxylic groups that are not engaged in ester bonds with the polyol are partially neutralized. In an embodiment, the polyacrylic acid polymers have a degree of neutralization in the range of from 40 to 95 mol-%, preferably 50 to 90 mol-%, more preferably 60 to 85 mol-%.Partially neutralized acrylic acids polymers swell faster with better water absorbing capacity due to the increment of total ionic charges inside the polymer network. This ionic charge difference between inside and outside the polymer network creates higher osmotic pressure difference. Therefore, more water molecules tend to diffuse inside the polymer network.Customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogencarbonates and also mixtures thereof. Instead of alkali metal salts, it is also possible to use ammonium salts. Particularly preferred alkali metals are sodium and potassium, but very particular preference is given to sodium hydroxide, sodium carbonate or sodium hydrogencarbonate and mixtures thereof.The invention further relates to a process for making a water-absorbent resin comprising a network of chain-extended or crosslinked polyacrylic acid polymers, comprising the following steps:(1) providing polyacrylic acid polymers having a weight-average molecular weight of from 1000 to 100000 g / mol, preferably 1000 to 5000 g / mol, more preferably 1000 to 2000 g / mol,(2) chain-extending and crosslinking polyacrylic acid polymers by esterification between polyol molecules and pendant carboxylic groups of the polyacrylic acid polymers.240163W001 5In an embodiment, the process additionally comprises the following step:(3) partially neutralizing the chain-extended or crosslinked polyacrylic acid polymers.Step (1) above involves providing polyacrylic acid polymers as described above. Preferably, the polyacrylic acid polymers are in an acidic, essentially unneutralized form. It was found that ester formation is more efficient with the polyacrylic acid polymers being in an acidic form.Step (2) above involves an esterification reaction between polyol molecules and carboxylic groups of the polyacrylic acid polymers provided in step 1. In an embodiment, esterification of step (2) comprises(2a) providing an aqueous solution containing the polyacrylic acid polymers and polyol, and heating the aqueous solution to effect esterification with concomitant evaporation of water to obtain solid matter.The aqueous solution may be heated with any heating device. Suitably, heating the aqueous solution may be carried out in an oven, preferably in a convection oven. For this purpose, the aqueous solution may be poured onto a surface of carrier, e.g. a tray, and the carrier may subsequently be placed in the oven, followed by providing heat with the oven. Suitably, the aqueous solution is heated to a temperature in the range of from 110 to 190 °C, preferably 130 to 170 °C, more preferably 140 to 160 °C. As a result, esterification is effected, together with evaporation of water from the aqueous solution. Solid matter of the esterification product is obtained. The solid matter preferably has a residual moisture content of less than 10 wt.-%, for example 1 to less than 10 wt.-%, preferably 1.5 to 8 wt.-%. The residual moisture content is determined by EDANA recommended test method No. WSP 230.2 (05) "Moisture Content - Weight loss upon Heating".In an embodiment, the process additionally comprises the following step:(2b) milling and, optionally, sieving the solid matter to obtain comminuted matter.Milling can typically be accomplished using one-stage or multistage roll mills, preferably roll mills, pinned disk mills, hammer mills or vibratory mills, more preferably one-stage roll mills. Sieving comprises at least two screens, in which case additional screens can be used as what are called guard screens, in order to avoid blockage of the screen pores. Sieving removes both comminuted matter particles having a particle size greater than a preset upper limit ("coarse particles") and comminuted matter particles having a particle size smaller than a preset lower limit ("fines"). A typical upper limit is, for example, 850 pm, or 700 pm, or 600 pm, a typical lower limit 200 pm, or 150 pm, or 100 pm.The mean particle size of the comminuted matter particles is preferably at least 200 pm, more preferably from 250 to 600 pm, most preferably from 300 to 600 pm. The mean particle size of the product fraction may be determined by means of EDANA recommended test method No. WSP 220.2 (05) "Particle Size Distribution", where the proportions by mass of the screen fractions are plotted in cumulated form and the mean particle size240163W001 6is determined graphically. The mean particle size here is the value of the hypothetical equivalent mesh size at which 50 wt.-% pass through the screen and 50 wt.-% remain on the screen. It is possible to determine the mean particle size practically by a screen analysis. The median particle diameter is the d50 of the cumulative particle size distribution.The proportion of particles having a particle size of at most 850 m, or 700 pm, or 600 pm is preferably at least 90 wt.-%, more preferably at least 95 wt.-%, most preferably at least 98 wt.-%.The proportion of particles having a particle size of greater than 200 pm, or 150 pm, or 100 pm is preferably at least 90 wt.-%, more preferably at least 95 wt.-%, most preferably at least 98 wt.-%.Partial neutralization may be implemented as follows:(3a) evenly moistening, e.g., by spraying, the solid matter or comminuted matter with an aqueous solution of a neutralization agent, and(3b) heating and drying the moistened neutralized comminuted matter to obtain the water-absorbent resin.For partial neutralization, the neutralizing agents as described above may be used. Preference is given to sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, and mixtures thereof. The resulting polyacrylic acid polymers suitably have a degree of neutralization in the range of from 40 to 95 mol-%, preferably 50 to 90 mol-%, more preferably 60 to 85 mol-%.Step (3b) involves heating and drying the moistened neutralized comminuted matter to obtain the waterabsorbent resin. Heating may be carried out with any heating device. Suitably, heating may be carried out in an oven, preferably in a convection oven. For this purpose, the moistened neutralized comminuted matter may be spread on a tray, and the tray may subsequently be placed in the oven, followed by providing heat with the oven. Solid matter of the water-absorbent resin is obtained. Alternatively, the moistened neutralized comminuted matter can be dried by means of any apparatus suitable for that purpose, such as a belt drier fluidized bed drier or a paddle drier.In an embodiment, the process additionally comprising the following step:(4) milling and sieving the water-absorbent resin.Milling and sieving of the water-absorbent resin obtained in step (4) may be carried out as described above for step (2b).To further improve the properties, the water-absorbent resin particles may be surface crosslinked. Suitable surface crosslinkers are compounds which comprise groups which can form covalent bonds with at least two carboxylate groups of the water-absorbent resin particles. Suitable compounds are, for example, polyfunctional240163W001 7amines, polyfunctional amido amines, polyfunctional epoxides, as described in EP 0083022 A2, EP 0543303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 35 23617 A1 and EP 0450922 A2, or p-hydroxyalkylamides, as described in DE 10204938 A1 and US 6,239,230.Further suitable surface crosslinkers are cyclic carbonates described in DE 4020780 C1, 2-oxazolidinone and derivatives thereof, such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 502 A1, bis- and poly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazine and derivatives thereof in DE 198 54 573 A1, N-acyl-2-oxazolidinones in DE 19854574 A1, cyclic ureas in DE 10204937 A1, bicyclic amido acetals in DE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 and morpholine-2, 3-dione and derivatives thereof in WO 03 / 031482 A1.Preferred surface crosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1 ,4-butanediol.Very particularly preferred surface crosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol.In addition, it is also possible to use surface crosslinkers which comprise additional polymerizable ethylenically unsaturated groups, as described in DE 37 13601 A1.The amount of surface crosslinker is preferably 0.001 to 2 wt.-%, more preferably 0.02 to 1 wt.-%, most preferably 0.05 to 0.2 wt.-%, based in each case on the water-absorbent resin particles.The present invention further provides hygiene articles comprising the water-absorbing resin as described above.The hygiene articles typically comprise a water-impervious backside, a water-pervious topside and an intermediate absorbent core composed of the inventive water-absorbing resin and fibers, preferably cellulose. In an embodiment, the proportion of the inventive water-absorbing resin in the absorbent core is 20 to 100 wt.-%, preferably 50 to 100 wt.-%.The present invention is illustrated by the examples that follow.Analysis methodsThe standard test methods described hereinafter and designated "WSP" are described in: "Standard Test Methods for the Nonwovens Industry", 2005 edition, published jointly by the Worldwide Strategic Partners EDANA (Avenue Eugene Plasky, 157, 1030 Brussels, Belgium, www.edana.org) and INDA (1100 Crescent240163W001 8Green, Suite 115, Cary, North Carolina 27518, U.S.A., www.inda.org). This publication is available both from EDANA and from INDA.The measurements should, unless stated otherwise, be conducted at an ambient temperature of 23 ± 2 °C and a relative air humidity of 50 ± 10%. The water-absorbing resin particles are mixed thoroughly before the measurement.FreeThe free swelling capacity (FSC) is determined by EDANA recommended test method No. WSP 240.2 (05) "Free Swell Capacity in Saline by Gravimetric Determination".Centrifuge retention capacity (CRC)The centrifuge retention capacity (CRC) is determined by EDANA recommended test method No. WSP 241.2 (05) "Gravimetric Determination of Fluid Retention Capacity in Saline Solution After Centrifugation".The absorption under a load of 0.1 psi is determined analogously to EDANA recommended test method No. WSP 242.2 (05) "Gravimetric Determination of Absorption Under Pressure", except that a load of 0.1 psi (7.0 g / cm2) is established rather than a load of 0.3 psi (21.1 g / cm2).The absorption under a load of 0.3 psi is determined by EDANA recommended test method No. WSP 242.2 (05) "Gravimetric Determination of Absorption Under Pressure".ExtractablesThe water soluble polymer content (extractables) is determined by EDANA recommended test method No. WSP 270.2 (05) "Determination of Extractable Polymer Content by Potentiometric Titration".ExamplesUnless stated otherwise, the following experiments are conducted in the same manner. First, a non-neutralized absorbent resin was synthesized and secondly, such resins were neutralized. In the first step, the stated polyol was added to a 250 mL beaker in a mass to fulfill the stated weight percentage. Secondly, an aqueous solution240163W001 9of the stated polyacrylic acid polymer with the stated molecular weight was added to the beaker. The resulting mixture was stirred at 400 rpm for 5 min. Then, the mixture was poured on parchment paper and heated in a lab air circulation (unless stated otherwise, 150 °C, 90 min). The resulting solid matter was comminuted using a mortar and sieved (300-600 m) to obtain a water-absorbent resin which was investigated for its FSC and CRC.For example, for example #5*, 0.5 g of glycerol were added to a beaker and mixed with 36 g of Sokalan PA 25 X S (49 wt.-% aqueous solution corresponding to 17.6 g of polyacrylic acid) resulting in a glycerol concentration of 2.8 wt.-%, based on the weight of the polyacrylic acid polymers. The mixture was stirred for 5 min at 400 rpm and then dried as described above.As the second step, 2 g of the non-neutralized resin was partially neutralized by evenly moistening with an aqueous 0.1 M Na2COa solution to reach a degree of neutralization of 75% (unless stated otherwise) and allowing to stand for 30 min. The resulting polymer gel was dried in a lab air circulation oven (140 °C, 120 min). The resulting water-absorbent resins were investigated for its FSC and CRC.The following polyacrylic acid polymers were used:- Sokalan NR 1000 (50 wt.-%): Polyacrylic acid polymer having a molecular weight of 1500 g / mol (available from BASF SE)- PAA 2000 (50 wt.-%): Polyacrylic acid polymer having a molecular weight of 2000 g / mol (available from Sigma Aldrich)- Sokalan PA 25 X S (49 wt.-%): Polyacrylic acid polymer having a molecular weight of 5000 g / mol (available from BASF SE)- Sokalan PA 80 S (35 wt.-%): Polyacrylic acid polymer having a molecular weight of 100000 g / mol (available from BASF SE)Example 1 - Varying the molecular weight of polyacrylic acid polymersExperiments were carried out with different polyacrylic acid solutions featuring polymers with different weightaverage molar masses in the range of from 1500 to 100000 g / mol. Due to the different molar weight, different weight percentages were chosen for the polyol, here glycerol. The resulting analytics of the non-neutralized and neutralized resin are summarized in Table 1.240163W001 10Table 1.1] polyacrylic acid polymer[2] molecular weight[3] wt.-% based on the weight of the polyacrylic acid polymers[4] degree of neutralization* comparative exampleInventive resins 2, 4, 6 and 8 showed improved FSC and CRC in comparison to non-neutralized, comparative resins 1*, 3*, 5* and 7*. Superior FSC and CRC results were obtained with PA 25 X S as polyacrylic acid polymer.Example 2 - Varying the polyolExperiments were carried out with different polyols with varying numbers of hydroxyl groups using a Sokalan PA 25 X S solution. The resulting analytics of the non-neutralized and neutralized resin are summarized in table 2. Examples 5* and 6 of table 1 are also included in table 2 for easier data comparability.240163W001 11Table 2.1] polyacrylic acid polymer[2] molecular weight[3] wt.-% based on the weight of the polyacrylic acid polymers[4] degree of neutralization[5] molecular weight 40 kg / mol[6] molecular weight 100 kg / mol* comparative exampleVarious polyols have proven useful for the preparation of the inventive resins.Example 3 - Varying the amount of polyolExperiments were carried out with different amounts of glycerol using a Sokalan PA 25 X S solution. The resulting analytics of the non-neutralized and neutralized resins are summarized in table 3. As the comparative240163W001 12sample 25 without glycerol and sample 26 with the lowest amount of glycerol showed no performance in the non-neutralized state, these samples were not neutralized. Examples 5* and 6 of table 1 are also included in table 3 for easier data comparability.Table 3.>1] polyacrylic acid polymer[2] molecular weight[3] wt.-% based on the weight of the polyacrylic acid polymers[4] degree of neutralization[5] not performed* comparative exampleUseful resins were obtained over a broad range of polyol amounts.240163W001 13Example 4 - Varying the drying temperatureExperiments were carried out at different drying temperatures. The resulting analytics of the non-neutralized and neutralized resins are summarized in table 4. Examples 5* and 6 of table 1 are also included in table 4 for easier data comparability.Table 4.1] polyacrylic acid polymer[2] wt.-% based on the weight of the polyacrylic acid polymers[3] degree of neutralization[4] drying temperature[5] molecular weight 5000 g / mol[6] not performed* comparative exampleFor PA 25 X S as polyacrylic acid polymer and glycerol as polyol, a drying temperature of 150 °C results in a particularly useful resin. The resin obtained at 150 °C shows superior FSC and CRC results compared to a resin obtained at 180 °C.Example 5 - Varying the degree of neutralizationExperiments were carried out in which the non-neutralized sample #5* was neutralized to different degrees of neutralization. The resulting analytics of the non-neutralized and neutralized resin are summarized in table 5. Examples 5* and 6 of table 1 are also included in table 5 for easier data comparability.240163W001 14Table 5.1] polyacrylic acid polymer[2] molecular weight[3] wt.-% based on the weight of the polyacrylic acid polymers[4] degree of neutralization* comparative exampleUseful resins were obtained over a broad range of degrees of neutralization with resins having superior FSC and CRC results at a degree of neutralization of 75%.Example 6 - Comparison with commercial biocompostable superabsorberInventive water-absorbent resin 6 as described above in table 1. Its performance was investigated compared to a commercial biocompostable superabsorber(Fairgreen, available from Fairwindel by GreenThingsGmbH). The results are shown in table 6.Table 6.1] not determined

Claims

240163W001 15Claims1. A water-absorbent resin comprising a network of partially neutralized, chain-extended or crosslinked polyacrylic acid polymers having a weight-average molecular weight of from 1000 to 100000 g / mol, wherein the polyacrylic acid polymers are chain-extended or crosslinked via ester bonds between pendant carboxylic groups of the polyacrylic acid polymers and polyol molecules.

2. The water-absorbent resin according to claim 1, wherein the polyol is selected from alkanepolyols and intramolecular or intermolecular dehydration products thereof, sugar polyols, dextrins, polyvinyl alcohols and acrylic polyols, and mixtures thereof.

3. The water-absorbent resin according to claim 2, wherein the polyol is a C2-C8-alkanepolyol.

4. The water-absorbent resin according to any one of the preceding claims, wherein the polyol comprises at least three hydroxyl groups per molecule of polyol.

5. The water-absorbent resin according to any one of the preceding claims, obtainable by chainextending and crosslinking the polyacrylic acid polymers with 1 to 25 wt.-%, preferably 2 to 10 wt.-% of the polyol, based on the weight of the polyacrylic acid polymers.

6. The water-absorbent resin according to any one of the preceding claims, wherein the polyacrylic acid polymers have a weight-average molecular weight of from 1000 to 5000 g / mol, preferably 1000 to 2000 g / mol.

7. The water-absorbent resin according to any one of the preceding claims, wherein the polyacrylic acid polymers have a degree of neutralization in the range of from 40 to 95 mol-%, preferably 50 to 90 mol-%, more preferably 60 to 85 mol-%.

8. A process for making a water-absorbent resin comprising a network of chain-extended or crosslinked polyacrylic acid polymers, comprising the following steps:(1) providing polyacrylic acid polymers having a weight-average molecular weight of from 1000 to 100000 g / mol,(2) chain-extending and crosslinking polyacrylic acid polymers by esterification between polyol molecules and pendant carboxylic groups of the polyacrylic acid polymers.

9. The process according to claim 8, additionally comprising the following step:(3) partially neutralizing the chain-extended or crosslinked polyacrylic acid polymers.240163W001 1610. The process according to claim 8 or 9, wherein step (2) comprises(2a) providing an aqueous solution containing the polyacrylic acid polymers and polyol, and heating the aqueous solution to effect esterification with concomitant evaporation of water to obtain solid matter.

11. The process according to claim 10, comprising heating the aqueous solution to a temperature in the range of from 110 to 190 °C, preferably 130 to 170 °C, more preferably 140 to 160 °C.

12. The process according to claim 10 or 11, additionally comprising the following step:(2b) milling and sieving the solid matter to obtain comminuted matter.

13. The process according to any one of claims 8 to 12, comprising the following steps:(3a) evenly moistening the solid matter or comminuted matter with an aqueous solution of a neutralization agent, and(3b) heating and drying the moistened neutralized matter to obtain the water-absorbent resin.

14. The process according to claim 13, additionally comprising the following step:(4) milling and sieving the water-absorbent resin.

15. A hygiene article comprising the water-absorbent resin according to any one of claims 1 to 7.

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