WO2014111321A1 - Process for the surface-postcrosslinking of superabsorbents - Google Patents

Abstract

In a process for surface-crosslinking superabsorbents, adding fatty alcohol polyglycol ethersurfactant to the surface-crosslinker solution improves quality of surface-crosslinking without showing negative effects of surface tension reduction of liquids in hygiene products comprising the surface-crosslinked superabsorbent.

Description

Process for the Surface-Postcrosslinking of Superabsorbents Description The present invention relates to a process for surface-postcrosslinking of superabsorbents.

Superabsorbents are known. Superabsorbents are materials that are able to take up and retain many times their weight in water, possibly up to several hundred times their weight, even under moderate pressure. Absorbing capacity is usually lower for salt-containing solutions compared to distilled or otherwise de-ionised water. Typically, a superabsorbent has a centrifugal retention capacity ("CRC", method of measurement see hereinbelow) of at least 5 g/g, preferably at least 10 g/g and more preferably at least 15 g/g. Such materials are also commonly known by designations such as "high-swellability polymer", "hydrogel" (often even used for the dry form), "hy- drogel-forming polymer", "water-absorbing polymer", "absorbent gel-forming material", "swella- ble resin", "water-absorbing resin" or the like. The materials in question are crosslinked hydrophilic polymers, in particular polymers formed from (co)polymerised hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, crosslinked ethers of cellulose or starch, crosslinked carboxymethylcellulose, partially crosslinked poly- alkylene oxide or natural products that are swellable in aqueous fluids, examples being guar derivatives, of which water-absorbing polymers based on partially neutralised acrylic acid are most widely used. Superabsorbents are usually produced, stored, transported and processed in the form of dry powders of polymer particles, "dry" usually meaning less than 5 wt.-% moisture content (method of measurement see hereinbelow), although forms in which superabsorbents particles are bound to a web, typically a nonwoven, are also known for some applications, as are superabsorbent fibres. A superabsorbent transforms into a gel on taking up a liquid, specifically into a hydrogel when as usual taking up water. By far the most important field of use of superabsorbents is the absorbing of body fluids. Superabsorbents are used for example in diapers for infants, incontinence products for adults or feminine hygiene products. Examples of other fields of use are as water-retaining agents in market gardening, as water stores for protec- tion against fire, for liquid absorption in food packaging or, in general, for absorbing moisture.

Processes for producing superabsorbents are also known. The acrylate-based superabsorbents which dominate the market are produced by radical polymerisation of acrylic acid in the presence of a crosslinking agent (the "internal crosslinker"), usually in the presence of water, the acrylic acid being neutralised to some degree in a neutralisation step conducted prior to or after polymerisation, or optionally partly prior to and partly after polymerisation, usually by adding a alkali, most often an aqueous sodium hydroxide solution. This yields a polymer gel which is comminuted (depending on the type of reactor used, comminution may be conducted concurrently with polymerisation) and dried. Usually, the dried powder thus produced (the "base poly- mer") is surface postcrosslinked (synonymously used with "surface-crosslinked" or with similar terms denoting a second crosslinking step focused on increasing crosslink density at the particle surface during production of superabsorbents) by adding further organic or polyvalent cati- onic crosslinkers to generate a surface layer which is crosslinked to a higher degree than the particle bulk. Most often, aluminium sulphate is being used as polyvalent cationic crosslinker. Applying polyvalent metal cations to superabsorbent particles is sometimes not regarded as surface crosslinking, but termed "surface complexing" or as another form of surface treatment, although it has the same effect of increasing the number of bonds between individual polymer strands at the particle surface and thus increases gel particle stiffness as organic surface cross- linkers have. Organic and polyvalent cation surface crosslinkers can be cumulatively applied, jointly or in any sequence. Surface crosslinking generally involves two process steps, the first one being the application of the surface crosslinking agents, typically by spraying a crosslinker solution on the base polymer particles, and the second one being a step of conducting the crosslinking reaction, typically a heating step. In rare cases, a step of destroying unconsumed, potentially hazardous surface crosslinker may ensue, such as removing unconsumed di- epoxide compounds by steam, but generally using non-hazardous surface crosslinkers is much preferred.

Surface crosslinking leads to a higher crosslinking density close to the surface of each super- absorbent particle. This addresses the problem of "gel blocking", which means that, with earlier types of superabsorbents, a liquid insult will cause swelling of the outermost layer of particles of a bulk of superabsorbent particles into a practically continuous gel layer, which effectively blocks transport of further amounts of liquid (such as a second insult) to unused superabsorbent below the gel layer. While this is a desired effect in some applications of superabsorbents (for example sealing underwater cables), it leads to undesirable effects when occurring in personal hygiene products. Increasing the stiffness of individual gel particles by surface crosslinking leads to open channels between the individual gel particles within the gel layer and thus facili- tates liquids transport through the gel layer. Although surface crosslinking decreases the CRC or other parameters describing the total absorption capacity of a superabsorbent sample, it may well increase the amount of liquid that can be absorbed by hygiene product containing a given amount of superabsorbent. It is well known that the penetration depth of the surface-crosslinking agent into the particle during application of the crosslinking solution and the evenness of the surface crosslinker distribution on the particle surface have tremendous influence on the quality of the surface crosslinking. Attempts have been made to control these parameters. Other means of increasing the permeability of a superabsorbent are also known. These include admixing of superabsorbent with fibres such as fluff in a diaper core or admixing other components that increase gel stiffness or otherwise create open channels for liquid transportation in a gel layer. Frederic L. Buchholz und Andrew T. Graham (Eds.) in:„Modern Superabsorbent Polymer

Technology", J. Wiley & Sons, New York, U.S.A. / Wiley-VCH, Weinheim, Germany, 1997, ISBN 0-471 -1941 1 -5, give a comprehensive overview over superabsorbents and processes for pro- ducing superabsorbents.

WO 95/27739 A1 relates to surface crosslinking of superabsorbents using water-diol mixtures as solvent for organic crosslinking agents to control the surface-crosslinking effect by adjusting the surface tension of the solvent. Optionally, surfactants may be used to reduce the surface tension of the crosslinker solution.

According to the teaching of US 2010/0 139 704 A1 , a cleansing composition may comprise non-ionic surfactants such as polyglycerolated fatty alcohols (also termed fatty alcohol polygly- col ethers) and a superabsorbent. US 6 136 873 discloses a superabsorbent foam where surfactants may be used to stabilise a foamed monomer mixture that will be polymerised to produce the superabsorbent foam. The foam may be surface-crosslinked in a subsequent process step. US 7 759 422 discloses the use of surfactant deagglomerating aids on the base polymer to be surface-crosslinked, in particular sorbitan esters and ethoxylated 2-propylheptanol. These de- agglomerating aids may be added to the surface crosslinker solution. US 7 473 739 and US 7 582 705 disclose surface crosslinking of superabsorbents, where some surfactant is added to the surface-crosslinking solution. Polyoxyethylene alkyl ethers are named as specific ex- amples of non-ionic surfactants.

Surfactants may be used to lower the surface tension of the surface-crosslinker solution, which helps in achieving a more even distribution of surface crosslinker on the superabsorbent particle surfaces, which in turn could improve surface-crosslinking quality. Adding surfactants, however, comes with a disadvantage. Surfactants will not only lower the surface tension of the surface crosslinker solution, but will remain in the superabsorbent and will lower the surface tension of any body fluid discharged into the hygiene product that comprises the surfactant-containing superabsorbent. This in turn causes highly undesired consequences. First, any fluid transport mechanism in the superabsorbent-containing part of the hygiene product that depends on the fluid's surface tension - in particular, capillary action (these processes are commonly termed "wicking") - will be impeded or made impossible. Such fluid transport mechanisms indeed play a significant role in distributing liquid in a bed of swollen superabsorbent. Second, modern hygiene products have breathable backsheets to improve comfort. Breathable fabrics that nevertheless are supposed to be impermeable to liquids generally achieve this by micro-pores that can be passed by gasses, but no by liquids. Drops of liquids cannot pass because surface tension ensures a minimum drop size. Adding a surfactant allows liquid to pass such fabrics. Both effects may lead to a loss of liquid from the hygiene product rather than liquid absorption by the superabsorbent. Consequently, surfactants must be used carefully and choices between optimising surface crosslinking and optimising hygiene products must be made.

It is an object of this invention to find a process for surface-crosslinking superabsorbents, using surfactants to improve surface-crosslinking quality, without encountering the known disad- vantages of surfactants in surface-crosslinking.

A process for surface-crosslinking a superabsorbent has been found that comprises a step of adding a surface-crosslinker solution to the superabsorbent, in which the solution also contains at least one fatty alcohol polyglycol ether. Further, the superabsorbent produced by this process has been found. Yet further, products for absorbing water that comprise that superabsorbent have been found.

In a preferred embodiment of the process of this invention, the fatty alcohol polyglycol ether has the formula:

HO-(CH₂-CH2-0)n-(CH2-CH2)y-CH₂CH3 where n and y independently are integers of from 1 to 20. In a particularly preferred embodi- ment, n is an integer of from 4 to 8 and y is an integer of from 2 to 6. In a most preferred embodiment, n is 6 and y is 4.

These fatty alcohol polyglycol ethers and methods for their production are well known. They are typical nonionic surfactants and commercially available. A typical process for their production involves the reaction of a fatty alcohol of formula H3C-H2C-(CH2-CH2)_y-OH with n mole equivalents of ethylene oxide.

The fatty alcohol polyglycol ether is added to the surface crosslinker solution in an amount of generally at least 0.01 wt.-%, preferably at least 0.03 wt.-% and more preferably at least 0.05 wt.-% and generally at most 1 wt.-%, preferably at most 0.8 wt.-% and more preferably at most 0.5 wt.-%, all based on polymer (to be precise, "based on polymer" means based on the amount of base polymer to be surface crosslinked).

The superabsorbent in the present invention is a superabsorbent capable of absorbing and re- taining amounts of water equivalent to many times its own weight under a certain pressure. Preferably, the superabsorbent is a crosslinked polymer based on partially neutralised acrylic acid and more preferably it is surface postcrosslinked. A "superabsorbent" can also be a mixture of chemically different individual superabsorbents in that it is not so much the chemical composition which matters as the superabsorbing properties.

Processes for producing superabsorbents, including surface-postcrosslinked superabsorbents, are known. Most synthetic superabsorbents on the market today are obtained by a process comprising polymerisation of a monomer solution comprising: a) at least one ethylenically unsaturated monomer which bears acid groups and may be at least partly neutralised,

b) at least one crosslinker, c) at least one initiator,

d) optionally one or more ethylenically unsaturated monomers copolymerisable with the monomers specified under a) and

e) optionally one or more water-soluble polymers.

The process typically further comprises drying, grinding, classifying and/or surface postcross- linking the resulting polymer.

The monomers a) are preferably water-soluble, i.e. 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 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). Impurities can have a considerable influence on the polymerisation. The raw materials used should therefore have a maximum purity. It is therefore often advantageous to specially purify the monomers a). Suitable purification processes are described, for example, in WO

2002/055469 A1 , WO 2003/078378 A1 and WO 2004/035514 A1 . A suitable monomer a) is, for example, acrylic acid purified according to WO 2004/035514 A1 comprising 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid, 0.0332% by weight of water, 0.0203% by weight of propionic acid, 0.0001 % by weight of furfurals, 0.0001 % by weight of maleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether. The proportion of acrylic acid and/or salts thereof in the total amount of monomers a) is preferably at least 50 mol%, more preferably at least 90 mol%, most preferably at least 95 mol%.

The monomers a) typically comprise polymerisation inhibitors, preferably hydroquinone half ethers, as storage stabilisers.

The monomer solution comprises preferably up to 250 ppm by weight, preferably 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, especially around 50 ppm by weight, of hydroquinone half ether, based in each case on the unneutralised monomer a). For example, the monomer solution can be prepared by using an ethylenically unsaturated monomer bearing acid groups with an appropriate content of hydroquinone half ether. Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and/or alpha- tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be polymerised free- radically into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomer a). In addition, polyvalent metal salts which can form coordinate bonds with at least two acid groups of the monomer a) are also suitable as crosslinkers b). Crosslinkers b) are preferably compounds having at least two polymerisable groups which can be polymerised free-radically into the polymer network. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1 , di- and triacrylates, as described in EP 0 547 847 A1 , EP 0 559 476 A1 , EP 0 632 068 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, as well as acrylate groups, comprise further ethylenically unsaturated groups, as described 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 2002/032962 A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether, tetraalloxyethane, methylenebismeth- acrylamide, 15-tuply ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine. Very particularly preferred crosslinkers b) are the polyethoxylated and/or propoxylated glycerols which have been esterified with acrylic acid or methacrylic acid to give di- or triacrylates, as described, for example, in WO 2003/104301 A1. 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. Most preferred are the triacry- lates of 3- to 5-tuply ethoxylated and/or propoxylated glycerol, especially the triacrylate of 3- tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably from 0.05 to 1 .5% by weight, more preferably from 0.1 to 1 % by weight, most preferably from 0.3 to 0.6% by weight, based in each case on mono- mer a). With rising crosslinker content, the centrifuge retention capacity (CRC) falls and the absorption under a pressure of 21.0 g/cm² (AUL 0.3 psi) passes through a maximum.

The initiators c) may be all compounds which generate free radicals under the polymerisation conditions, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodium perox- odisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite. Preference is given to using mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate/hydrogen peroxide/ ascorbic acid. The reducing component used is, however, preferably a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures are obtainable as Bruggolite^® FF6 and Bruggolite^® FF7 (Bruggemann Chemicals; Heilbronn; Germany).

Ethylenically unsaturated monomers d) copolymerisable with the ethylenically unsaturated monomers a) bearing acid groups are, for example, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacry- late, diethylaminoethyl methacrylate.

The water-soluble polymers e) used may be polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.

Typically, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75% by weight, more preferably from 45 to 70% by weight, most preferably from 50 to 65% by weight. It is also possible to use monomer suspensions, i.e. monomer solutions with excess monomer a), for example sodium acrylate. With rising water content, the energy requirement in the subsequent drying rises, and, with falling water content, the heat of polymerisation can only be removed inadequately.

For optimal action, the preferred polymerisation inhibitors require dissolved oxygen. Before the polymerisation, the monomer solution can therefore be freed of dissolved oxygen, and the polymerisation inhibitor present in the monomer solution can be deactivated, by inertisation. A comfortable method for inertisation is passing a flow of an inert gas through the monomer solution, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution is preferably lowered before the polymerisation to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

Suitable reactors are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel formed in the polymerisation of an aqueous monomer solution or suspension is comminuted continuously by, for example, contrarotatory stirrer shafts, as described in WO

2001/038402 A1. Polymerisation on a belt is described, for example, in DE 38 25 366 A1 and US 6,241 ,928. Polymerisation in a belt reactor forms a polymer gel, which has to be comminuted in a further process step, for example in an extruder or kneader.

However, it is also possible to generate droplets of an aqueous monomer solution and to polymerise the droplets obtained in a heated carrier gas stream. This allows the process steps of polymerisation and drying to be combined, as described in WO 2008/040715 A2 and WO 2008/052971 A1. The acid groups of the resulting polymer gels have typically been partially neutralised. Neutralisation is preferably carried out at the monomer stage. This is typically done by mixing in the neutralising agent as an aqueous solution or preferably also as a solid. The degree of neutralisation is preferably from 25 to 95 mol%, more preferably from 30 to 80 mol%, most preferably from 40 to 75 mol%, for which the customary neutralising agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogencar- bonates 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 also mixtures thereof.

However, it is also possible to carry out neutralisation after the polymerisation, at the stage of the polymer gel formed in the polymerisation. It is also possible to neutralise up to 40 mol%, preferably from 10 to 30 mol% and more preferably from 15 to 25 mol% of the acid groups be- fore the polymerisation by adding a portion of the neutralising agent actually to the monomer solution and setting the desired final degree of neutralisation only after the polymerisation, at the polymer gel stage. When the polymer gel is neutralised at least partly after the polymerisation, the polymer gel is preferably comminuted mechanically, for example by means of an extruder, in which case the neutralising agent can be sprayed, sprinkled or poured on and then carefully mixed in. To this end, the gel mass obtained can be repeatedly extruded for homoge- nisation.

The polymer hydrogel is then typically dried with a belt dryer until the residual moisture content is preferably from 0.5 to 15% by weight, more preferably from 1 to 10% by weight, most prefer- ably from 2 to 8% by weight. In the case of too high a residual moisture content, the dried polymer gel has too low a glass transition temperature Tg and can be processed further only with difficulty. In the case of too low a residual moisture content, the dried polymer gel is too brittle and, in the subsequent comminution steps, undesirably large amounts of polymer particles with an excessively low particle size are obtained (fines). The solids content of the gel before the drying is preferably from 25 to 90% by weight, more preferably from 35 to 70% by weight, most preferably from 40 to 60% by weight. Optionally, it is, however, also possible to use a fluidised bed dryer or a paddle dryer for the drying operation.

The dried hydrogel (which is no longer a gel (even though often still called that) but a dry poly- mer having superabsorbing properties, which comes within the term "super-absorbent") is typically ground and sieved to produce a particulate superabsorbent "base polymer" of the desired particle size distribution. Useful grinding apparatus typically including single or multistage roll mills, pin mills, hammer mills, cutting mills or swing mills.

The mean particle size of the polymer particles collected as the product fraction is preferably at least 200 μηη, more preferably from 250 to 600 μηη, very particularly from 300 to 500 μηη. For economic reasons, it is desirable to select conditions of grinding and other steps of mechanical treatment to achieve a proportion of particles with a particle size of at least 150 μηη of preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight.

Polymer particles with too small a particle size lower the permeability (SFC). The proportion of excessively small polymer particles (fines) should therefore be small.

Excessively small polymer particles are therefore typically removed and recycled into the pro- cess. This is preferably done before, during or immediately after the polymerisation, i.e. before the drying of the polymer gel. The excessively small polymer particles can be moistened with water and/or aqueous surfactant before or during the recycling.

It is also possible to remove excessively small polymer particles in later process steps, for ex- ample after the surface postcrosslinking or another coating step. In this case, the excessively small polymer particles recycled are surface postcrosslinked or coated in another way, for example with fumed silica.

When a kneading reactor is used for the polymerisation, the excessively small polymer particles are preferably added during the last third of the polymerisation.

When the excessively small polymer particles are added at a very early stage, for example actually to the monomer solution, this lowers the centrifuge retention capacity (CRC) of the resulting water-absorbing polymer particles. However, this can be compensated, for example, by ad- justing the amount of crosslinker b) used.

When the excessively small polymer particles are added at a very late stage, for example not until within an apparatus connected downstream of the polymerisation reactor, for example an extruder, the excessively small polymer particles can be incorporated into the resulting polymer gel only with difficulty. Excessively small polymer particles which have been insufficiently incorporated, however, become detached again from the dried polymer gel during the grinding, and are therefore removed again in the classification and increase the amount of excessively small polymer particles to be recycled. For economic reasons, it is desirable to select conditions of grinding and other steps of mechanical treatment to achieve a proportion of particles with a particle size of at most 850 μηη of preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight. Advantageously, the proportion of polymer particles with a particle size of at most 600 μηη is preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight. Polymer particles with too great a particle size lower the swell rate. The proportion of excessively large polymer particles should therefore likewise be small. Excessively large polymer particles are therefore typically removed and recycled into the grinding of the dried polymer gel.

To further improve the properties, the base polymer particles are optionally, and in the process of this invention are, surface postcrosslinked. Suitable surface postcrosslinkers are compounds which comprise groups which can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunc- tional amido amines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1 , DE 35 23 617 A1 and EP 0 450 922 A2, or Hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230.

Additionally described as suitable surface postcrosslinkers are cyclic carbonates in DE 40 20 780 C1 , 2-oxazolidone and its derivatives, such as 2-hydroxyethyl-2-oxazolidone in DE 198 07 502 A1 , bis- and poly-2-oxazolidinones in DE 198 07 992 C1 , 2-oxotetrahydro-1 ,3-oxazine and its derivatives in DE 198 54 573 A1 , N-acyl-2-oxazolidones in DE 198 54 574 A1 , cyclic ureas in DE 102 04 937 A1 , bicyclic amide acetals in DE 103 34 584 A1 , oxetanes and cyclic ureas in EP 1 199 327 A2 and morpholine-2,3-dione and its derivatives in WO 2003/031482 A1 .

Preferred surface postcrosslinkers are glycerol, 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 postcrosslinkers are 2-hydroxyethyloxazolidin-2-one, oxazol- idin-2-one and 1 ,3-propanediol.

In addition, it is also possible to use surface postcrosslinkers which comprise additional polymerisable ethylenically unsaturated groups, as described in DE 37 13 601 A1 .

The amount of surface postcrosslinker is preferably from 0.001 to 2% by weight, more prefera- bly from 0.02 to 1 % by weight, most preferably from 0.05 to 0.2% by weight, based in each case on the polymer particles.

The surface postcrosslinking is typically performed in such a way that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. After the spraying, the polymer par- tides coated with surface postcrosslinker are dried thermally, and the surface postcrosslinking reaction can take place either before or during the drying. This drying step may also be referred to as "curing". The spraying of a solution of the surface postcrosslinker is preferably performed in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Particular preference is given to horizontal mixers such as paddle mixers, very particular preference to vertical mixers. The distinction between horizontal mixers and vertical mixers is made by the position of the mixing shaft, i.e. horizontal mixers have a horizontally mounted mixing shaft and vertical mixers a vertically mounted mixing shaft. Suitable mixers are, for example, horizontal

Pflugschar^® mixers (Gebr. Lodige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixers (Hosokawa Micron BV; Doetinchem; the Netherlands), Processall Mixmill Mixers (Processall Incorporated; Cincinnati; US) and Schugi Flexomix^® (Hosokawa Micron BV; Doetinchem; the Netherlands). However, it is also possible to spray the surface postcrosslinker solution into a fluidised bed of base polymer.

The surface postcrosslinkers are typically used in the form of an aqueous solution. The penetra- tion depth of the surface postcrosslinker into the polymer particles can be adjusted via the content of nonaqueous solvent and total amount of solvent, and is adjusted by adding the surfactant in the process of this invention.

Water may be used as the single solvent, however, preference is given to using solvent mix- tures, for example isopropanol/water, 1 ,3 propanediol/water and propylene glycol/water, where the mixing ratio in terms of mass is preferably from 20:80 to 40:60.

The thermal drying is preferably carried out in contact dryers, more preferably paddle dryers, most preferably disk dryers. Suitable dryers are, for example, Hosokawa Bepex^® Horizontal Paddle Dryers (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex^® Disc Dryers (Hosokawa Micron GmbH; Leingarten; Germany) and Nara Paddle Dryers (NARA Machinery Europe; Frechen; Germany). Moreover, it is also possible to use fluidised bed dryers.

The drying can be effected in the mixer itself, by heating the jacket or blowing in warm air.

Equally suitable is a downstream dryer, for example a shelf dryer, a rotary tube oven or a heat- able screw. It is particularly advantageous to mix and dry in a fluidised bed dryer.

Preferred drying temperatures are in the range from 130 to 250°C, preferably from 140 to 220°C, more preferably from 150 to 210°C, most preferably from 160 to 200°C. The preferred residence time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typically at most 100 minutes, preferably at most 80 minutes.

Quite usually, but not necessarily, water-soluble polyvalent metal salts comprise bi- or more highly valent ("polyvalent") metal cations capable of reacting with the acid groups of the polymer to form complexes are added. Examples of polyvalent cations are or metal cations such as Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺, Mn²⁺, Fe^2+/3+, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Y³⁺, Zr*⁺, La³⁺, Ce⁴⁺, Hf⁴⁺, and Au³⁺. Preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺, Ti⁴⁺, Zr⁴* and La³⁺, and particularly preferred metal cations are Al³⁺, Ti⁴⁺ and Zr⁴*. The metal cations can be used not only alone but also in admixture with each other. Of the metal cations mentioned, any metal salt can be used that has sufficient solubility in the solvent to be used. Metal salts with weakly complexing anions such as for example chloride, nitrate and sulphate, hydrogen sulphate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate and lactate, are particularly suitable. It is particularly preferred to use aluminium sulfate, aluminium lactate or aluminium dihydroxi monoacetate. The amount of polyvalent cation used is, for example, 0.001 to 1 .5% by weight, preferably 0.005 to 1 % by weight, more preferably 0.02 to 0.8% by weight, based in each case on the polymer particles.

The treatment of the superabsorbent polymer with solution of a polyvalent cation is car-ried out in the same way as that with surface postcrosslinker, including the selective drying step. Useful solvents for the metal salts include water, alcohols, DMF, DMSO and also mixtures thereof. Particular preference is given to water and water-alcohol mixtures such as for example water- methanol, water-1 ,2-propanediol, water-2-propanol and water-1 ,3-propanediol. In a preferred embodiment of the present invention, the complexing agent is applied to a super- absorbent that is surface crosslinked, or concurrently with surface crosslinking, or partly simultaneously and partly after surface crosslinking. For example, a suitable method of applying a complexing agent is applying a polyvalent metal cation such as Al³⁺ concurrently with a surface crosslinker.

If surface crosslinker and complexing agent are added as one solution, the fatty alcohol polygly- col ether surfactant is added to that solution in the amount stated above. If surface crosslinker and complexing agent are added in separate solutions, the fatty alcohol polyglycol ether surfactant may be added to either one or preferably to both. If these solutions are added concurrently or subsequently, but without any intermediate heating step, the amount of surfactant stated above generally will be sufficient to ensure even distribution of both components on the particles. If these solutions are added subsequently with an intermediate heating step, each solution will need to contain an amount of surfactant in the range stated above. The surface-crosslinked superabsorbent produced using the process of the instant inventions is optionally ground and/or sieved in a conventional manner. Grinding is typically not necessary, but the sieving out of agglomerates which are formed or undersize is usually advisable to set the desired particle size distribution for the product. Agglomerates and undersize are either discarded or preferably returned into the process in a conventional manner and at a suitable point; agglomerates after comminution.

To improve some properties such as brittleness, the surface-crosslinked polymer particles may be moistened. Moistening is carried out preferably at from 30 to 80°C, more preferably at from 35 to 70°C and most preferably at from 40 to 60°C. At excessively low temperatures, the water- absorbing polymer particles tend to form lumps, and, at higher temperatures, water already evaporates noticeably. The amount of water used for subsequent moistening is preferably from 1 to 10% by weight, more preferably from 2 to 8% by weight and most preferably from 3 to 5% by weight. The subsequent moistening increases the mechanical stability of the polymer particles and reduces their tendency to static charging.

If the superabsorbent is produced by another method, some of the process steps described above may be unnecessary. For example, emulsion or droplet polymerisation produces particulate superabsorbents that may not need grinding or classification or surface crosslinking. Further, droplet polymerisation is typically performed in reactors that also dry the product due to the gas streams typically necessary to conduct droplet polymerisation, so a separate drying step may not be necessary. The method of producing the superabsorbent plays no role in this inven- tion and any particulate superabsorbent produced in any manner may be treated according to this invention.

After any drying, curing or other heating step, it may be advantageous but not absolutely necessary to cool the product. Cooling can be carried out continuously or discontinuously, convenient- ly by conveying the product continuously into a cooler downstream of the dryer. Any apparatus known for removing heat from pulverulent solids can be used, in particular any apparatus mentioned above as a drying apparatus, provided it is supplied not with a heating medium but with a cooling medium such as for example with cooling water, so that heat is not introduced into the superabsorbent via the walls and, depending on the design, also via the stirrer elements or oth- er heat-exchanging surfaces, but removed from the superabsorbent. Preference is given to the use of coolers in which the product is agitated, i.e., cooled mixers, for example shovel coolers, disk coolers or paddle coolers, for example Nara^® or Bepex^® coolers. The superabsorbent can also be cooled in a fluidised bed by blowing a cooled gas such as cold air into it. The cooling conditions are set such that a superabsorbent having the temperature desired for further pro- cessing is obtained. Typically, the average residence time in the cooler will be in general at least 1 minute, preferably at least 3 minutes and more preferably at least 5 minutes and also in general not more than 6 hours, preferably 2 hours and more preferably not more than 1 hour, and cooling performance will be determined such that the product obtained has a temperature of generally at least 0°C, preferably at least 10°C and more preferably at least 20°C and also generally not more than 100°C, preferably not more than 80°C and more preferably not more than 60°C.

Optionally, the superabsorbent is provided with further customary additives and auxiliary materials to influence storage or handling properties. Examples thereof are colorations, opaque addi- tions to improve the visibility of swollen gel, which is desirable in some applications, surfactants, de-dusting agents, colour stabilisers, flowability aids, anticaking additives or the like. These additives and auxiliary materials can each be added in separate processing steps, but one con- venient method may be to add them to the superabsorbent in the cooler, for example by spraying the superabsorbent with a solution or adding them in finely divided solid or in liquid form, if this cooler provides sufficient mixing quality. The inventive water-absorbing polymer particles have a moisture content of typically 0 to 15% by weight, preferably 0.2 to 10% by weight, more preferably 0.5 to 8% by weight, most preferably 1 to 5% by weight, and/or a centrifuge retention capacity (CRC) of typically at least 20 g/g, preferably at least 26 g/g, more preferably at least 28 g/g, most preferably at least 30 g/g, and/or an absorption under a pressure of 49.2 g/cm² (AUL 0.7 psi) of typically at least 12 g/g, preferably at least 16 g/g, more preferably at least 18 g/g, most preferably at least 20 g/g, and/or a saline flow conductivity (SFC) of typically at least 20^■ 10^"7 cm³s/g, preferably at least 40^■ 10^"7 cm³s/g, more preferably at least 50^■ 10^"7 cm³s/g, most preferably at least 60^■ 10^"7 cm³s/g. The centrifuge retention capacity (CRC) of the water-absorbing polymer particles is typically less than 60 g/g. The absorption under a pressure of 49.2 g/cm² (AUL 0.7 psi) of the water- absorbing polymer particles is typically less than 35 g/g. The saline flow conductivity (SFC) of the water-absorbing polymer particles is typically less than 200^■ 10^"7 cm³s/g. We have further found water-absorbing products, in particular hygiene articles comprising the superabsorbent of the present invention. Hygiene articles in accordance with the present invention are for example those intended for use in mild or severe incontinence, such as for example inserts for severe or mild incontinence, incontinence briefs, also diapers, training pants for babies and infants or else feminine hygiene articles such as liners, sanitary napkins or tampons. Hygiene articles of this kind are known. The hygiene articles of the present invention differ from known hygiene articles in that they comprise the superabsorbent of the present invention. We have also found a process for producing water-absorbing products, in particular hygiene articles, this process comprising adding at least one superabsorbent of the present invention to the in the manufacture of the water-absorbing-product in particular hygiene article in question dur- ing its manufacture. Processes for producing water-absorbing products, in particular hygiene articles comprising superabsorbent are otherwise known.

The present invention further provides for the use of the composition of the present invention in training pants for children, shoe inserts and other hygiene articles to absorb body fluids. The composition of the present invention can also be used in other technical and industrial fields where liquids, in particular water or aqueous solutions, are absorbed. These fields are for example storage, packaging, transportation (as constituents of packaging material for water- or moisture-sensitive articles, for example for flower transportation, also as protection against mechanical impacts); animal hygiene (in cat litter); food packaging (transportation of fish, fresh meat; absorption of water, blood in fresh fish or meat packs); medicine (wound plasters, water- absorbing material for burn dressings or for other weeping wounds), cosmetics (carrier material for pharmachemicals and medicaments, rheumatic plasters, ultrasonic gel, cooling gel, cosmetic thickeners, sun protection); thickeners for oil-in-water and water-in-oil emulsions; textiles (moisture regulation in textiles, shoe inserts, for evaporative cooling, for example in protective clothing, gloves, headbands); chemical engineering applications (as a catalyst for organic reactions, to immobilise large functional molecules such as enzymes, as adhesion agent in relation to ag- glomerations, heat storage media, filter aids, hydrophilic component in polymeric laminates, dispersants, superplasticisers); as auxiliaries in powder injection moulding, in building construction and engineering (installation, in loam-based renders, as a vibration-inhibiting medium, auxiliaries in tunnel excavations in water-rich ground, cable sheathing); water treatment, waste treatment, water removal (deicing agents, reusable sandbags); cleaning; agritech (irrigation, retention of melt water and dew deposits, composting additive, protection of forests against fungal/insect infestation, delayed release of active components to plants); for firefighting or for fire protection; coextrusion agents in thermoplastic polymers (for example to hydrophilise multi- layered films); production of films and thermoplastic mouldings able to absorb water (for example rain and dew water storage films for agriculture; superabsorbent-containing films for keeping fruit and vegetables fresh which are packed in moist films; superabsorbent-polystyrene coextru- dates, for example for food packaging such as meat, fish, poultry, fruit and vegetables); or as carrier substance in formulations of active components (pharma, crop protection).

Test Methods

The standard test methods referred to as "WSP" described below are described in: "Standard Test Methods for the Nonwovens Industry", 2010 edition, published jointly by the Worldwide Strategic Partners EDANA (European Disposables and Nonwovens Association, Avenue Herrmann Debroux 46, 1 160 Brussels, Belgium, www.edana.org) and INDA (Association of the Nonwoven Fabrics Industry, 1 100 Crescent Green, Suite 1 15, Cary, North Carolina 27518, U.S.A., www.inda.org). This publication is obtainable both from EDANA and from INDA.

The measurements should, unless stated otherwise, be carried out at an ambient temperature of 23 ± 2°C and a relative air humidity of 50 ± 10%. The water-absorbing polymer particles are mixed thoroughly before the measurement.

Centrifuge retention capacity ("CRC")

The centrifuge retention capacity (CRC) is determined by test method No. WSP 214.2 (10) "Centrifuge Retention Capacity".

Absorption under a pressure of 21 .0 g/cm² ("AUL 0.3 psi")

The absorption under a pressure of 49.2 g/cm² (commonly referred to as "AUL 0.3 psi") is de- termined by test method No. WSP 242.2 (10) "Absorption under Pressure"

Absorption under a pressure of 49.2 g/cm² ("AUL 0.7 psi") The absorption under a pressure of 49.2 g/cm² (commonly referred to as "AUL0.7 psi") is determined by test method No. WSP 242.2 (10) "Absorption under Pressure", however, with a pressure setting of 49.2 g/cm² (AUL0.7 psi) instead of 21 .0 g/cm² (that corresponds to the AUL0.3 psi).

Absorption under a pressure of 63.3 g/cm² ("AUL 0.9 psi")

The absorption under a pressure of 63.3 g/cm² (commonly referred to as "AUL0.9 psi") is de- termined by test method No. WSP 242.2 (10) "Absorption under Pressure", however, with a pressure setting of 63.3 g/cm² (AUL0.9 psi) instead of 21 .0 g/cm² (that corresponds to the AUL0.3 psi).

Water or Moisture Content

Water or moisture content is determined by test method No. WSP 230.2 (10) "Moisture Content".

Saline flow conductivity ("SFC")

The saline flow conductivity (SFC) of a swollen gel layer under a pressure of 0.3 psi (2070 Pa) is, as described in EP 0 640 330 A1 (page 19, line 13 to page 21 , line 35), determined as the gel layer permeability of a swollen gel layer of water-absorbing polymer particles, with modification of the apparatus described in figure 8 in that the glass frit (40) is not used, the plunger (39) consists of the same plastic material as the cylinder (37), and now has 21 bores of equal size distributed homogeneously over the entire contact area. The procedure and evaluation of the measurement remain unchanged from EP 0 640 330 A1 . The flow is detected automatically.

The saline flow conductivity (SFC) is calculated as follows:

SFC [cm³s/g] = (Fg(t=0)xL0)/(dxAxWP) where Fg(t=0) is the flow of NaCI solution in g/s, which is obtained using a linear regression analysis of the Fg(t) data of the flow determinations by extrapolation to t=0, L0 is the thickness of the gel layer in cm, d is the density of the NaCI solution in g/cm³, A is the area of the gel layer in cm², and WP is the hydrostatic pressure over the gel layer in dyn/cm².

Free Swell Gel Bed Permeability ("FS-GBP") The method for determination of the Free Swell Gel Bed Permeability (Free Swell GBP) is described in US patent application no. US 2005/0 256 757 A1 , paragraphs [0061] through [0075]. Centrifuge Retention Capacity, alternate method ("CRC alternate")

The method for determination of the CRC alternate is described in US patent application no. US 2002/0 165 288 A1 , paragraphs [0105] and [0106].

Wicking Index

The method for determination of the Wicking Index is described in EP 532 002 A1 , page 6, line 36, to page 7, line 26. However, the angle of the trough sheet to horizontal was set to 30° instead of 20°.

Examples The base polymer (superabsorbent prior to surface crosslinking) used in the following examples was Hysorb^® T 8760 available from BASF SE, 67056 Ludwigshafen, Germany). The Wicking Index of this product is 15.4 cm.

The surfactant used was obtained as Unifroth^® 0520 from Unichem Inc., 916 West Main Street, Haw River, NC 27258, U.S.A. and is an ethoxylated Decanol with an average number of 6 ethylene oxide units per molecule, i.e. HO-(CH₂-CH2-0)6-(CH2-CH2)4-CH₂CH3.

Aluminium dihydroxi monoacetate was used as 20 wt.-% aqueous solution (stabilised with 4 wt- % aluminium lactate), obtained as Lohtragon^® 200 solution from Dr. Paul Lohmann GmbH KG, Haupts^e 2, 31860 Emmerthal, Germany.

The abbreviation "bop" means "based on polymer".

Example 1 :

25.0 g of Lohtragon^® 200 solution were placed in a 200 ml. beaker with a stir bar. 1.20 g of a mixture of 50 wt.-% 2-Hydroxyethyloxazolidin-2-one and 50 wt.-% 1 ,3-propanediol and 8.0 g propylene glycol were added. The amount of Unifroth^® 0520 specified in Table 1 was then added. The resulting surface crosslinker solution was stirred for 20 minutes.

1000 g (±1 g) of base polymer were heated to 50 °C in a laboratory Ploughshare^® mixer with a heated jacket (model M 5; manufactured by Gebruder Lodige Maschinenbau GmbH; Paderborn; Germany). At a mixer speed of 450 rpm, the surface crosslinker solution was added to the polymer powder using a disposable syringe to coat the polymer powder particles with surface crosslinker solution. After coating was completed, the product was transferred to another, identical laboratory Ploughshare^® mixer and cured by heating to 180 °C while mixing at 150 rpm. Mixing was interrupted every 20 minutes to collect a 50 g sample. Once the run was completed, the polymer was cooled to room temperature. The properties of the samples are summarised in Table 1 .

Table 1

FS-GBP is a measure of general permeability in a swollen superabsorbent gel bed to be expected in a diaper, by all fluid transport mechanisms including flow by gravitation. Since permeability gets better with surface crosslinking, a higher FS-GBP number indicates better surface crosslinking, or the desired effect of surfactants. The Wicking Index is a measure of fluid transport in the swollen gel bed by capillary action only. Since this type of fluid transport gets worse with breakdown of surface tension due to surfactants, a higher wicking index indicates better fluid transport by capillary action, or less undesired effects of surfactants. The data in Table 1 demonstrate that the superabsorbents of example 1 , crosslinked in the presence of 0.100 % of Unifroth^® 0520 surfactant, exhibit superior surface crosslinking combined with no undesired breakdown of surface tension.

Example 2 Two superabsorbents were prepared following the general procedure of Example 1. In both cases, however, only 2 % of Lohtragon^® 200 solution were used, the curing temperature was set to 182 °C, samples were taken at 15 min intervals. Further, in the preparation of one super- absorbent, no Unifroth^® 0520 was added while 0.100 % thereof was added in the preparation of the other. The properties of the samples are summarised in Table 2. Table 2

The data in Table 2 show that the addition of surfactant increases the maximum FS-GBP, demonstrating the improved surface crosslinking. Furthermore, the FS-GBP develops more rapidly, which corresponds to a shorter curing time, which means the surface crosslinking process will be more efficient. Example 3

Three superabsorbents were prepared following the general procedure of Example 1. The amount of Unifroth^® 0520, however, was kept constant at 0.100 %, and the amount of Loh- tragon^® 200 was varied as indicated in Table 3, while the total volume of the solution was kept constant by adding water to compensate for any amount of Lohtragon^® 200 below 3.00 wt.-% bop. The properties of the samples are summarised in Table 3. Table 3

The data show that only 2.0 % aluminium acetate were sufficient to achieve a FS-GBP of over 60 Darcy without suppressing the wicking index.

Example 4

Two superabsorbents were prepared following the general procedure of Example 1. In both cases, however, 2.00 wt.-% bop of aluminium sulphate (in the form of a 26.7 wt.-% aqueous solution) were added instead of Lohtragon^® 200 and samples were taken at 15 min intervals. Further, in the preparation of one superabsorbent, no Unifroth^® 0520 was added while 0.100 % thereof was added in the preparation of the other. The properties of the samples are summarised in Table 2.

Table 4

Unifroth^® Curing time CRC AUL 0.9 psi FS-GBP

0520 alternate

[wt.-% bop] [min] [g/g] [g/g] [Darcy]

- 15 36.0 15.3 32

- 30 34.1 17.6 62

- 45 32.7 18.5 59

- 60 31.9 18.6 65

0.100 15 36.3 13.7 24

30 34.9 17.4 55

45 32.9 18.9 77

60 32.5 18.9 87 The data in Table 4 show that the addition of surfactant improves surface crosslinking irrespective of the complexing agent.

Claims

We claim:

1 . A process for surface-crosslinking a superabsorbent that comprises a step of adding a surface-crosslinker solution to the superabsorbent, in which the solution also contains at least one fatty alcohol polyglycol ether.

2. The process of claim 1 , where the fatty alcohol polyglycol ether has the formula

HO-(CH₂-CH2-0)n-(CH2-CH2)y-CH₂CH3 where n and y independently are integers of from 1 to 20.

3. The process of claim 2, where n is an integer of from 4 to 8 and y is an integer of from 2 to 6.

4. The process of claim 3, where n is 6 and y is 4.

5. The process of any of claims 1 to 4, where the surfactant is added in an amount of from 0.01 to 1 wt.-% based on polymer.

6. The process of any of claims 1 to 5, where the superabsorbent prior to surface crosslinking is produced by a process comprising polymerisation of a monomer solution comprising: a) at least one ethylenically unsaturated monomer which bears acid groups and may be at least partly neutralised,

b) at least one crosslinker,

c) at least one initiator,

d) optionally one or more ethylenically unsaturated monomers copolymerisable with the monomers specified under a) and

e) optionally one or more water-soluble polymers,

the process further comprising drying and optionally grinding and classifying.

7. The superabsorbent obtained by the process of any of claims 1 to 6.

8. A water-absorbing product comprising the superabsorbent of claim 7.

9. The water-absorbing product of claim 8 that is a hygiene article.

10. A process for producing the water-absorbing product of any of claims 8 or 9 that comprises adding the superabsorbent of claim 7 to the water-absorbing product during its production.

SB/TB 19.07.2013 0 Fig/0 Seq.