The present invention relates to a permeable superabsorbent, to a process for producing it and to its use and to hygiene articles com prising it.

Superabsorbents are known. For such materials, names such as "high ly swel lable polymer", "hydrogel" (often also used for the dry form) , "hyd rogel-forming polymer", "water-absorbing polymer", "absorbent gel-forming material", "swel lable resin", "water-absorbing resin" or the like are also commonly used. These materials are crosslinked hydrophilic polymers, more particu larly polymers formed from (co) polymerized hydrophilic monomers, graft (co) poly- mers of one or more hydrophilic monomers on a suitable graft base, crosslinked cel lulose ethers or starch ethers, crosslin ked carboxymethylcel lu lose, partly crosslinked polyal kylene oxide or natu ral products swel lable in aqueous liquids, for exam ple guar derivatives, the most com mon being water-absorbing polymers based on partly neutralized acrylic acid. The essential properties of su perabsorbents are their abilities to absorb several times their own weight of aqueous liquids and not to release the liquid again even under a certain pressu re. The superabsorbent, which is used in the form of a d ry powder, is converted to a gel when it absorbs fluid, and correspondingly to a hyd rogel when it absorbs water as usual. Crosslin k ing is essential for synthetic superabsorbents and is an important difference from custom ary straightforward thickeners, since it leads to the insolubility of the polymers in water. Soluble su bstances wou ld be u nusable as superabsorbents. By far the most important field of use of su perabsorbents is the absorption of body fluids. Su perabsorbents are used, for example, in diapers for infants, incontinence products for adults or feminine hygiene prod ucts. Other fields of use are, for example, as water-retaining agents in market gardening, as means of water storage for protection from fire, for fluid absorption in food packaging, or quite general ly for absorbing moistu re.

Superabsorbents are capable of absorbing several times their own weight of water and of retaining it under a certain pressu re. I n general, such a superabsorbent has a CRC ("centri fuge retention capacity", see below for test method) of at least 5 g/g, preferably at least 10 g/g and more preferably at least 15 g/g. A "superabsorbent" may also be a mixtu re of differ ent individual su perabsorbent substances or a mixtu re of com ponents which exhibit super absorbent properties on ly when they interact; it is not so much the physical com position as the superabsorbent properties that are important here.

I m portant featu res for a su perabsorbent are not on ly its absorption capacity, but also the ability to retain fluid under pressure (retention, usual ly expressed as“Absorption u nder Load” (“AU L”) or“Absorption against Pressu re” (“AAP”) , for test method see below) and the permeability, i.e. the ability to conduct fluid in the swol len state (usual ly expressed as “Saline Flow Conductivity” (“SFC”) or as“Gel Bed Permeability” (“GBP”) , for test method see below (although changes to the su perabsorbent do not necessarily alter both its SFC and GBP values, or alter them to the same degree)) . Swol len gel can hinder or prevent fluid conductivity to as yet unswollen superabsorbent ("gel blocking"). Good conductivity proper ties for fluids are possessed, for example, by hydrogels which have a high gel strength in the swollen state. Gels with only a low gel strength are deformable under an applied pres sure (body pressure), block pores in the superabsorbent / cellulose fiber absorbent core and thus prevent fluid conductivity to as yet unswollen or incompletely swollen superabsor bent and fluid absorption by this as yet unswollen or incompletely swollen superabsorbent. An increased gel strength is generally achieved through a higher degree of crosslinking, but this reduces the absorption capacity of the product. An elegant method of increasing the gel strength is that of increasing the degree of crosslinking at the surface of the superabsor bent particles compared to the interior of the particles. To this end, superabsorbent parti cles which have usually been dried in a surface postcrosslinking step and have an average crosslinking density are subjected to additional crosslinking in a thin surface layer of the particles thereof. The surface postcrosslinking increases the crosslinking density in the shell of the superabsorbent particles, which raises the absorption under compressive stress to a higher level. While the absorption capacity in the surface layer of the superabsorbent particles falls, their core, as a result of the presence of mobile polymer chains, has an im proved absorption capacity compared to the shell, such that the shell structure ensures im proved permeability, without occurrence of gel blocking. It is likewise known that super absorbents which are relatively highly crosslinked overall can be obtained, and that the de gree of crosslinking in the interior of the particles can subsequently be reduced compared to an outer shell of the particles.

Processes for producing superabsorbents are also known. Superabsorbents based on acrylic acid, which are the most common on the market, are produced by free-radical polymerization of acrylic acid in the presence of a crosslinker (the "inner crosslinker"), the acrylic acid being neutralized to a certain degree before, after or partly before and partly af ter the polymerization, typically by adding alkali, usually an aqueous sodium hydroxide solu tion. The polymer gel thus obtained is comminuted (according to the polymerization reactor used, this can be done simultaneously with the polymerization) and dried. The dry powder thus obtained (the "base polymer") is typically postcrosslinked on the surface of the parti cles, by reacting it with further crosslinkers, for instance organic crosslinkers or polyvalent cations, for example aluminum (usually used in the form of aluminum sulfate) or both, in or der to obtain a more highly crosslinked surface layer compared to the particle interior.

Fredric L. Buchholz and Andrew T. Graham (editors), in: "Modern Superabsorbent Polymer Technology", J. Wiley & Sons, New York, U.S.A. / Wiley-VCH, Weinheim, Germany, 1997, ISBN 0-471-19411-5, give a comprehensive review of superabsorbents, the properties thereof and processes for producing superabsorbents.

Treating superabsorbents with aluminum compounds is known. For example, superabsor bents are powdered with inorganic fine particles in order to lower the tendency to caking and to increase the flowability of the powder or else its permeability. Usually, precipitated silicon dioxide is used for this purpose, but US 7795345 B2, US 3932322 or WO

2013/076031 A1 also disclose the addition of pyrogenic silicon or aluminum oxides to su perabsorbents. According to the teaching of WO 01/68 156 Al, aluminosilicates, for ex ample zeolite, are added to su perabsorbents in order to increase permeability and bind un pleasant odors. WO 2007/74 108 Al teaches coating su perabsorbents with non-reactive and non-fil m-forming compounds, for exam ple with water-insolu ble salts. Hydrated alumi num oxide is mentioned among a num ber of such salts. WO 2007/121 941 A2 discloses sim ilar superabsorbents coated with inorganic powders, where the powders may also be pro vided with a binder. Aluminiu m hydroxide is mentioned among a nu mber of inorganic pow ders.

Also known, although extremely rarely if ever practiced, is the use of polyvalent cations such alu minu m as internal crosslin ker of superabsorbents. By contrast, the addition of poly valent cations to su perabsorbents in the cou rse of surface postcrosslin king with su rface postcrosslinkers which form covalent bonds between the polymer chains is customary.

WO 99/55 767 Al discloses superabsorbents to which, before, du ring or after the polymeri zation, alu minates of the formula M _n [H _2n+2 Al n0 _3n+1 ] with M = K or Na and n = an integer from 1 to 10 are added. WO 98/48 857 Al describes su perabsorbents which are crosslinked with Al, Fe, Zr, Mg or Zn cations and then mixed with a liquid such as water, mineral oil or polyols. WO 01/74 913 Al relates to the regeneration of superabsorbents, specifical ly to the increase in a permeability reduced by attrition, by addition of a solution of an at least triva- lent cation, typical ly of an aqueous alu minu m sulfate solution. US 6 620 889 B1 discloses su perabsorbents which are su rface postcrosslin ked with a combination of a polyol and a salt of a polyvalent metal in aqueous solution. The anion of the salt may be chloride, bro mide, sulfate, carbonate, nitrate, phosphate, acetate or lactate. The use of aluminum su lfate is preferred.

According to the teaching of WO 2006/111 402 A2, a base polymer is treated with a perme ability im prover selected from silicon-oxygen com pou nds, salts of polyvalent, especial ly tri - valent, cations or mixtu res thereof. The salt of a trivalent cation is preferably an alu minum salt, which is selected from a group of salts including alu minum lactate, oxalate, citrate, gly- oxylate, succinate, tartrate and other organic and inorganic aluminum salts. WO 2005/108 472 Al discloses a process which com prises the treatment of a base polymer with a water- solu ble salt of a polyvalent metal and an organic acid or salt thereof. The salt of a polyva lent metal is preferably alu minum su lfate. The organic acid or salt thereof is selected from a group of acids including citric acid, glyoxylic acid, glutaric acid, succinic acid, tartaric acid, lactic acid and the al kali metal or ammoniu m salts of these acids.

WO 2004/113 452 Al describes su perabsorbents which are treated with concentrated solu tions of polyvalent metal salts, especial ly sodium alu minate. WO 2013/156 281 Al teaches the treatment of su perabsorbents with alu minum glycinate. WO 2010/108 875 Al, WO 2012/045 705 Al and WO 2013/156 330 Al teach the treatment of superabsorbents with basic alu minum salts such as basic alu minum acetate or aluminu m lactate. WO 2009/080611 A2 discloses the treatment of superabsorbents with mixtures of alumi num salts, one of which comprises a chelating anion, for example dicarboxylates or hy- droxycarboxylates, particular preference being given to lactate, and the other a weakly com- plexing anion, for example chloride, nitrate, sulfate, hydrogensulfate, carbonate, hydrogen- carbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate or carboxylate, particular preference being given to sulfate. Prior application 17200963.1 at the European Patent Office teaches a superabsorbent complexed with aluminum ions, where the alumi num ions are applied in the form of an aqueous solution comprising aluminum ions, which has the feature that it comprises aluminum ions in a proportion within the range of 0.5%- 15% by weight (optionally converted to Al ³⁺ ), based on the total mass of the solution, and further comprises anions of lactic acid (lactate ions) and phosphoric acid (phosphate ions), where the molar proportion of the lactate ions is within the range of 0.01-2.99 times the mo lar amount of Al ³⁺ and the molar proportion of the phosphate ions is within the range of 0.01-2.99 times the molar amount of Al ³⁺ . These solutions are prepared by adding acid to amorphous aluminum hydroxide.

EP 233067 A2 discloses the surface postcrosslinking of superabsorbents with aluminum salts that can react with the superabsorbent in the presence of polyol and water. Among a number of aluminum salts, aluminum hydroxide is also mentioned. The use of freshly pre cipitated aluminum hydroxide sol or gel is recommended. According to the teaching of US 5 145906, undried polymerized gel is treated with a surface postcrosslinker; the surface post crosslinking reaction takes place in the course of heating for drying or during the drying. Aluminum hydroxide is one of the possible surface postcrosslinkers mentioned. JP 09/124 879 A likewise mentions, among a number of compounds, aluminum hydroxide for this pur pose, but this, like the other water-soluble compounds mentioned, is to be applied as a so lution. EP 780424 A1 mentions the hydroxides and chlorides of a number of metals, includ ing aluminum, as surface postcrosslinkers. US 5684106 mentions aluminum sulfate, so dium aluminate or other polyvalent metal compounds for this purpose. JP 3121934 B teaches, for this purpose, the use of polyaluminum hydroxides of the formula [Al (O H) ₃ ] _n AICI ₃ with n = 10-21).

WO 03/049778 Al teaches, in the case of superabsorbents postcrosslinked with either co valent surface postcrosslinkers or polyvalent metal ions, after a first absorption of liquid, re dissolving the postcrosslinking with metal ions, for example complexing agents, in order to obtain further absorption capacity thereby. Aluminum hydroxide is mentioned among a number of possible polyvalent metal salts as postcrosslinkers.

According to the teaching of WO 2012/143215 Al, a solution of a neutralized polyvalent metal salt, preferably of an aluminum salt which is formed from an aluminum compound and an organic acid with a chelate-forming anion, and wherein the solution is neutralized to a pH between 5 and 9 with acid or base, is added to the superabsorbent. The aluminum compound that has thus been reacted with an acid having a chelate-forming anion may also be aluminum hydroxide. The organic chelate-forming anions mentioned form a water- soluble complex with the metal ion. WO 2013/72311 A1 teaches surface postcrosslinking using a complex of a metal salt, for instance aluminum hydroxide, and a 2-hydroxycarbox- amide.

WO 2014/167036 Al, WO 2014/167040 and WO 2014/168858 A1 disclose, like EP 233067 A2 already cited above, the application of freshly precipitated aluminum hydroxide sol in the surface postcrosslinking.

US 2016/0235882 Al teaches, prior to application of a solution of a covalent postcross linker, mixing of aluminum hydroxide powder into the superabsorbent base polymer. The mixing-in is preferably effected in dry form since there is a rise in the tendency to caking on application of suspensions.

International Patent Application No. PCT/EP2019/058165 filed 01 ^st April 2019 by the same applicant discloses adding x-ray amorphous aluminium hydroxide powder to a superabsor bent. Optionally, 0.1% to 10% by weight, based on the amount of polymer prior to the addi tion of the x-ray-amorphous aluminum hydroxide powder, of water is added to the super absorbent.

There still remains the problem of finding different or even improved superabsorbents, in particular permeable superabsorbents, and very substantially simplified or improved pro cesses for production thereof. There should be at least only insignificant, if any, impairment of the use properties of the superabsorbent, especially its absorption capacity for liquid, in cluding under pressure, and its swelling kinetics, quantified as volumetric absorption under pressure“VAULT Further objects of the invention are uses of this superabsorbent, such as hygiene products comprising this superabsorbent and processes for production thereof.

The object was achieved by a process for producing a superabsorbent, comprising polymer izing an aqueous monomer solution comprising

a) at least one ethylenically unsaturated monomer which bears acid groups and is op tionally at least partly in salt form,

b) at least one crosslinker,

c) at least one initiator,

d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers mentioned under a), and

e) optionally one or more water-soluble polymers, drying the resulting polymer, optionally grinding the dried polymer and sieving the ground polymer, optionally surface postcrosslinking of the dried and optionally ground and sieved polymer, adding x-ray-amorphous alu minum hyd roxide powder after d rying, grinding or sieving, and, if su rface postcrosslinking is conducted, du ring or after this su rface postcrosslin king, adding 0.1% to 10% by weight, based on the amou nt of polymer prior to the addition of the x-ray-amorphous alu minum hydroxide powder, of water after the addition of x-ray-amor- phous aluminu m hydroxide powder, wherein the water is added to the su perabsorbent in at least three portions.

The su perabsorbents of the invention are obtainable by the process of the invention and are preferably produced by the process of the invention. They show su rprisingly good permea bility, without any significant impairment in their other use properties such as CRC or AU L.

Articles for absorption of fluids have additional ly been fou nd, especial ly hygiene articles for absorption of fluid excretions or fluid com ponents of excretions, which comprise the su per absorbent of the invention. Processes for production of such articles for absorption of fluids have also been fou nd, the production of these articles involving addition of the su perabsor bent of the invention thereto.

I n the production of the superabsorbent of the invention in the process of the invention, x- ray-amorphous alu minu m hydroxide is added.

The addition of polyvalent metal salts to su perabsorbents has long been known. Usual ly, aluminu m salts are added. The addition is typical ly effected in the course of surface post crosslinking with a covalent surface postcrosslin king, i.e. a compound that can form cova lent bonds with fu nctional grou ps at the su rface of the su perabsorbent particles, typical ly the acid grou ps of the customary acrylic acid-based su perabsorbents. U ltimately, the addi tion of polyvalent metal salts also leads to crosslin king sites at the surface of the su per absorbent particles, but by ionic bonding. The treatment of superabsorbents with polyvalent metal ions in the cou rse of su rface postcrosslin king is often referred to as "com plexation". "Complexation" is thus, strictly speaking, solely a specific term for the special case of su r face postcrosslinking in which polyvalent metal ions produce ionic bonds between several polar grou ps at the su rface of the superabsorbent particles, and complexation is often also discussed u nder "su rface postcrosslin king". I n the context of this invention, "complexation" is u nderstood to mean su rface postcrosslinking with polyvalent metal ions, especial ly alu mi num, in order to delimit it from su rface postcrosslinking with postcrosslin kers which form covalent bonds with polar groups at the surface of the su perabsorbent particles.

I n com plexation, the added metal salt is reacted with the superabsorbent. The x-ray-amor- phous aluminu m hydroxide added here to the superabsorbent in accordance with the inven tion is therefore not or at least not completely conserved as such in the su perabsorbent. What is therefore essential is that the superabsorbent of the invention has been treated with x-ray-amorphous alu minu m hydroxide - or in other words that it has been added thereto. It may but need not still comprise added x-ray-amorphous aluminum hydroxide as such.

Aluminum hydroxide is understood here to mean aluminum trihydroxide. The nomenclature is not treated uniformly in the literature. More particularly, aluminum hydroxide is often also counted among the aluminum hydroxide hydrates or else referred to as hydrated aluminum oxide. Aluminum hydroxide is precipitated in a known manner from aluminum salt solutions by addition of base or from aluminate solutions by addition of acid, in each case to the neu tral pH region. For preparation of amorphous aluminum hydroxide, precipitation has to be effected rapidly since, in the case of slow precipitation, crystalline gamma-AI(OH) ₃ (“hydrar- gillite” or“gibbsite”) forms from aluminate solutions or alpha-AI(OH) ₃ (“bayerite”) from alu minum salt solutions. Bayerite is gradually converted to hydrargillite over time. These hy droxides can condense slowly to form the aluminum hydroxide hydrates of the formula AI(0)0H (diaspore or boehmite), which are occasionally also referred to as "hydroxides". In the production of x-ray-amorphous aluminum trihydroxide, therefore, rather than seeding with crystalline aluminum hydroxide as in the Bayer process for preparation of aluminum hydroxide hydrates, there is rapid precipitation and rapid drying. Preference is given to dry ing freshly precipitated aluminum hydroxide gel by means of spray drying. The chemistry and preparation of aluminum hydroxide, oxide hydrate and oxide are well-known; see, for example, Holleman/Wiberg, Lehrbuch der Anorganischen Chemie [Inorganic Chemistry], Walter de Gruyter & Co., Berlin, 103rd edition 2016, ISBN 978-3-11-051854-2, section 2.4 “Sauerstoffverbindungen des Aluminiums” [Oxygen Compounds of Aluminum], or Ullmann's Encyclopedia of Industrial Chemistry, under“Aluminum Oxide”, Wiley-VCH Verlag Gmbh & Co. KGaA, Weinheim 2012, DOI 10.1002/14356007. a01_557. Some structural differences be tween amorphous and crystalline aluminum hydroxide at the atomic/molecular level are elucidated by T. Isobe et a I . , J. Colloid and Interface Science 261 (2003) 320-324, which in cidentally also mentions further known methods of production of amorphous aluminum hy droxide by way of introduction. X-ray-amorphous aluminum hydroxide in powder form is also commercially available, for example as "aluminum hydroxide dried gel", catalog number 511066100, from Dr. Paul Lohmann GmbH KG, Hauptstrasse 2, 31860 Emmerthal, Germany.

Aluminum hydroxide is x-ray-amorphous when it does not show any signals ("lines") in an x-ray powder diffractogram. Crystalline substances diffract electromagnetic radiation having a wavelength in the order of magnitude of the interatomic distances - x-radiation is typically used - such that a diffraction pattern of the radiation can be measured. When single crys tals are sufficiently large, they result in diffraction patterns in the form of dots, in which the crystal structure including the position of the atoms in the crystal lattice can be calculated from the position and intensity of the dots. It is possible to measure a diffractogram on a powder of a crystalline substance that shows maxima of the scattered x-radiation as a function of the deflection angle as signals ("lines"), where the half-height width of the max ima correlates with the size of the crystals in the powder. All of this is well-known; see, for example, Ullmann's Encyclopedia of Industrial Chemistry, under“Structure Analysis by Dif fraction”, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2012, DOI d

10.1002/14356007. b05_341. The method is established, and instruments for measurement of powder diffractograms are commercially available.

Accordingly, powders show no signals in the x-ray diffractogram when they have no crystal structure, the atoms or molecules are thus in entirely or largely unordered form, or the crys tals are so small that the number of unit cells in the crystal that are arranged to form the crystal is insufficient for focused diffraction of radiation and the half-height width of the maxima in the diffractogram becomes so great that they disappear in the baseline and it is possible to measure only diffuse radiation. What is crucial here is the size of the crystals in the powder, not the particle size of the powder. In one powder particle, it is possible for nu merous smaller crystals ("primary crystals" or "primary crystallites") to be agglomerated or bonded in some other way. For the x-ray-amorphous aluminum hydroxide used here in ac cordance with the invention, that the primary crystallites present therein - if any are present - have a size of not more than 2 nm.

The x-ray-amorphous aluminum hydroxide can be used in a mixture with crystalline alumi num hydroxide. In this case, lines attributable to the crystalline component may occur in the diffractogram of the mixture. The proportion of crystalline aluminum hydroxide can also be determined, for example, by calibrating a powder diffractogram, by adding a known amount of crystalline aluminum hydroxide to a comparative sample of the powder and expressing the increase in the total area of all lines in the diffractogram that results therefrom in rela tion to the area of all lines in the diffractogram of the actual sample. All of this is known, and is no different in the calibration of chromatographs for instance.

In general, the total aluminum hydroxide added has a proportion of at least 50% by weight of x-ray-amorphous aluminum hydroxide, preferably of at least 70% by weight and more preferably of at least 90% by weight. What is desirable is pure x-ray-amorphous aluminum hydroxide without crystalline components that occur in an x-ray diffractogram, or at least aluminum hydroxide which, apart from unavoidable small crystalline fractions (as occur, for instance, in the course of aging), consists solely of x-ray-amorphous aluminum hydroxide.

According to the invention, the x-ray-amorphous aluminum hydroxide is added generally in an amount of at least 0.01% by weight, preferably at least 0.1% by weight and more prefera bly of at least 0.2% by weight, even more preferably of at least 0.3% by weight, and gener ally of at most 1% by weight, preferably at most 0.8% by weight and more preferably of at most 0.7% by weight, even more preferably at most 0.5% by weight, based in each case on the superabsorbent prior to the addition. One benefit of the present invention over the teaching of International Patent Application No. PCT/EP2019/058165 is that less aluminum hydroxide is needed to achieve similar GBP enhancement.

According to the invention, the x-ray-amorphous aluminum hydroxide is added as powder after drying, grinding or sieving, and, if surface postcrosslinking is conducted, during or after this surface postcrosslinking. The total amount of x-ray-amorphous aluminum hydroxide to be added can also be divided between mu ltiple addition sites or ju nctu res. Preference is given to addition at one site and at one ju nctu re.

I n the process of the invention, the x-ray-amorphous alu minum hyd roxide is accordingly mixed d ry into the su perabsorbent powder after drying thereof, and, if a sieve fraction of the su perabsorbent is obtained after drying, into this sieving step. This can be effected by any known mixing apparatus and process and is preferably conducted in mixers with moving mixing tools, for exam ple screw mixers, disk mixers, padd le mixers or shovel mixers, or mix ers with other mixing tools. Suitable mixers are obtainable, for exam ple, as Pflugschar ⁰ (plowshare) mixers from Gebr. Lodige Maschinen bau Gm bH , Elsener-Strasse 7-9, 33102 Paderborn, Germany.

According to the invention, after the x-ray-amorphous alu minu m hyd roxide has been mixed in, the su perabsorbent thus produced is moistened, i.e. its water content is increased. For this purpose, in a dedicated apparatus or conveniently in the same mixer, water is added, for instance by spray application via a nozzle, likewise in the man ner described below for the spray application of a postcrosslinker solution. I n general, water is added to the su per absorbent in an amount of at least 0.1% by weight, preferably of at least 0.5% by weight and more preferably at least 1% by weight, and general ly at most 10% by weight, preferably at most 7% by weight and more preferably at most 5% by weight, based in each case on the su perabsorbent prior to the addition.

According to the invention, the water is added to the superabsorbent in at least three por tions. That means the total amount of water is not added at one time, but at least in th ree separate portions at different times. Between adding portions of water, the superabsorbent is preferably mixed.

Typical ly, the second and any consecutive portions of water are added after a time of gen eral ly at least 1 second, for exam ple at least 5 seconds, at least 10 seconds, at least 15 sec onds, at least 20 seconds or at least 40 seconds fol lowing the com plete addition of the pre vious portion. The optimu m time between additions can easily be determined for any given su perabsorbent and mixing conditions by a few routine experiments. If a shorter or longer time before the addition of the next portion of water does not further im prove the su per absorbent’s properties, there is no need to shorten or extend such times. Very gentle mixing may require longer times between water additions. General ly, once the time is chosen long enough to achieve the desired increase in properties, in particu lar G BP, com pared to adding al l of the water in one amount at once, longer times between additions of water portions do not further im prove the su perabsorbent, but also do not worsen the su perabsorbent or any of its properties. The time between adding the first and the second and any consecutive portions of water is therefore not critical and may be chosen dependent on commercial as pects and what is achievable using the available equipment. Commercial aspects typical ly favour a shorter time while available mixing equipment may need longer times to mix the su perabsorbent. On standard com mercial powder mixing equipment, the time wil l be gener al ly be most sixty minutes, preferably at most three minutes and more preferably at least two minutes fol lowing the addition of the previous portion, for example one minute fol lowing the addition of the previous portion.

The water is added in at least three portions. Preferably, the water is added in at least four portions. It is possible to add water in any nu mber of portions above fou r, for exam ple five, six, seven, eight, nine, ten or above. These portions of water may be equal or not. Prefera bly, the total amount of water is added in equal portions.

For every specific su perabsorbent to be produced according to the process of the invention, there wil l be a nu mber of portions which wil l not resu lt in a fu rther increased technical ben efit. This num ber can easily be determined by routine experimentation. It is possible, and in some cases - for example due to constraints of existing apparatus - even be reasonable to add water in more portions than the minimum nu m ber needed to achieve the desired effect of GBP en hancement, but general ly it wil l be economical ly disadvantageous to use more than that minimu m nu m ber of portions.

Water addition in portions can easily be done in batch processing by adding water, in sepa rate portions, to an amou nt of su perabsorbent in a vessel, preferably a stirred vessel. More preferably, water is added to the su perabsorbent in a vessel where the su perabsorbent is continuously flowing or transported th rough the vessel. Yet more preferably, the su per absorbent is also stirred in this vessel. Water may be added in the same type of apparatus that is also used for adding alu minum hyd roxide and conveniently may be added in the very same piece of equipment that is used for alu minu m hydroxide addition. Most conveniently, the water portions are sprayed on the mixed superabsorbent via nozzles, likewise in the man ner described below for the spray application of a postcrosslinker solution.

I n continuously operated apparatus, the superabsorbent continuously travels through the apparatus and adding portions of water to su perabsorbent separately from each other, at different times, is equivalent to adding the portions of water at different places along the su perabsorbent’s overal l path of travel, so that each portion of water reaches the moving su perabsorbent one by one at a different time du ring its travel th rough the apparatus. I n other words, the fact that the portions of water may be continuously and permanently con currently sprayed from nozzles stil l means adding these portions separately to the su per absorbent if the nozzles are distant from each other along the path of travel of the super absorbent. Adding water in portions can therefore be achieved by spacing the nozzles ac cordingly in the apparatus so that the water portions are added at the desired time intervals of the su perabsorbents’ particles total residence time in the apparatus. I n a mixing appa ratus, the average residence time of a particle counts. Methods for determining the average residence time of particles in specific continuous apparatus are known, one convenient method is adding a sample of particles of different color as the main amou nt and determin ing the moment at which the nu mber of differently colored particles in the total particles discharged from the apparatus reaches a maximu m. According to this invention, water can easily be added in portions by locating nozzles for water addition the length of the continu ous apparatus. One position along the apparatus’s length wil l deliver one portion of water. One portion of water may be added through more than one nozzle located at that position along the length of the apparatus. As the su perabsorbent moves th rough the continuous ap paratus, it wil l reach the next nozzle position along the length of the apparatus at a later time, consequently the next portion of water wil l be added at that later time. I n this embodi ment of the invention, the nu mber of nozzle positions along the length of the apparatus is equal to the nu mber of portions of water added and the distance between nozzle positions along the length of the apparatus, jointly with the average residence time particles deter mines the times of addition of the individual portions of water. I n other words, the nozzles are positioned along the length of the apparatus where they wil l add the portions of water at the desired points of time in view of the average residence time of the su perabsorbent in the specific apparatus.

If the su perabsorbent is not warm or hot (for instance after a preceding d rying operation) at the time of addition of water, it may be advantageous to add heated water in this remoistur izing operation. I n general, water is added at a tem perature of at least 5° C, preferably at least 20° C and more preferably at least 25° C, for example water at a tem peratu re of 30° C, 40° C, 50° C, 60° C, 70° C, 80° C or 90° C. It is also possible to use steam.

The addition of x-ray-amorphous alu minu m hydroxide and optional ly that of water can be effected before, du ring or after su rface postcrosslin king. If the su rface postcrosslin ker solu tion to be applied comprises water, it is conveniently possible to conduct the addition of x- ray-amorphous alu minum hyd roxide beforehand, then add the portions of water except the last one and replace the addition of the last portion of water by the addition of the su rface postcrosslin ker solution.

For the rest, the process of the invention for production of superabsorbents is known. It is a process for aqueous solution polymerization of a monomer mixtu re comprising the fol low ing: a) at least one ethylenical ly unsatu rated monomer which bears at least one acid group and is optional ly at least partly in salt form,

b) at least one crosslinker,

c) at least one initiator,

d) optional ly one or more ethylenical ly u nsaturated monomers copolymerizable with the monomers mentioned u nder a) , and

e) optional ly one or more water-solu ble polymers.

The monomers a) are preferably water-solu ble, i.e. their solu bility in water at 23° C is typi cal ly 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 and most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for exam ple, ethylenical ly unsaturated carboxylic acids or salts thereof, such as acrylic acid, methacrylic acid, maleic acid or salts thereof, maleic anhydride and itaconic acid or salts thereof. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particu lar preference is given to acrylic acid.

Fu rther suitable monomers a) are, for example, ethylen ica I ly unsatu rated su lfonic acids, such as styrenesu lfonic acid and 2-acrylamido-2-methyl propanesu lfonic acid (AM PS) .

I m pu rities can have a considerable influence on the polymerization. The raw materials used should therefore have a maximu m pu rity. It is therefore often advantageous to special ly pu rify the monomers a) . Suitable pu rification processes are described, for exam ple, in WO 2002/055469 Al, WO 2003/078378 A1 and WO 2004/035514 Al. A suitable monomer a) is, for example, an acrylic acid pu rified according to WO 2004/035514 Al and com prising 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 fu rfurals, 0.0001% by weight of maleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hyd roquinone 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 monomer solution com prises preferably at most 250 ppm by weight, preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight and preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight, especial ly arou nd 50 ppm by weight, of hydroquinone monoether, based in each case on the un neutralized monomer a) ; neutralized monomer a), i.e. a salt of the monomer a) , is considered for arithmetic pu rposes to be u n neutralized monomer. For example, the monomer solution can be prepared by using an ethylenical ly u nsatu rated monomer bearing acid groups with an appropriate content of hyd roquinone monoether.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether (M EHQ) and/or al pha-tocopherol (vitamin E) .

Suitable crosslin kers b) are com pou nds having at least two grou ps suitable for crosslin king. Such grou ps are, for example, ethylenical ly u nsatu rated groups which can be polymerized f ree-rad ica I ly into the polymer chain, and fu nctional grou ps which can form covalent bonds with the acid grou ps of the monomer a) . I n addition, polyvalent metal salts which can form coordinate bonds with at least two acid grou ps of the monomer a) are also suitable as crosslin kers b) .

Crosslin kers b) are preferably compou nds having at least two polymerizable groups which can be polymerized free-radical ly into the polymer network. Suitable crosslin kers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol di acrylate, a I ly I methacrylate, trimethylol propane triacrylate, trial lylamine, tetraal lylam monium chloride, tetraal lyloxyethane, as described in EP 530 438 Al, di- and triacrylates, as described in EP 547 847 Al, EP 559 476 Al, EP 632 068 Al, WO 93/21237 Al,

WO 2003/104299 Al, WO 2003/104300 Al, WO 2003/104301 Al and DE 103 31 450 Al, mixed acrylates which, as well as acrylate groups, comprise further ethylenically unsatu rated groups, as described in DE 103 31 456 Al and DE 103 55 401 Al, or crosslinker mix tures, as described, for example, in DE 195 43 368 Al, DE 196 46 484 Al, WO 90/15830 Al and WO 2002/32962 A2.

Preferred crosslinkers b) are pentaerythrityl trial lyl ether, tetraallyloxyethane, meth- ylenebismethacrylamide, 15- to 20-tuply ethoxylated trimethylolpropane triacrylate, 15-20- tuply ethoxylated glyceryl triacrylate, polyethylene glycol diacrylate having between 4 and 45 -CH ₂ CH ₂ 0 units in the molecule chain, trimethylolpropane triacrylate and trial lylamine.

Very particularly preferred crosslinkers b) are the polyethoxylated and/or propoxylated glyc erols which have been esterified with acrylic acid or methacrylic acid to give di- or triacry lates, as described, for example, in WO 2003/104301 Al. 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 pre ferred are the triacrylates of 3- to 5-tuply ethoxylated and/or propoxylated glycerol, espe cial ly the triacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably 0.05% to 1.5% by weight, more preferably 0.1% to 1% by weight, most preferably 0.3% to 0.6% by weight, based in each case on monomer a). With rising crosslinker content, there is a fall in the centrifuge retention capacity (CRC) and a rise in the absorption under load (AUL).

The initiators c) used may be all compounds which generate free radicals under the polymerization conditions, for example thermal initiators, redox initiators and/or photoinitia tors. Suitable redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen perox ide/ascorbic acid, sodium peroxodisulfate/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. However, the reducing component used is preferably also a sulfonic acid derivative, for example a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonato- acetic acid and sodium bisulfite, obtainable, for example, from L. Bmggemann KG (Salz- strasse 131, 74076 Heilbronn, Germany, www.brueggemann.com) under the BRLIGGOLIT ⁰ FF6M or BRLIGGOLIT ⁰ FF7, or alternatively BRUGGOLITE ⁰ FF6M or BRUGGOLITE ⁰ FF7 names, or the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, obtainable, for example, from L. Bmggemann KG under the BLANCOLEN ⁰ HP name. The initiators are, incidentally, used in customary amounts. The customary amount of the reducing component of a redox initiator is generally at least 0.00001% by weight, preferably at least 0.0001% by weight and more preferably at least 0,001% by weight, and generally at most 0.2% by weight and prefer ably at most 0.1% by weight, based in each case on the amount of monomers a) and d). If, however, the sole reducing component used in the redox initiator is sulfonic acid derivative, the added amount thereof is generally at least 0.001% by weight, preferably at least 0.01% by weight and more preferably at least 0.03% by weight, and general ly at most 1.0% by weight, preferably at most 0.3% by weight and more preferably at most 0.2% by weight, based in each case on the amount of monomers a) and d) . The customary amou nt of the oxidizing com ponent of a redox initiator is general ly 0.0001% by weight and more preferably at least 0.001% by weight, and general ly at most 2% by weight and preferably at most 1.0% by weight, based in each case on the amou nt of monomers a) and d). The customary amount of the thermal initiators is general ly 0.01% by weight and more preferably at least 0.1% by weight, and general ly at most 2% by weight and preferably at most 1.0% by weight, based in each case on the amount of monomers a) and d) . The customary amou nt of the photoinitiators is general ly 0.001% by weight and more preferably at least 0.01% by weight, and general ly at most 1.0% by weight and preferably at most 0.2% by weight, based in each case on the amou nt of monomers a) and d) .

Ethylen ical ly u nsaturated monomers d) copolymerizable with the ethylenical ly u nsaturated monomers a) bearing acid groups are, for exam ple, acrylamide, methacrylamide, hydroxy- ethyl acrylate, hyd roxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylami- noethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylami noethyl methacrylate, diethylaminoethyl methacrylate, maleic acid or salts thereof and ma leic an hyd ride.

The water-solu ble polymers e) used may be polyvinyl alcohol, polyvinyl pyrrolidone, starch, starch derivatives, modified cel lu lose, such as methylcel lulose or hydroxyethylcel lu lose, gel atin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cel lu lose.

Typical ly, an aqueous monomer solution is used. The water content of the monomer solu tion is preferably from 40% to 75% by weight, more preferably from 45% to 70% by weight and most preferably from 50% to 65% by weight. It is also possible to use monomer suspen sions, i.e. oversatu rated monomer solutions. As the water content rises, the energy expendi tu re in the su bsequent d rying rises and, as the water content fal ls, the heat of polymeriza tion can on ly be removed inadequately.

For optimal action, the preferred polymerization in hibitors require dissolved oxygen. The monomer solution can therefore be freed of dissolved oxygen before the polymerization by inertization, i.e. flowing an inert gas th rough, preferably nitrogen or carbon dioxide. The oxy gen content of the monomer solution is preferably lowered before the polymerization 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.

The monomer mixture may comprise fu rther components. Exam ples of fu rther com ponents used in such monomer mixtu res are, for instance, chelating agents in order to keep metal ions in solution, or inorganic powders in order to increase the stiffness of the su perabsor bent in the swol len state, or recycled u ndersize from a later grinding operation. It is possible here to use al l known additions to the monomer mixture. Even though on ly "solution" is discussed here in con nection with the monomer mixture, this also means the polymerization of a suspension, for instance when insoluble constituents are added to the monomer mix tu re.

The acid grou ps of the polymer gels resu lting from the polymerization have typical ly been partly neutralized. Neutralization is preferably carried out at the monomer stage; in other words, salts of the monomers bearing acid grou ps or, to be precise, a mixtu re of monomers bearing acid groups and salts of the monomers bearing acid grou ps ("partly neutralized acid") are used as component a) in the polymerization. This is typical ly accom plished by mixing the neutralizing agent as an aqueous solution or preferably also as a solid into the monomer mixture intended for polymerization or preferably into the monomer bearing acid groups or a solution thereof. The degree of neutralization is preferably from 25 to 95 mol%, more preferably from 50 to 80 mol% and most preferably from 65 to 72 mol%, for which the customary neutralizing agents can be used, preferably al kali metal hydroxides, al kali metal oxides, al kali metal carbonates or al kali metal hyd rogencarbonates and also mixtures thereof. I nstead of al kali metal salts, it is also possible to use am monium salts. Particu larly preferred al kali metals are sodiu m and potassium, but very particu lar preference is given to sodium hydroxide, sodium carbonate or sodiu m hydrogencarbonate and also mixtures thereof.

However, it is also possible to carry out neutralization after the polymerization, at the stage of the polymer gel formed in the polymerization. It is also possible to neutralize u p to 40 mol%, preferably 10 to 30 mol% and more preferably 15 to 25 mol% of the acid grou ps be fore the polymerization by adding a portion of the neutralizing agent directly to the monomer solution and setting the desired final degree of neutralization only after the polymerization, at the polymer gel stage. When the polymer gel is at least partly neutralized after the polymerization, the polymer gel is preferably com minuted mechanical ly, for example by means of an extruder, in which case the neutralizing agent can be sprayed, sprin kled or pou red on and then carefu l ly mixed in. For this pu rpose, the gel material obtained can be extruded several times more for homogenization.

However, preference is given to performing the neutralization at the monomer stage. I n other words, in a very particu larly preferred em bodiment, the monomer a) used is a mixtu re of 25 to 95 mol%, more preferably from 50 to 80 mol% and most preferably from 65 to 75 mol% of salt of the monomer bearing acid groups, and the remainder to 100 mol% of mono mer bearing acid grou ps. This mixtu re is, for exam ple, a mixture of sodiu m acrylate and acrylic acid or a mixtu re of potassiu m acrylate and acrylic acid.

I n a preferred em bodiment, the neutralizing agent used for the neutralization is one whose iron content is general ly below 10 ppm by weight, preferably below 2 ppm by weight and more preferably below 1 ppm by weight. Likewise desired is a low content of chloride and anions of oxygen acids of ch lorine. A suitable neutralizing agent is, for exam ple, the 50% by weight sodium hyd roxide solution or potassiu m hyd roxide solution which is typical ly traded as“mem brane grade”; even more pu re and likewise suitable, but also more expensive, is the 50% by weight sodiu m hydroxide solution or potassium hydroxide solution typical ly traded as“amalgam grade” or“mercu ry process”.

Processes for production of su perabsorbents from monomer mixtu res, such as those de scribed by way of example above, are known in principle. Suitable polymerization reactors are, for example, kneading reactors or belt reactors. I n the kneader, the polymer gel formed in the polymerization of an aqueous monomer solution or suspension is com minuted contin uously by, for exam ple, contrarotatory stirrer shafts, as described in WO 2001/38402 Al. Polymerization on a belt is described, for exam ple, in EP 955 086 A2, DE 38 25 366 Al and US 6,241,928. Polymerization in a belt reactor forms, like the likewise known polymerization in batchwise operation or in a tubu lar reactor, as described, for exam ple, in EP 445 619 A2 and DE 19 846 413 Al, a polymer gel which has to be comminuted in a further process step, for example in a meat grinder, extruder or kneader. It is also possible to produce spherical or differently shaped su perabsorbent particles by suspension or emu lsion polymerization, as described, for exam ple, in EP 457 660 Al, or by spray or d roplet polymerization processes, as described, for exam ple, in EP 348 180 Al, EP 816 383 Al, WO 96/404 27 Al, US 4 020 256, US 2002/0 193 546 Al, DE 35 19 013 Al, DE 10 2005 044 035 Al, WO 2007/093531 Al, WO 2008/086 976 Al or WO 2009/027 356 Al. Likewise known are processes in which the monomer mixture is applied to a substrate, for exam ple a nonwoven web, and polymerized, as described, for instance, in WO 02/94 328 A2 and WO 02/94 329 Al.

It is optional ly possible in a known manner to add a su lfonic acid derivative, including in a mixtu re with sulfite or sulfinic acid derivative, to the superabsorbent or else to the monomer mixtu re before or after d rying, but preferably before d rying. These mixtures are standard commercial products and are available, for example, in the form of mixtu res of the sodium salt of 2-hydroxy-2-su lfinatoacetic acid, the disodiu m salt of 2-hyd roxy-2-su lfonatoacetic acid and sodiu m bisulfite from L. Bmggemann KG (Salzstrasse 131, 74076 Heil bron n, Ger many, www.brueggeman n.com) under the BRLIGGOLIT ⁰ FF6M or BRLIGGOLIT ⁰ FF7 names, or alternatively BRUGGOLITE ⁰ FF6M or BRUGGOLITE ⁰ FF7. Preference is given to the use of the su lfonic acid derivatives in pu re form. These too are standard com mercial products. For example, the disodiu m salt of 2-hydroxy-2-su lfonatoacetic acid is available from L. Bmggeman n KG (Salzstrasse 131, 74076 Heil bron n, Germany, www.brueggeman n.com) un der the BLANCOLEN ⁰ H P name.

The su lfonic acid derivative is general ly used in an amou nt of at least 0.0001% by weight, preferably at least 0.001% by weight and more preferably at least 0.025% by weight, for ex ample at least 0.05% by weight or at least 0.1% by weight, and general ly at most 3% by weight, preferably at most 2% by weight and more preferably at most 0.5% by weight, for ex ample at most 0.35% by weight or 0.2% by weight, based in each case on the total weight of the su perabsorbent.

Just like the su lfonic acid derivative, it is optional ly also possible in a known manner, in ad dition thereto or on its own, to add at least one phosphonic acid derivative to the su per absorbent or else to the monomer mixtu re before or after d rying, but preferably before drying. Particular preference is given to the addition of preferably (l-hydroxyethane-1,1- diyl)bisphosphonic acid (“etidronic acid”) or a salt thereof, especially the sodium salt, the potassium salt, the disodium salt, the dipotassium salt or the sodium potassium salt. Phos- phonic acid derivatives of this kind are standard commercial products and are available, for example, under the ModosoP (formerly Cublen ⁰ ) brand from Zschimmer & Schwarz

Mohsdorf GmbH & Co KG, Chemnitztalstrasse 1, 09217 Burgstadt, Germany.

The phosphonic acid derivative is generally added in an amount of at least 0.01% by weight, preferably at least 0.1% by weight and more preferably at least 0.2% by weight, and gener ally at most 1.9% by weight, preferably at most 1.3% by weight and more preferably at most 0.6% by weight, based in each case on the total amount of the anhydrous superabsorbent.

The polymer gel obtained from the aqueous solution polymerization and optional subse quent neutralization is then preferably dried with a belt drier until the residual moisture content is preferably 0.5 to 15% by weight, more preferably 1 to 10% by weight and most preferably 2 to 8% by weight (see below for test method for the residual moisture or water content). 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 dry ing is generally from 25 to 90% by weight, preferably from 30 to 80% by weight, more prefer ably from 35 to 70% by weight and most preferably from 40 to 60% by weight. Optionally, however, it is also possible to dry using a fluidized bed drier or a heatable mixer with a me chanical mixing unit, for example a paddle drier or a similar drier with mixing tools of differ ent design. Optionally, the drier can be operated under nitrogen or another nonoxidizing in ert gas or at least under reduced partial oxygen pressure in order to prevent oxidative yel lowing processes. As a rule, however, sufficient aeration and removal of the steam will also lead to an acceptable product. In general, a minimum drying time is advantageous with re gard to color and product quality.

During the drying, the residual monomer content in the polymer particles is also reduced, and last residues of the initiator are destroyed.

Thereafter, the dried polymer gel is optionally - and preferably - ground and classified, in which case the apparatus used for grinding may typically be single or multistage roll mills, preferably two- or three-stage roll mills, pin mills, hammer mills or vibratory mills. Oversize gel lumps which often still have not dried on the inside are elastomeric, lead to problems in the grinding and are preferably removed before the grinding, which can be done in a simple manner by wind sifting or by means of a sieve ("guard sieve" for the mill). In view of the mill used, the mesh size of the sieve should be selected such that a minimum level of disruption resulting from oversize, elastomeric particles occurs. Excessively large, insufficiently finely grou nd superabsorbent particles are perceptible as coarse particles in their predominant use, in hygiene products such as diapers; they also lower the mean initial swel l rate of the su perabsorbent. Both are u ndesired. Advanta geously, coarse-grain polymer particles are therefore separated from the product. This is done by conventional classification processes, for example wind sifting, or by sieving th rough a sieve with a mesh size of at most 1000 pm, preferably at most 900 pm, more pref erably at most 850 pm and most preferably at most 800 pm. For exam ple, sieves of mesh size 700 pm, 650 pm or 600 pm are used. The coarse polymer particles ("oversize") removed may, for cost optimization, be sent back to the grinding and sieving cycle or be processed fu rther separately.

Polymer particles with too smal l a particle size lower the permeability (SFC) . Advanta geously, this classification therefore also removes fine polymer particles. If sieving is ef fected, this can conveniently be done through a sieve of mesh size at most 300 pm, prefera bly at most 200 pm, more preferably at most 150 pm and most preferably at most 100 pm. The fine polymer particles ("u ndersize" or "fines") removed can, for cost optimization, be sent back as desired to the monomer stream, to the polymerizing gel, or to the fu l ly pol ymerized gel before the d rying of the gel.

The mean particle size of the polymer particles removed as the product fraction is general ly at least 200 pm, preferably at least 250 pm and more preferably at least 300 pm, and gener al ly at most 600 pm and more preferably at most 500 pm. The proportion of particles with a particle size of at least 150 pm is general ly at least 90% by weight, more preferably at least 95% by weight and most preferably at least 98% by weight. The proportion of particles with a particle size of at most 850 pm is general ly at least 90% by weight, more preferably at least 95% by weight and most preferably at least 98% by weight.

I n some other known production processes for superabsorbents, especial ly in the case of suspension polymerization, spray or d ropletization polymerization, the selection of the pro cess parameters defines the particle size distribution. These processes directly give rise to particulate su perabsorbents of the desired particle size, such that grinding and sieving steps can often be dispensed with. I n some processes (especial ly in the case of spray or d ropletization polymerization), a dedicated d rying step can often also be dispensed with.

The polymer thus prepared has su perabsorbent properties and is covered by the term "su perabsorbent". Its CRC is typical ly comparatively high, but its AU F or SFC com paratively low. A su rface nonpostcrosslinked su perabsorbent of this type is often referred to as "base polymer" to distinguish it from a su rface postcrosslinked su perabsorbent produced there from.

If no su rface postcrosslin king takes place or the x-ray-amorphous aluminu m hydroxide is added prior to the su rface postcrosslinking, it is added to this base polymer as described above. The base polymer is optional ly su rface postcrosslin ked. Surface postcrosslin kers for su per absorbents and processes for surface postcrosslin king of su perabsorbents are wel l-known. Suitable postcrosslin kers are compounds which comprise grou ps which can form bonds with at least two fu nctional groups of the su perabsorbent particles. I n the case of the acrylic acid/sodiu m acrylate-based su perabsorbents prevalent on the market, suitable sur face postcrosslin kers are com pounds which comprise grou ps which can form bonds with at least two carboxylate groups. Rather than "surface postcrosslin ker" or "surface postcross lin king", merely "postcrosslinker" or "postcrosslinking" are often also used.

Preferred su rface postcrosslin kers are di- or triglycidyl compounds, for exam ple glycidyl ethers, for instance ethylene glycol diglycidyl ether and glycerol di- or triglycidyl ether.

Preferred su rface postcrosslinkers are also 2-oxazolidones such as 2-oxazolidone and N-(2- hyd roxyethyl)-2-oxazolidone, N-methyl-2-oxazolidone, N-acyl-2-oxazolidones such as N- acetyl-2-oxazolidone, 2-oxotetrahydro-l,3-oxazine, bicyclic amide acetals such as 5-methyl- l-aza-4,6-dioxabicyclo[3.3.0]octane, l-aza-4,6-dioxabicyclo[3.3.0]octane and 5-isopropyl-l- aza-4,6-dioxabicyclo[3.3.0]octane, bis-2-oxazolidones and poly-2-oxazolidones. Among these, particu lar preference is given to 2-oxazolidone, N-methyl-2-oxazolidone, N-(2-hy- d roxyethyl) -2-oxazolidone and N-hyd roxypropyl-2-oxazolidone.

Fu rther preferred postcrosslinkers are propane-1, 3-diol, pentane-1, 5-diol, hexane-1, 6-diol and heptane-1, 7-diol, butane-1, 3-diol, octane-1, 8-diol, nonane-1, 9-diol and decane-1, 10- diol. Among these, particular preference is given to those that are water-solu ble at 23° C to an extent of at least 30% by weight, preferably to an extent of at least 40% by weight, more preferably to an extent of at least 50% by weight, most preferably at least to an extent of 60% by weight, for exam ple propane-1, 3-diol and heptane-1, 7-diol. Even more preferred are those that are liquid at 25° C.

Fu rther preferred postcrosslin kers are butane-1, 2, 3-triol, butane-1, 2, 4-triol, glycerol, trime- thylol propane, trimethylolethane, pentaeryth ritol, 1- to 3-tu ply (per molecule) ethoxylated glycerol, trimethylolethane or trimethylol propane and 1- to 3-tu ply (per molecu le) propox- ylated glycerol, trimethylolethane or trimethylol propane. Additional ly preferred are 2-tu ply ethoxylated or propoxylated neopentyl glycol. Particu lar preference is given to 2-tu ply and 3-tu ply ethoxylated glycerol, neopentyl glycol, 2-methyl propane-l, 3-diol and trimethylol pro pane. Among these, particu lar preference is given to those that have a viscosity at 23° C of less than 3000 mPas, preferably less than 1500 mPas, more preferably less than 1000 m Pas, especial ly preferably less than 500 m Pas and very especial ly preferably less than 300 m Pas.

Fu rther preferred postcrosslinkers are ethylene carbonate and propylene carbonate.

A further preferred postcrosslinker is 2,2’-bis(2-oxazoline) . Likewise preferred postcrosslin kers are oxetanes, especial ly 3-ethyloxetane-3-methanol or 3,3‘-[oxybis(methylene)] bis [(3-ethyl) oxetane] .

These preferred postcrosslin kers minimize side reactions and su bsequent reactions which lead to volatile and hence malodorous com pou nds. The su perabsorbents produced with the preferred postcrosslin kers are therefore odor-neutral even in the moistened state.

It is possible to use an individual postcrosslinker or any desired mixtu res of different post crosslin kers.

The postcrosslinker is general ly used in an amount of at least 0.001% by weight, preferably of at least 0.02% by weight, more preferably of at least 0.05% by weight, and general ly at most 2% by weight, preferably at most 1% by weight, more preferably at most 0.3% by weight, for example at most 0.15% by weight or at most 0.095% by weight, based in each case on the mass of the base polymer contacted therewith (for example the sieve fraction in question) .

The postcrosslinking is typical ly performed in such a way that a solution of the postcross lin ker is sprayed onto the d ried base polymer particles. After the spray application, the poly mer particles coated with postcrosslinker are dried thermal ly, and the postcrosslinking reac tion can take place either before or during the d rying. If su rface postcrosslin kers with polymerizable grou ps are used, the surface postcrosslin king can also be effected by means of free- rad ica I ly induced polymerization of such grou ps by means of com mon free-radical formers or else by means of high-energy radiation, for example UV light. This can be done in paral lel with or instead of the use of postcrosslinkers which form covalent or ionic bonds to functional groups at the su rface of the base polymer particles.

The spray application of the postcrosslin ker solution is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers, padd le mixers or shovel mixers, or mixers with other mixing tools. Particu lar preference is given, however, to vertical mixers. It is also possible to spray on the postcrosslin ker solution in a fluidized bed. Suitable mixers are obtainable, for example, as Pflugschar ⁰ (plowshare) mixers from Gebr. Lodige Maschi- nen bau Gm bH , Elsener-Strasse 7 - 9, 33102 Paderborn, Germany, or as Schugi ⁰ Flexomix ⁰ mixers, Vrieco-Nauta ⁰ mixers or Tu rbulizer ⁰ mixers from Hosokawa Micron BV, Gildenstraat 26, 7000 AB Doetinchem, the Netherlands.

The spray nozzles usable are not su bject to any restriction. Suitable nozzles and atomization systems are described, for exam ple, in the fol lowing references: Zerstau ben von Flu- ssigkeiten [Atomization of Liquids] , Expert-Verlag, vol. 660, Reihe Kontakt & Studiu m, Thomas Richter (2004) and in Zerstaubungstech nik [Atomization Technology] , Springer- Verlag, VDI-Reihe, GOnter Wozniak (2002) . It is possible to use mono- and polydisperse spray systems. Among the polydisperse systems, one-phase pressurized nozzles (jet- or la mel la-forming), rotary atomizers, two-phase atomizers, u ltrasou nd atomizers and im pinge ment nozzles are suitable. I n the case of the two-phase atomizers, the liquid phase can be mixed with the gas phase either internal ly or external ly. The spray profile of the nozzles is u ncritical and may assume any desired form, for exam ple a rou nd jet, flat jet, wide angle rou nd jet or circu lar ring spray profile. It is advantageous to use a nonoxidizing gas if two- phase atomizers are used, particu lar preference being given to nitrogen, argon or carbon di oxide. The liquid to be sprayed can be supplied to such nozzles u nder pressu re. The atomi zation of the liquid to be sprayed can be effected by expanding it in the nozzle bore on at tainment of a particu lar minimu m velocity. I n addition, it is also possible to use one-phase nozzles for the inventive pu rpose, for exam ple slit nozzles or swirl cham bers (fu l l-cone noz zles) (for exam ple from Ddsen-Schlick G mbH, Germany, or from Spraying Systems Germany Gm bH, Germany). Such nozzles are also described in EP 0 534 228 A1 and EP 1 191 051 A2.

The postcrosslinkers are typical ly used in the form of an aqueous solution. If exclusively wa ter is used as the solvent, a surfactant or deagglomeration assistant is advantageously added to the postcrosslinker solution or actual ly to the base polymer. This improves the wetting characteristics and reduces the tendency to form lu mps.

Al l anionic, cationic, nonionic and am photeric su rfactants are suitable as deagglomeration assistants, but preference is given to nonionic and am photeric su rfactants for skin com pati bility reasons. The su rfactant may also com prise nitrogen. For exam ple, sorbitan monoes ters, such as sorbitan monococoate and sorbitan monolau rate, or ethoxylated variants thereof, for example Polysorbat 20 ^® , are added. Fu rther suitable deagglomeration assistants are the ethoxylated and al koxylated derivatives of 2-propyl heptanol, which are sold u nder the Lutensol XL ⁰ and Lutensol XP ⁰ brands (BASF SE, Carl-Bosch-Strasse 38, 67056 Fud- wigshafen, Germany) .

The deagglomeration assistant can be metered in separately or added to the postcross lin ker solution. Preference is given to simply adding the deagglomeration assistant to the postcrosslin ker solution.

The amou nt of the deagglomeration assistant used, based on base polymer, is, for exam ple, 0% to 0.1% by weight, preferably 0% to 0.01% by weight, more preferably 0% to 0.002% by weight. The deagglomeration assistant is preferably metered in such that the su rface ten sion of an aqueous extract of the swol len base polymer and/or of the swol len postcross- lin ked water-absorbing polymer at 23° C is at least 0.060 N/m, preferably at least 0.062 N/m, more preferably at least 0.065 N/m, and advantageously at most 0.072 N/m.

The aqueous postcrosslinker solution may, as wel l as the at least one postcrosslin ker, also com prise a cosolvent. The content of nonaqueous solvent or total amou nt of solvent can be used to adjust the penetration depth of the postcrosslin ker into the polymer particles. I n dustrial ly readily available cosolvents are C1-C6 alcohols such as methanol, ethanol, n-pro- panol, isopropanol, n-butanol, sec-butanol, tert-butanol or 2-methyl-l-propanol, C ₂ -C ₅ diols such as ethylene glycol, 1,2-propylene glycol or butane-1, 4-diol, ketones such as acetone, or carboxylic esters such as ethyl acetate. A disadvantage of some of these cosolvents is that they have typical intrinsic odors. The cosolvent itself is ideal ly not a postcrosslin ker under the reaction conditions. However, it may arise in the boundary case and depending on the residence time and temperatu re that the cosolvent contributes partly to crosslinking. This is the case especial ly when the postcrosslin ker is relatively slow to react and can therefore also constitute its own cosol vent, as is the case, for exam ple, when cyclic carbonates, diols or polyols are used. Such postcrosslinkers can also be used in the fu nction as a cosolvent in a mixture with more re active postcrosslin kers, since the actual postcrosslin king reaction can then be performed at lower tem peratu res and/or with shorter residence times than in the absence of the more re active crosslin ker. Since the cosolvent is used in relatively large amou nts and some also re mains in the product, it must not be toxic.

I n the process of the invention, the abovementioned diols and polyols and also the cyclic carbonates are also suitable as cosolvents. They fu lfil l this fu nction in the presence of a com paratively reactive postcrosslin ker and/or of a di- or triglycidyl com pou nd. Preferred cosolvents in the process of the invention are, however, especial ly the diols mentioned, es pecial ly when a reaction of the hydroxyl grou ps is sterical ly hindered by neighboring grou ps. Although such diols are also suitable in principle as postcrosslin kers, this requires signifi cantly higher reaction temperatu res or optional ly higher use amou nts than for sterical ly u n hindered diols.

Particu larly preferred com binations of low-reactivity postcrosslin ker as a cosolvent and re active postcrosslin ker are combinations of the polyhyd ric alcohols, diols and polyols men tioned with the stated amide acetals or carbamates. Suitable combinations are, for exam ple, 2-oxazolidone/propane-l,2-diol and N-(2-hyd roxyethyl)-2-oxazolidone/propane-l,2-diol, and also ethylene glycol diglycidyl ether/propane-1, 2-diol. Very particularly preferred com binations are 2-oxazolidone/propane-l,3-diol and N-(2-hyd roxyethyl) -2-oxazolidone/pro- pane-l,3-diol. Fu rther preferred com binations are those with ethylene glycol diglycidyl ether or glyceryl di- or triglycidyl ether with the fol lowing solvents, cosolvents or cocrosslin kers: isopropanol, propane-1, 3-diol, 1,2-propylene glycol or mixtures thereof. Fu rther preferred com binations are those with 2-oxazolidone or (2-hyd roxyethyl) -2-oxazolidone in the fol low ing solvents, cosolvents or cocrosslin kers: isopropanol, propane-1, 3-diol , 1,2-propylene gly col, ethylene carbonate, propylene carbonate or mixtures thereof.

Frequently, the concentration of the cosolvent in the aqueous postcrosslin ker solution is from 15 to 50% by weight, preferably from 15 to 40% by weight and more preferably from 20 to 35% by weight, based on the postcrosslin ker solution. I n the case of cosolvents which have on ly limited miscibility with water, the aqueous postcrosslinker solution wil l advanta geously be adjusted such that on ly one phase is present, optional ly by lowering the concen tration of the cosolvent.

I n a preferred em bodiment, no cosolvent is used. The postcrosslinker is then em ployed only as a solution in water, optional ly with addition of a deagglomeration assistant. The concentration of the at least one postcrosslin ker in the aqueous postcrosslin ker solu tion is typical ly from 1 to 20% by weight, preferably from 1.5 to 10% by weight and more preferably from 2 to 5% by weight, based on the postcrosslinker solution.

The total amount of the postcrosslin ker solution based on base polymer is typical ly from 0.3 to 15% by weight and preferably from 2 to 6% by weight.

The actual su rface postcrosslin king by reaction of the su rface postcrosslinker with fu nc tional groups at the surface of the base polymer particles is usual ly carried out by heating the base polymer wetted with surface postcrosslin ker solution, typical ly referred to as "d ry ing" (but not to be confused with the above-described drying of the polymer gel from the polymerization, in which typical ly very much more liquid has to be removed). The d rying can be effected in the mixer itself, by heating the jacket, by means of heat exchange su rfaces or by blowing in warm gases. Simu ltaneous admixing of the su perabsorbent with su rface post crosslinker and drying can be effected, for example, in a fluidized bed drier. The d rying is, however, usual ly carried out in a downstream drier, for exam ple a tray d rier, a rotary tu be oven, a paddle or disk drier or a heatable screw. Suitable d riers are obtainable, for example, as Solidair ⁰ or Torusdisc ⁰ d riers from Bepex I nternational LLC, 333 N .E. Taft Street, Min ne apolis, M N 55413, U.S.A., or as padd le or shovel driers or else as fluidized bed d riers from Nara Machinery Co., Ltd., European office, Eu ropaal lee 46, 50226 Frechen, Germany.

It is possible to heat the polymer particles by means of contact su rfaces in a downstream d rier for the pu rpose of d rying and performing the su rface postcrosslinking, or by means of warm inert gas su pply, or by means of a mixture of one or more inert gases with steam, or only with steam alone. I n the case of su pply of the heat by means of contact surfaces, it is possible to perform the reaction under inert gas at slightly or completely reduced pressure. I n the case of use of steam for direct heating of the polymer particles, it is desirable in ac cordance with the invention to operate the drier u nder standard pressu re or elevated pres su re. I n this case, it may be advisable to split u p the postcrosslin king step into a heating step with steam and a reaction step under inert gas but without steam. This can be achieved in one or more apparatuses. According to the invention, the polymer particles can be heated with steam as early as in the postcrosslin king mixer. The base polymer used may stil l have a temperatu re of from 10 to 120° C from preceding process steps; the postcross lin ker solution may have a tem peratu re of from 0 to 70° C. I n particu lar, the postcrosslin ker solution can be heated to reduce the viscosity.

Preferred d rying temperatures are in the range of 100 to 250° C, preferably 120 to 220° C, more preferably 130 to 210° C and most preferably 150 to 200° C. The preferred residence time at this temperatu re 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 60 minutes. Typical ly, the drying is conducted such that the su perabsorbent has a re sidual moistu re content of general ly at least 0.1% by weight, preferably at least 0.2% by weight and most preferably at least 0.5% by weight, and general ly at most 15% by weight, preferably at most 10% by weight and more preferably at most 8% by weight. The postcrosslinking can take place u nder standard atmospheric conditions. "Standard at mospheric conditions" means that no technical precautions are taken in order to reduce the partial pressu re of oxidizing gases, such as that of the atmospheric oxygen, in the apparatus in which the postcrosslin king reaction predominantly takes place (the "postcrosslin king re actor", typical ly the drier) . However, preference is given to performing the postcrosslin king reaction u nder reduced partial pressu re of oxidizing gases. Oxidizing gases are su bstances which, at 23° C, have a vapor pressure of at least 1013 mbar and act as oxidizing agents in com bustion processes, for example oxygen, nitrogen oxide and nitrogen dioxide, especial ly oxygen. The partial pressu re of oxidizing gases is preferably less than 140 m bar, preferably less than 100 mbar, more preferably less than 50 mbar and most preferably less than 10 m bar. When the thermal postcrosslin king is carried out at ambient pressu re, i.e. at a total pressure around 1013 mbar, the total partial pressu re of the oxidizing gases is determined by their proportion by volu me. The proportion of the oxidizing gases is preferably less than 14% by volu me, preferably less than 10% by volu me, more preferably less than 5% by vol u me and most preferably less than 1% by volu me.

The postcrosslinking can be carried out under reduced pressure, i.e. at a total pressure of less than 1013 mbar. The total pressu re is typical ly less than 670 mbar, preferably less than 480 mbar, more preferably less than 300 m bar and most preferably less than 200 mbar. When drying and postcrosslinking are carried out under air with an oxygen content of 20.8% by volu me, the partial oxygen pressu res corresponding to the abovementioned total pres su res are 139 m bar (670 m bar), 100 mbar (480 mbar) , 62 mbar (300 mbar) and 42 mbar (200 m bar), the respective total pressu res being in the brackets. Another means of lowering the partial pressure of oxidizing gases is the introduction of nonoxidizing gases, especial ly inert gases, into the apparatus used for postcrosslin king. Suitable inert gases are su b stances that are in gaseous form in the postcrosslin king drier at the postcrosslin king tem perature and the given pressu re and do not have an oxidizing action on the constituents of the d rying polymer particles under these conditions, for exam ple nitrogen, carbon dioxide, argon, steam, preference being given to nitrogen. The amou nt of inert gas is general ly from 0.0001 to 10 m ³ , preferably from 0.001 to 5 m ³ , more preferably from 0.005 to 1 m ³ and most preferably from 0.005 to 0.1 m ³ , based on 1 kg of superabsorbent.

I n the process of the invention, the inert gas, if it does not comprise steam, can be blown into the postcrosslin king d rier via nozzles; however, particu lar preference is given to adding the inert gas to the polymer particle stream via nozzles actual ly within or just u pstream of the mixer, by ad mixing the su perabsorbent with su rface postcrosslin ker.

It wil l be appreciated that vapors of cosolvents removed from the d rier can be condensed again outside the d rier and optional ly recycled.

One way of adding the x-ray-amorphous alu minum hydroxide during the su rface postcross lin king is mixing-in du ring the surface postcrosslinking reaction by addition to an apparatus used for the su rface postcrosslinking reaction if the contents are mixed in this apparatus. Suitable apparatuses are, for instance, the apparatuses mentioned above for the purpose except for the tray drier. Preference is given to using paddle and shovel driers for the pur pose.

Before, during or after the postcrosslinking, in addition to the organic postcrosslinkers men tioned that form covalent bonds with carboxyl groups in the superabsorbent and in addition to the x-ray-amorphous aluminum hydroxide, polyvalent metal ions are optionally applied to the surfaces of the superabsorbent of the invention, or, if no surface postcrosslinking with one of the organic postcrosslinkers mentioned is conducted, in lieu thereof. As already stated above, this application of polyvalent metal ions is in principle an (optionally addi tional) surface postcrosslinking by ionic, noncovalent bonds and is referred to in the context of this invention, for distinction from surface postcrosslinking by means of covalent bonds, as "complexation" with the metal ions in question.

This application of polyvalent cations is typically effected by spray application of solutions of di- or polyvalent cations, usually d i - , tri- or tetravalent metal cations, but also polyvalent cations such as polymers formed, in a formal sense, entirely or partly from vinylamine mon omers, such as partly or fully hydrolyzed polyvinylamide (so-called "polyvinylamine"), whose amine groups are always - even at very high pH values - present partly in protonated form to give ammonium groups. Examples of usable divalent metal cations are especially the di valent cations of metals of groups 2 (especially Mg, Ca, Sr, Ba), 7 (especially Mn), 8 (espe cially Fe), 9 (especially Co), 10 (especially Ni), 11 (especially Cu) and 12 (especially Zn) of the Periodic Table of the Elements. Examples of usable trivalent metal cations are espe cially the trivalent cations of metals of groups 3 including the lanthanides (especially Sc, Y, La, Ce), 8 (especially Fe), 11 (especially Au), 13 (especially Al) and 14 (especially Bi) of the Periodic Table of the Elements. Examples of usable tetravalent cations are especially the tetravalent cations of metals from the lanthanides (especially Ce) and group 4 (especially Ti, Zr, Hf) of the Periodic Table of the Elements. The metal cations can be used either alone or as a mixture with one another. Particular preference is given to the use of trivalent metal cations. Very particular preference is given to the use of aluminum cations.

Among the metal cations mentioned, suitable metal salts are all of those which possess sufficient solubility in the solvent to be used. Particularly suitable metal salts are those with weakly complexing anions, for example chloride, nitrate and sulfate, hydrogensulfate, car bonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate, or dihydrogenphos- phate. Preference is given to salts of mono- and dicarboxylic acids, hydroxy acids, keto ac ids and amino acids, or basic salts. Preferred examples include acetates, propionates, tar trates, maleates, citrates, lactates, malates, succinates. Likewise preferred is the use of hy droxides, provided that they are soluble. Particular preference is given to the use of 2-hy- droxycarboxylic salts such as citrates and lactates. Examples of particularly preferred metal salts are alkali metal and alkaline earth metal aluminates and hydrates thereof, for instance sodium aluminate and hydrates thereof, alkali metal and alkaline earth metal lactates and citrates and hydrates thereof, aluminum acetate, aluminum propionate, aluminum citrate and aluminum lactate. The cations and salts mentioned can be used in pu re form or as a mixtu re of different cati ons or salts. The salts of the di- and/or trivalent metal cation used may com prise further secondary constituents such as stil l un neutralized carboxylic acid and/or al kali metal salts of the neutralized carboxylic acid. Preferred al kali metal salts are those of sodium and po tassiu m, and those of ammoniu m. They are typical ly used in the form of an aqueous solu tion which is obtained by dissolving the solid salts in water, or is preferably obtained directly as such, which avoids any d rying and pu rification steps. It is advantageously also possible to use the hyd rates of the salts mentioned, which often dissolve more rapidly in water than the an hydrous salts.

The amou nt of metal salt used is general ly at least 0.001% by weight, preferably at least 0.01% by weight and more preferably at least 0.1% by weight, for example at least 0.4% by weight, and general ly at most 5% by weight, preferably at most 2.5% by weight and more preferably at most 1% by weight, for exam ple at most 0.7% by weight, based in each case on the mass of the base polymer.

The salt of the trivalent metal cation can be used in the form of a solution or suspension. Solvents used for the metal salts may be water, alcohols, DM F, DMSO and mixtu res of these com ponents. Particular preference is given to water and water/alcohol mixtures, for example water/methanol, water/propane-1, 2-diol and water/propane-1, 3-diol.

The treatment of the base polymer with solution of a di- or polyvalent cation is effected in the same manner as that with su rface postcrosslinker, including the d rying step. Su rface postcrosslinker and polyvalent cation can be sprayed on in a combined solution or as sepa rate solutions. The spray application of the metal salt solution to the su perabsorbent parti cles may either precede or fol low the su rface postcrosslinking. I n a particularly preferred process, the spray application of the metal salt solution is effected in the same step to gether with the spray application of the crosslinker solution, in which case the two solutions are sprayed on separately in succession or simultaneously via two nozzles, or crosslinker solution and metal salt solution can be sprayed on jointly via one nozzle.

It is also possible to add al l fu rther additives known in the surface postcrosslin king of su perabsorbents. Examples are basic salts of a divalent metal cation such as calcium or stron tiu m, usual ly in the form of hyd roxides, hyd rogencarbonates, carbonates, acetates, propio nates, citrates, gluconates, lactates, tartrates, malates, succinates, maleates and/or fumarates. Further exam ples are reducing compou nds such as hypophosphites, phosphonic acid derivatives, su lfinates or sulfites.

More preferably, aside from the addition of x-ray-amorphous alu minu m hyd roxide, no fur ther polyvalent metal ions are added.

If a drying step is carried out after the surface postcrosslin king and/or treatment with com- plexing agent, it is advantageous but not absolutely necessary to cool the product after the d rying. The cooling can be effected continuously or batchwise; to this end, the product is conveniently conveyed continuously into a cooler arranged downstream of the drier. Any ap paratus known for removal of heat from pulveru lent solids can be used for this pu rpose, more particu larly any device mentioned above as drying apparatus, except that it is charged not with a heating medium but with a cooling mediu m, for example with cooling water, such that no heat is introduced into the superabsorbent via the wal ls and, according to the con struction, also via the stirring elements or other heat exchange surfaces, and is instead re moved therefrom. Preference is given to the use of coolers in which the product is moved, i.e. cooled mixers, for example shovel coolers, disk coolers or padd le coolers. The su per absorbent can also be cooled in a fluidized bed by injecting a cooled gas such as cold air. The cooling conditions are adjusted so as to obtain a su perabsorbent with the temperatu re desired for fu rther processing. Typical ly, a mean residence time in the cooler of general ly at least 1 minute, preferably at least 3 minutes and more preferably at least 5 minutes, and general ly at most 6 hou rs, preferably at most 2 hou rs and more preferably at most 1 hou r is established, and the cooling performance is such that the product obtained has a tempera tu re of general ly at least 0° C, preferably at least 10° C and more preferably at least 20° C, and general ly at most 100° C, preferably at most 80° C and more preferably at most 60° C.

The surface postcrosslin ked superabsorbent is optional ly grou nd and/or sieved in a cus tomary man ner. Grinding is typical ly not required here, but the removal by sieving of ag glomerates or fines formed is usual ly appropriate for establish ment of the desired particle size distribution of the product. Agglomerates and fines are either discarded or preferably recycled into the process in a known man ner at a suitable point, agglomerates after com mi nution. The particle sizes desired for su rface postcrosslin ked superabsorbents are the same as for base polymers.

If x-ray-amorphous alu minum hyd roxide is added after a su rface postcrosslinking, this can conveniently be done in the downstream cooler, but also in a dedicated mixing apparatus. The x-ray-amorphous alu minum hyd roxide can be added in each case either before or after a sieving and grinding step that fol lows a su rface postcrosslinking operation.

Optional ly, the su perabsorbents of the invention that have been produced by the process of the invention are provided with fu rther additions, non limiting examples being those that provide stabilization against discoloration, reduce the tendency to caking or fu rther increase the permeability. For this pu rpose, al l known additives can be used in the man ner known for each in the process of the invention. Examples of known additions that provide stabilization against discoloration are the abovementioned sulfonic acid or phosphonic acid derivatives, which can also be applied after the production of the su perabsorbent of the invention rather than or as wel l as the addition during the production thereof. Examples of known additions that reduce the caking tendency of the superabsorbent or fu rther increase the permeability are water-insoluble inorganic powders.

Usual ly, the x-ray-amorphous alu minum hydroxide added in accordance with the invention wil l also sufficiently reduce the caking tendency of the superabsorbent. If the amou nt added in accordance with the invention is insufficient for this purpose, it can also be increased above the abovementioned limits to the extent that the desired reduction in the caking ten dency is achieved. This additional amou nt of x-ray-amorphous alu minum hyd roxide can also be added in a second portion, like other water-solu ble inorganic powders as wel l. Typical ly, the additional amou nt of x-ray-amorphous alu minu m hyd roxide is at least as large as the other water-insoluble inorganic powders.

If a further inorganic powder is added, more preferably, precipitated silicon dioxide or silicon dioxide produced by pyrolysis and alu minum oxide produced by pyrolysis is used. Pyrogenic silicon dioxide is available, for exam ple, u nder the AEROSI L ⁰ brand, and pyrogenic alu minu m oxide, for exam ple, under the AEROXI DE ⁰ Alu brand from Evonik I ndustries AG, I norganic Materials, Roden bacher Chaussee 4, 63457 Hanau-Wolfgang, Germany. Silicon dioxide pro duced by precipitation is available, for exam ple, under the SI PERNAT ⁰ brand from Evonik I n dustries AG, I norganic Materials, Roden bacher Chaussee 4, 63457 Hanau-Wolfgang, Ger many. The water-insoluble inorganic powders can also be hydrophobized by suitable sur face treatment and are often supplied by manufacturers both in hydrophobized and in hy d rophilic form. I n the context of this invention, the use of hyd rophilic water-insolu ble inor ganic powders is preferred.

I n general, the water-insoluble inorganic powder is added to the superabsorbent in an amou nt of at least 0.005% by weight, preferably of at least 0.03% by weight and more pref erably of at least 0.05% by weight, and general ly of at most 6.0% by weight, preferably at most 1.0% by weight and more preferably at most 0.5% by weight, based in each case on the total weight of the an hyd rous su perabsorbent comprising inorganic powder.

Superabsorbents can be mixed with the optional additives by any known mixing process. When in solid form, they are incorporated by mixing in su bstance or as a suspension in a solvent or suspension medium, and, when in dissolved or liquid form, optional ly also in solu tion or liquid form. Due to easier homogeneous distribution, the additives are preferably in corporated into the su perabsorbent by mixing as a powder or suspension. This does not necessarily produce a physical mixtu re separable in a simple manner by mechanical measu res. The additives may quite possibly enter into a more definite bond with the su per absorbent, for example in the form of a comparatively firmly adhering su rface layer or in the form of particles adhering firm ly to the su rface of the su perabsorbent particles. The mixing of the additives into the known su perabsorbent can quite possibly also be understood and referred to as“coating”.

If a solution or suspension is used for coating, the solvent or suspension mediu m used is a solvent or suspension medium which is chemical ly com patible both with the su perabsorbent and with the additive, i.e. does not enter into any undesired chemical reactions therewith. Typical ly, water or an organic solvent is used, for example an alcohol or polyol, or mixtures thereof. Examples of suitable solvents or suspension media are water, isopropanol/water, propane-1, 3-diol/water and propylene glycol/water, where the mixing ratio by mass is pref erably from 20:80 to 40:60. If a suspension mediu m is used for the stabilizers to be used in accordance with the invention or the inorganic particu late solid, water is preferred. A su r factant can be added to the solution or suspension.

Optional additives are, if they are not added to the monomer mixtu re or the polymerizing gel, general ly mixed with the su perabsorbent in exactly the same way as the solution or suspen sion which comprises a surface postcrosslin ker and is applied to the su perabsorbent for su rface postcrosslinking. The additive can be applied as a constituent of the solution ap plied for su rface postcrosslin king or of one of the com ponents thereof to an (as yet) non postcrosslinked su perabsorbent (a“base polymer”) , i.e. the additive is added to the so lution of the surface postcrosslin ker or to one of the com ponents thereof. The superabsor bent coated with su rface postcrosslinker and additives then passes through the fu rther pro cess steps required for surface postcrosslin king, for example a thermal ly induced reaction of the su rface postcrosslin ker with the superabsorbent. This process is comparatively sim ple and economical ly viable.

If the su perabsorbent is su bjected to a cooling step after the surface postcrosslin king or the com plexation, the optional additions can conveniently be mixed in in the cooler. If additives are applied as a solution or suspension, they can also be applied to the al ready su rface postcrosslinked su perabsorbent in the same mixing apparatuses as described for the appli cation of the su rface postcrosslin ker to the base polymer. Usual ly, but not necessarily, this is fol lowed by heating, just like in the su rface postcrosslinking step, in order to dry the su perabsorbent again. The tem perature established in this drying operation is then, however, general ly at most 110° C, preferably at most 100° C and more preferably at most 90° C, in order to prevent undesired reactions of the additive. The tem peratu re is adjusted such that, in view of the residence time in the d rying unit, the desired water content of the su per absorbent is achieved. It is also entirely possible and convenient to add additives individu al ly or together with other customary assistants, for example dust binders, anticaking agents or water for remoisturization of the su perabsorbent. The tem peratu re of the polymer particles in this case is between 0° C and 190° C, preferably less than 160° C, more pref erably less than 130° C, even more preferably less than 100° C and most preferably less than 70° C. The polymer particles are optional ly cooled rapid ly after coating to tem pera tu res below any decomposition tem peratu re of the additive.

It is optional ly possible to additional ly apply to the su rface of the superabsorbent particles, whether u npostcrosslinked or postcrosslinked, in any process step of the preparation pro cess, if required, al l known coatings, such as fil m-forming polymers, thermoplastic poly mers, dendrimers, polycationic polymers (for example polyvinylamine, polyethyleneimine or polyallylamine) , or al l water-solu ble mono- or polyvalent metal salts known to those skil led in the art, for example alu minu m sulfate, sodium salts, potassiu m salts, zirconium salts or iron salts. Examples of useful al kali metal salts are sodiu m and potassium su lfate, and so dium and potassiu m lactates, citrates and sorbates. This al lows additional effects, for ex ample a reduced caking tendency of the end product or of the intermediate in the particu lar process step of the production process, improved processing properties or a fu rther en hanced permeability (SFC) , to be achieved. When additives are used and sprayed on in the form of dispersions, they are preferably used in the form of aqueous dispersions, and pref erence is given to additional ly applying an antidusting agent to fix the additive on the sur face of the su perabsorbent. The antidusting agent is then either added directly to the dis persion of the inorganic pulveru lent additive; optional ly, it can also be added as a separate solution before, during or after the application of the inorganic pulveru lent additive by spray application. Most preferred is the simu ltaneous spray application of postcrosslin king agent, antidusting agent and pulveru lent inorganic additive in the postcrosslin king step. I n a fur ther preferred process variant, the antidusting agent is, however, added separately in the cooler, for exam ple by spray application from above, below or from the side. Particu larly suitable antidusting agents which can also serve to fix pulveru lent inorganic additives on the surface of the water-absorbing polymer particles are polyethylene glycols with a molec u lar weight of 400 to 20 000 g/mol, polyglycerol, 3- to 100-tu ply ethoxylated polyols, such as trimethylol propane, glycerol, sorbitol and neopentyl glycol. Particularly suitable are 7- to 20- tu ply ethoxylated glycerol or trimethylol propane, for example Polyol TP 7O ⁰ (Perstorp, Swe den) . The latter have the advantage, more particu larly, that they lower the su rface tension of an aqueous extract of the water-absorbing polymer particles only insignificantly.

It is likewise possible to adjust the su perabsorbent of the invention to a desired water con tent by adding water. It may also be advantageous to slightly swel l the su perabsorbent by addition of water and then adjust it back to the desired water content by drying.

Al l coatings, solids, additives and assistants can each be added in separate process steps, but the most convenient method is usual ly to add them - if they are not added du ring the admixing of the base polymer with surface postcrosslin king agent - to the su perabsorbent in the cooler, for instance by spray application of a solution or addition in fine solid form or in liquid form.

The su perabsorbents of the invention general ly have a centrifuge retention capacity (CRC, for test method see below) of at least 5 g/g, preferably of at least 10 g/g and more prefera bly of at least 20 g/g. Typical ly, it is not more than 40 g/g for surface postcrosslin ked su per absorbents, but it is often higher for base polymers.

The su perabsorbents of the invention, if they have been su rface postcrosslin ked, typical ly have an absorption u nder load (AU L0.9psi, for test method see below) of at least 10 g/g, preferably at least 14 g/g, more preferably at least 18 g/g and most preferably at least 22 g/g, and typical ly not more than 30 g/g.

The present invention fu rther provides hygiene articles com prising su perabsorbent of the invention, preferably u ltrathin diapers, com prising an absorbent layer consisting of 50 to 100% by weight, preferably 60 to 100% by weight, more preferably 70 to 100% by weight, es pecial ly preferably 80 to 100% by weight and very especial ly preferably 90 to 100% by weight of su perabsorbent of the invention, of cou rse not including the envelope of the ab sorbent layer. Very particu larly advantageously, the superabsorbents of the invention are also suitable for production of laminates and composite structu res, as described, for exam ple, in US

2003/0181115 and US 2004/0019342. I n addition to the hotmelt ad hesives described in both docu ments for production of such novel absorbent structu res, and especial ly the fi bers, described in US 2003/0181115, composed of hotmelt adhesives to which the su per absorbent particles are bou nd, the superabsorbents of the invention are also suitable for production of entirely analogous structu res using UV-crosslinkable hotmelt adhesives, which are sold, for exam ple, as AC-Resin ^® (BASF SE, Ludwigshafen, Germany). These UV- crosslinkable hotmelt adhesives have the advantage of al ready being processible at 120 to 140° C; they therefore have better com patibility with many thermoplastic su bstrates. A fu r ther significant advantage is that UV-crosslinkable hotmelt ad hesives are very benign in toxicological terms and also do not cause any vaporization in the hygiene articles. A very significant advantage in con nection with the su perabsorbents of the invention is the prop erty of the UV-crosslin kable hotmelt adhesives of lacking any tendency to yel low du ring processing and crosslin king. This is especial ly advantageous when u ltrathin or partly trans parent hygiene articles are to be produced. The combination of the su perabsorbents of the invention with UV-crosslinkable hotmelt adhesives is therefore particularly advantageous. Suitable UV-crosslinkable hotmelt adhesives are described, for example, in EP 0 377 199 A2, EP 0 445 641 Al, US 5,026,806, EP 0 655 465 A1 and EP 0 377 191 A2.

The superabsorbent of the invention can also be used in other fields of industry in which fluids, especial ly water or aqueous solutions, are absorbed. These fields are, for exam ple, storage, packaging, transport (as constituents of packaging material for water- or moisture- sensitive articles, for instance for flower transport, and also as protection against mechani cal effects) ; animal hygiene (in cat litter); food packaging (transport of fish, fresh meat; ab sorption of water, blood in fresh fish or meat packaging); medicine (wou nd plasters, water absorbing material for burn dressings or for other weeping wounds), cosmetics (carrier ma terial for pharmaceutical chemicals and medicaments, rheumatic plasters, u ltrasonic gel, cooling gel, cosmetic thickeners, su nscreen) ; thickeners for oil/water or water/oil emu l sions; textiles (moistu re regu lation in textiles, shoe insoles, for evaporative cooling, for in stance in protective clothing, gloves, head bands) ; chemical engineering applications (as a catalyst for organic reactions, for immobilization of large functional molecu les such as en zymes, as an adhesive in agglomerations, heat stores, filtration aids, hydrophilic com po nents in polymer laminates, dispersants, liquefiers); as assistants in powder injection mold ing, in the building and construction industry (instal lation, in loam-based renders, as a vi bration-in hibiting mediu m, assistants in tu nnel excavations in water-rich ground, cable sheathing); water treatment, waste treatment, water removal (deicers, reusable sand bags) ; cleaning; agrochemical industry (irrigation, retention of melt water and dew deposits, com posting additive, protection of forests from fu ngal/insect infestation, retarded release of ac tive ingredients to plants) ; for firefighting or for fire protection; coextrusion agents in ther moplastic polymers (for exam ple for hyd rophilization of multilayer fil ms); production of fil ms and thermoplastic moldings which can absorb water (e.g. fil ms which store rain and dew for agricu lture; fil ms comprising su perabsorbents for maintaining freshness of fruit and vegeta bles which are packaged in moist fil ms; superabsorbent-polystyrene coextrudates, for example for packaging foods such as meat, fish, pou ltry, fruit and vegetables) ; or as a car rier su bstance in active ingredient formu lations (pharmaceuticals, crop protection) .

The articles of the invention for absorption of fluid differ from known examples in that they com prise the su perabsorbent of the invention.

A process for producing articles for absorption of fluid, especial ly hygiene articles, has also been fou nd, said process comprising using at least one su perabsorbent of the invention in the production of the article in question. I n addition, processes for producing such articles using su perabsorbents are known.

Test methods

The superabsorbent is tested by the test methods described below.

The standard test methods described hereinafter and designated "NWSP" are described in: "Nonwovens Standards Procedures”, 2015 edition, published jointly by EDANA (European Disposables and Nonwovens Association, Avenue Herrmann Debroux 46, 1160 Brussels, Belgium, www.edana.org) and IN DA (Association of the Nonwoven Fabrics Industry, 1100 Crescent Green, Suite 115, Cary, North Carolina 27518, U.S.A., www.inda.org). This publica tion is obtainable both from EDANA and from I NDA.

All measurements described below should, unless stated otherwise, be conducted at an ambient temperature of 23 ± 2° C and a relative air humidity of 50 ± 10%. The superabsor bent particles are mixed thoroughly before the measurement unless stated otherwise.

Centrifuge retention capacity (CRC)

The centrifuge retention capacity of the superabsorbent is determined by the method de scribed in US 2007/0 135 785 Al, paragraphs [0153] to [0157].

Absorbency under a load of 0.9 psi (AUL0.9psi)

The absorbency under a load of 6205 Pa (0.9 psi) (AUL0.9psi) of the superabsorbent is de termined by the method described in US 2014/0 306 156 Al, paragraphs [0124] to [0143].

Absorbency under a load of 0.3 psi (AUL0.3psi)

The absorbency under a load of 2068 Pa (0.3 psi) (AUL0.3psi) of the superabsorbent is de termined by standard test method No. NWSP 242.0 R2 (15)“Gravimetric Determination of Absorption Against Pressure”, but with a weight (see point 6.5 of the method description) with which a pressure of 2068 Pa rather than 4826 Pa is established (corresponding to 21 g/cm ² rather than 49 g/cm ² ).

Volumetric Absorbency under Load (VAUL)

The volumetric absorbency under load of the superabsorbent is determined by the method described in US 2015/0 299 404 Al, paragraphs [0386] to [0398]. Table 1 reports the r value ascertained at a pressure of 2068 Pa (0.3 psi). Moisture content of the superabsorbent (residual moisture, water content)

The water content of the superabsorbent is determined by standard test method No. NWSP 230.0 R2 (15) "Estimation of the Moisture Content as Weight Loss Upon Heating".

Particle size distribution

The particle size distribution of the superabsorbent is determined by standard test method No. NWSP 220.0 R2 (15) ..Determination of Polyacrylate Superabsorbent Powders and Par ticle Size Distribution - Sieve Fractionation".

Extractables

The extractables in the superabsorbent are determined by standard test method No. NWSP 270.0 R2 (15) ..Determination of Extractable Polymer Content by Potentiometric Titration”.

Permeability (SFC, "Saline Flow Conductivity")

The permeability of a swollen gel layer formed by the superabsorbent as a result of liquid absorption is determined under a pressure of 0.3 psi (2068 Pa), as described in EP 0 640 330 Al, as the gel layer permeability of a swollen gel layer of superabsorbent particles, the apparatus described in the aforementioned patent application on page 19 and in Figure 8 being modified to the effect that the glass frit (40) is not used, and the plunger (39) consists of the same polymer material as the cylinder (37) and now comprises 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 Al. The flow is detected automati cally.

The permeability (SFC) is calculated as follows:

SFC [cmVg] = (Fg(t=0)xL0)/(dxAxWP) where Fg(t=0) is the flow of NaCI solution in g/s, which is obtained using linear regression analysis of the Fg(t) data of the flow determinations by extrapolation to t=0, L0 is the thick ness of the gel layer in cm, d is the density of the NaCI solution in g/cm ³ , A is the area of the gel layer in cm ² , and WP is the hydrostatic pressure over the gel layer in dyn/cm ² .

Permeability (GBP,“Gel Bed Permeability”)

The gel bed permeability is measured by the method in published patent application No. US 2005/0 256 757 Al, paragraphs [0061] to [0075] Examples

The base polymer used in the examples 1-7 was prepared by polymerizing an aqueous mon omer solution that comprised sodium acrylate and acrylic acid (corresponding to a neutrali zation level of the acrylic acid 71 mol%) in a concentration of 41% by weight (sodium acry late plus acrylic acid based on the total amount), and also 0.75% by weight (based on un neutralized acrylic acid) of polyethylene glycol-4000 (polyethylene glycol having an average molar mass of 4000 g/mol) and 0.46% by weight (based on unneutralized acrylic acid) of tri acrylate of triethoxylated glycerol. The initiator system used (based in each case on unneu tralized acrylic acid) was 0.184% by weight of sodium persulfate, 0.0007% by weight of hy drogen peroxide and 0.0026% by weight of ascorbic acid. Polymerization was effected in a kneader. For better drying, the gel obtained was extruded and then dried and ground, and the sieve cut from 150 to 710 pm was obtained therefrom. The base polymer thus prepared had a CRC of 36.5 g/g and an AUL 0.3 psi of 14.6 g/g, and comprised 13.0% by weight of ex- tractables. The particle size distribution obtained by means of sieve analysis was:

> 850 pm < 0.1% by weight

600 - 850 pm 10.6% by weight

300 - 600 pm 70.8% by weight

100 - 300 pm 18.0% by weight

< 100 pm < 0.5% by weight

Base polymers of this kind are standard and also commercially available, for example from BASF SE, Fudwigshafen, Germany.

The mixer used in the examples was a Pflugschar ⁰ 5R-MK plowshare mixer with capacity 5 F, model VT 5R-MK, with a heating jacket from Gebr. Fodige Maschinenbau GmbH;

Elsener Strasse 7-9, 33102 Paderborn, Germany. To measure the temperature of the prod uct in the mixer, a thermocouple was introduced into the opening provided for the purpose in the mixer to such an extent that its tip was at a distance from the heated inner wall of the mixer and was within the product, but could not be impacted by the mixing tools. For addi tional aluminum hydroxide in examples 1-6, an identical mixer but without heating jacket and thermocouple was used.

The x-ray-amorphous aluminum hydroxide used in the examples was aluminum hydroxide dried gel, catalog no. 511066100, batch number 3048632 from Dr. Paul Fohmann GmbH KG, Hauptstrasse 2, 31860 Emmerthal, Germany. By scanning electron microscope, the powder is found to be in the form of spherical particles having diameters in the region of 20-25 pm, but also some smaller spheres in the region of 5-10 pm. By x-ray diffractogram (measured with a D8 Advance Serie 2 diffractometer from Bruker Corporation, 40 Manning Road, Biller ica, MA 01821, U.S.A., with multiple sample changer, Cu anode, divergence slit 0.1° with ASS and Fynx-Eye, 3° aperture), no diffraction mines were measured, which indicates a size of the primary crystallites of distinctly smaller than 2 nm. Exam ple 1 (comparative)

1.2 kg of su perabsorbent base polymer were initial ly charged in the mixer. At 23° C and a shaft speed of 200 revolutions per minute, by means of a nitrogen-d riven two-phase spray nozzle, a solution of 0.08% by weight of ethylene glycol diglycidyl ether, 2.5% by weight of propane-1, 2-diol and 3% by weight of water, based in each case on the base polymer, was sprayed on. Su bsequently, the shaft speed was reduced to 60 revolutions per minute, and the product tem perature was increased to 130° C and then maintained for 30 minutes.

Directly thereafter (the product tem perature at that point was about 100° C) , the super absorbent obtained was mixed in a fu rther mixer at a shaft speed of 200 revolutions per mi nute with 0.1% by weight, based on the su perabsorbent, of x-ray-amorphous alu minum hy d roxide (mixing time about one minute) . After the aluminu m hydroxide had been mixed in, by means of a nitrogen-d riven two-phase nozzle, 4.0% by weight, based on the superabsor bent, of water was also sprayed on (at a rate of 38 g/min) and mixed in. Thereafter, the sieve cut of 150-710 pm was obtained.

The su perabsorbent thus obtained was analyzed; the measu rements obtained are reported in table 1.

Exam ple 2 (comparative)

Exam ple 1 was repeated, except that the total amou nt of water was added in two portions of 2.0% by weight each, the second after mixing in the first one (about fifteen seconds of mixing time). Sam ples were taken after each water addition step. The measurements ob tained are reported in table 1.

Example 3

Exam ple 1 was repeated, except that the total amount of water was added in four portions of 1.0% by weight each, each one after mixing in the previous one (about fifteen seconds of mixing time). The measu rements obtained are reported in table 1.

Evaluation of exam ples 1 - 3

Com parison between exam ples 1 to 3 shows that adding the same amou nt of water in at least three portions rather than at once im proves GBP without significantly affecting CRC and AU L.

Exam ple 4

Example 3 was repeated, except that the amount of alu minu m hyd roxide was increased to 0.3% by weight. Example 5

Example 3 was repeated, except that the amount of aluminum hydroxide was increased to 0.5% by weight.

Evaluation of examples 3 -5

Comparison between examples 3 to 5 show the influence of aluminum hydroxide amount on GBP. For the superabsorbent and water addition method of this example, more than 0.3% by weight of aluminum hydroxide do not further increase GBP.

Example 6 (comparative)

Example 3 was repeated, except that the amount of aluminum hydroxide was increased to 0.3% by weight.

Example 7 (comparative)

Example 3 was repeated, except that the amount of aluminum hydroxide was increased to 0.3% by weight.

Evaluation of Examples 6 -7

Examples 6 and 7, as compared to 4 and 5, respectively, show that adding the same amount of water in at least three portions rather than at once improves GBP without significantly affecting CRC and AUL also at higher aluminium hydroxide levels.

Example 8

1.2 kg of a standard commercial surface-postcrosslinked superabsorbent polymer was charged in the mixer.

At 23° C and a shaft speed of 200 revolutions per minute, 0.3% by weight, based on the su perabsorbent, of x-ray-amorphous aluminum hydroxide were added to the superabsorbent and mixed for fifteen seconds. Then, by means of a nitrogen-driven two-phase nozzle, four portions of 1.0% by weight each, based on the super-absorbent, of water were also sprayed on (at a rate of 38 g/min) and mixed in. The mixing time following the addition of each por tion of water but the last was fifteen seconds. Following the last portion, the superabsor bent was mixed for 300 seconds. Following each addition of water and mixing, samples were taken and the sieve cut of 150-710 pm thereof was obtained.

The samples thus obtained were analyzed; the measurements obtained are reported in ta ble 2. Example 9

Example 8 was repeated, except that the mixing time following the addition of each portion of water but the last was sixty seconds.

Example 10

Example 8 was repeated, except that the mixing time following the addition of each portion of water but the last was three hundred seconds.

Example 11

Example 8 was repeated, except that the mixing time following the addition of each portion of water but the last was six hundred seconds.

Evaluation of Examples 8 - 11

Examples 8 - 11 show that extending the time between adding the portions of water does not further improve or decrease GBP.

able 1

able 2