WO2013076031A1 - Superabsorbent polymer with pyrogenium aluminum oxide - Google Patents

Abstract

The invention relates to a superabsorbent polymer with pyrogenium aluminum oxide having low caking tendency with good absorption properties and rapid water absorption.

Description

 Superabsorber with pyrogenic alumina

The present invention relates to a superabsorbent with pyrogenic alumina, a process for its preparation and its use and hygiene articles containing it.

Superabsorbents are known. Also, for such materials, terms such as "high swellable polymer" "hydrogel" (often used for the dry form), "hydrogel-forming polymer", "water-absorbent polymer", "absorbent gelling material",

"Swellable resin", "water-absorbent resin" or the like in common use. These are crosslinked hydrophilic polymers, in particular polymers of (co) polymerized hydrophilic monomers, graft (co) polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose or starch ethers, crosslinked carboxymethylcellulose, partially crosslinked polyalkylene oxide or Swellable in aqueous liquids natural products, such as guar derivatives, with superabsorbents based on partially neutralized acrylic acid are the most widespread. The essential properties of superabsorbers are their ability to absorb many times their own weight in aqueous liquids and to not release the liquid under some pressure. The superabsorber, which is used in the form of a dry powder, transforms into a gel when it absorbs liquid, with the usual absorption of water corresponding to a hydrogel. Crosslinking is essential for synthetic superabsorbents and an important difference to conventional pure thickeners, as it leads to the insolubility of the polymers in water. Soluble substances would not be useful as superabsorbent. By far the most important application of superabsorbers is the absorption of body fluids. Superabsorbents are used, for example, in infant diapers, adult incontinence products or feminine hygiene products. Other fields of application are, for example, those used as water-retaining agents in agricultural horticulture, as water storage for protection against fire, for liquid absorption in food packaging or, more generally, for the absorption of moisture.

Superabsorbents can absorb several times their own weight in water and retain them under some pressure. In general, such a superabsorbent has a centrifuge retention capacity ("CRC", "Centrifuge Retention Capacity", measuring method see below) of at least 5 g / g, preferably at least 10 g / g and in a particularly preferred form at least 15 g / g. A "superabsorber" may also be a mixture of materially different individual superabsorbers or a mixture of components which only show superabsorbent properties when interacting, it is less important on the material composition than on the superabsorbent properties.

Important for a superabsorbent is not only its absorption capacity, but also the ability to retain liquid under pressure (retention) as well as the liquid transport in the air swollen state, ie the permeability for liquids in the swollen gel. Swollen gel can hinder fluid transport to superabsorbers that are not yet swollen ("gel blocking") .Good transport properties for liquids include, for example, hydrogels which have a high gel strength in the swollen state Gels with only low gel strength are under an applied pressure (body pressure) deforms, clogs pores in the superabsorbent / cellulose fiber absorber and prevents further absorption of fluid Increased gel strength is usually achieved by a higher degree of cross-linking, but this reduces the absorption capacity of the product An elegant method for increasing the gel strength is to increase the Degree of crosslinking on the surface of the superabsorbent particles in relation to the interior of the particles. For this purpose, in a surface postcrosslinking step, dried superabsorbent particles having an average crosslinking density are usually added to subjected to additional crosslinking in a thin surface layer of their particles. Surface postcrosslinking increases the crosslink density in the shell of the superabsorbent particles, raising the absorption under pressure to a higher level. While the absorption capacity in the surface layer of the superabsorbent particles decreases, its core has an improved absorption capacity compared to the shell due to the presence of mobile polymer chains, so that the shell construction ensures improved fluid transfer without gel blocking occurring. It is also known to produce overall crosslinked superabsorbents and to subsequently reduce the degree of crosslinking in the interior of the particles compared to an outer shell of the particles.

Also, processes for the production of superabsorbents are known. Acrylic acid-based superabsorbents, which are most commonly used in the marketplace, are prepared by free-radical polymerization of acrylic acid in the presence of a crosslinker (the "internal crosslinker"), the acrylic acid before, after or partly before, partly after the polymerization The polymer gel obtained in this way is comminuted (depending on the polymerization reactor used, this can take place simultaneously with the polymerization) and dried. or "base polymer") is usually post-crosslinked on the surface of the particles by reacting with other crosslinkers, such as organic crosslinkers or polyvalent cations, for example, aluminum (usually used as aluminum sulfate) or both, to produce a more highly crosslinked surface layer to the particle interior ,

A problem often encountered in superabsorbents is the tendency to caking, which, by their very nature, exhibits moisture-absorbing substances, especially in humid air. Especially when storing or processing superabsorbers in tropical or subtropical countries, this is a common problem that makes the storage and processing of superabsorbers considerably more difficult. While storage and processing can take place in rooms supplied with dehumidified air, this is energy intensive and expensive. Fredric L. Buchholz and Andrew T. Graham (ed.), In: "Modern Superabsorbent Polymer Technology", J. Wiley & Sons, New York, USA / Wiley-VCH, Weinheim, Germany, 1997, ISBN 0-471 -1941 1 -5, a summary of superabsorbents, their properties and processes for the production of superabsorbers, Chapter 3.2.8.2. "Additives for Improved Handling" teaches that the tendency to caking can be counteracted by additives known as common additives oils , as also used in hygroscopic fertilizers, polymeric soaps as drying aids for acrylamide copolymers, and particulate silica in combination with polyols or polyalkylene glycols as flow aids for poly (acrylamide) polymers and copolymers. Furthermore, quaternary surfactants are mentioned, alone or in combination with other additives, to reduce dust formation, which is a disadvantage of the addition of silica. All of these additives can be added in a variety of types of commercial mixers.

The addition of silicon dioxide or other inorganic powders to reduce the tendency to bake (ie as "anticaking agent"), in addition to increased dust formation, often has the disadvantage that some properties of the superabsorber are impaired as a result: in particular, the conveyance in screw conveyors and in the measurement of properties decreases; presuppose the swelling of the superabsorbent under pressure, sometimes inferiorly degrade such treated superabsorbents, presumably because the inorganic particulate superabsorbent particles may be less likely to slip past each other during production or pressurized swelling, but this in turn increases liquid permeability in the swollen gel. because open pores and passages are retained, which may also be a desirable effect Dusting, poor pumpability and reduced swelling under pressure can be achieved by the addition of dedusting agents (also known as "dust binders") tel ") are counteracted again. The polyols and polyalkylene glycols usually used as dedusting agents not only bind dust, but also act as lubricants between the superabsorbent particles. In the case of commercially available superabsorbers, if their tendency to cake at the storage and processing site is a problem, the addition of silica powder alone or in combination with dedusting agents such as polyols or polyalkylene glycols is the most widespread.

WO 2004/069 915 A2 teaches a superabsorbent which contains 0.01 to 5% of a water-insoluble inorganic powder such as silicon dioxide. WO 2008/055 935 A2 discloses a superabsorber containing optimized amounts of inorganic powder and dedusting agents such as polyols, for example 1,2-propylene glycol, 1,3-propanediol, 1,2,2,1,3 or 1,4-butanediol or Glycerol or polyglycols such as polyethylene glycol, polypropylene glycol or polybutylene glycol, these typically having a molecular weight of up to 5000 g / mol. JP 63/039 934 teaches the addition of a mixture of water-insoluble inorganic powder such as silica and organic compounds such as polyethylene glycol or its ethers.

It is also known to treat superabsorbents with particles such as microcrystalline cellulose or inorganic particles such as silica or clays in order to increase the water absorption capacity. speed, which improves the absorption of water from cell-containing fluids, such as blood, as taught in WO 00/62 825 A2.

WO 2004/018 005 A1 and WO 2004/018 006 A1 describe clay-added superabsorbents. WO 2005/097 881 A1 and WO 02/060 983 A2 disclose superabsorbents with water-insoluble phosphates and WO 2006/058 683 A2 relates to superabsorbents with insoluble metal sulfates.

WO 94/22 940 A1 teaches the dedusting of superabsorbers with aliphatic polyols having an average molecular weight of more than 200 g / mol or polyalkylene glycols having an average molecular weight between 400 and 6000 g / mol. Also polyether polyols are called. The superabsorbent thus dedusted can furthermore be mixed with flow aids (there so called), such as silicon dioxide.

No. 7,995,345 and US Pat. No. 3,932,322 disclose the addition of pyrogenic silicon or aluminum oxides to superabsorbers.

There is also the task of finding new or improved superabsorbents with reduced caking tendency. The performance characteristics of the superabsorber, in particular its ability to absorb liquid, even under pressure, as well as its ability to transfer liquid, but also its ability to be conveyed, are not or at least not substantially impaired.

The problem was solved by a superabsorbent with pyrogenic alumina. Furthermore, a process for the preparation of this superabsorbent were found, uses of this superabsorbent and hygiene products containing this superabsorbent.

The superabsorber according to the invention shows low tendency to caking without its use properties being significantly impaired. The superabsorber according to the invention contains pyrogenic alumina. Pyrogenic alumina is alumina produced by a pyrogenic process, unlike most aluminas by precipitation. Pyrogenic processes are processes in which an oxide is prepared by flame oxidation or flame hydrolysis of a suitable starting compound in a flame, in flame hydrolysis usually a blast gas flame. Pyrogenic alumina is usually produced by flame oxidation of a vaporizable aluminum compound or by flame hydrolysis of a vaporizable aluminum compound in an oxyhydrogen flame. The vaporizable aluminum compound used is typically aluminum chloride, which produces fumed alumina and hydrogen chloride in the oxyhydrogen gas flame. A process for the preparation of pyrogenic aluminum oxide and pyrogenic aluminum oxide are known commodity and is a common ^®, for example, under the brand name AEROXIDE Alu by Evonik Industries AG, Inorganic Materials, Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang, Germany, available. In comparison with aluminum oxide produced by precipitation, it is usually purer, more finely divided and has a higher surface area.

Pyrogenic alumina typically has a BET surface area of at least 20 m ² / g, preferably at least 30 m ² / g and more preferably at least 50 m ² / g and typically at most 200 m ² / g, preferably at most 180 m ² / g and in a particularly preferred form at most 150 m ² / g. (The BET surface area is the specific surface area of a solid as determined by the method given by Stephen Brunauer, Paul Hugh Emmett, and Edward Teller, for the first time in J. Am. Chem. Soc., 60 (1938) 309, and is determined according to DIN ISO 9277: 2003-05 ("Determination of the specific surface area of solids by gas adsorption using the BET method") A simplified procedure which generally yields comparable results within the scope of measurement accuracy is described in DIN 66132: 1975-07 ( In the case of deviations, the first-mentioned standard is applicable in the context of this invention.The BET method is one in the field of porous solids, including catalysts, a well-known and routinely used method.)

In general, the fumed alumina is added to the SAP in an amount of at least 0.005 wt%, preferably at least 0.03 wt%, and most preferably at least 0.05 wt%, and generally at most 6.0 wt .-%, preferably at most 1, 0 wt .-% and in a particularly preferred form at most 0.5% by weight added, each based on the total weight of the superabsorbent with pyrogenic alumina. The superabsorbent is prepared in the usual way except for the addition of fumed alumina. A preferred process for the preparation of the market dominating acrylate superabsorbent is the aqueous solution polymerization of a monomer mixture comprising a) at least one ethylenically unsaturated, acid group-carrying monomer which is optionally present at least partially as a salt,

b) at least one crosslinker,

c) at least one initiator,

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

e) optionally one or more water-soluble polymers.

The monomers a) are preferably water-soluble, ie the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, more preferably at least 25 g / 100 g of water, most preferably at least 35 g / 100 g of water. Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids or their salts, such as acrylic acid, methacrylic acid, maleic acid, maleic anhydride, and itaconic acid or its salts. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a significant influence on the polymerization. Therefore, the raw materials used should have the highest possible purity. It is therefore often advantageous to purify the monomers a) specifically. Suitable purification processes are described, for example, in WO 2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitable monomer a) is, for example, an acrylic acid purified according to WO 2004/035514 A1 with 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid,

0.0332% by weight of water, 0.0203% by weight of propionic acid, 0.0001% by weight of furfurals, 0.0001

% By weight of maleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether.

The proportion of acrylic acid and / or salts thereof in the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%.

The monomer solution preferably contains 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, in particular by 50% by weight. ppm, hydroquinone half ethers, based in each case on the unneutralized monomer a), where neutralized monomer a), ie a salt of the monomer a) is mathematically taken into account as unneutralized monomer. For example, an ethylenically unsaturated, acid group-carrying monomer having a corresponding content of hydroquinone half-ether can be used to prepare the monomer solution.

Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and / or alpha tocopherol (vitamin E). Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be radically copolymerized into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts which can form coordinative bonds with at least two acid groups of the monomer a) are also suitable as crosslinking agents b). Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be incorporated in the polymer network in free-radically polymerized form. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 530 438 A1, di- and triacrylates, as in EP 547 847 A1, EP 559 476 A1, EP 632,068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated groups, as in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, as described, for example, in DE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/32962 A2.

Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, trimethylolpropane triacrylate 10 to 20 times ethoxylated, trimethylolethane triacrylate 10 to 20 times ethoxylated, particularly preferably 15-times ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylates having 4 to 30 ethylene oxide units in the polyethylene glycol chain, trimethylolpropane triacrylate, di and triacrylates of 3 to 30 times ethoxylated glycerol, more preferably di- and triacrylates of 10-20-fold ethoxylated glycerol, and triallylamine. The polyesters which are not completely esterified with acrylic acid can also be present here as Michael adducts with themselves, as a result of which tetra-, penta- or even higher acrylates may also be present.

Very particularly preferred crosslinkers b) are the polyethyleneglyoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form diioder triacrylates, as described, for example, in WO 2003/104301 A1. Particularly advantageous are di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol. Very particular preference is given to diacrylates or triacrylates of 1 to 5 times ethoxylated and / or propoxylated glycerol. Most preferred are the triacrylates of 3 to 5 times ethoxylated and / or propoxylated glycerol, in particular the triacrylate of 3-times ethoxylated glycerol. The amount of crosslinker b) is preferably from 0.05 to 1, 5 wt .-%, particularly preferably 0.1 to 1 wt .-%, most preferably 0.3 to 0.6 wt .-%, each based on Monomer a). As the cross-linker content increases, the centrifuge retention capacity (CRC) decreases and the absorbance increases under a pressure of 0.7 psi ("AAP (0.7 psi)", below). As initiators c), all compounds which generate free radicals under the polymerization conditions can be used, for example Thermal initiators, redox initiators, photoinitiators Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / 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 As ascending component, however, a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and Sodium bisulfite used (as Brüggolit ^® FF6M or Brüggolit ^® FF7, alternatively BRUGGOLITE ^® FF6M or BRUGGOLITE ^® FF7 available from L. Brüggemann KG, Salzstraße 131, 74076 Heilbronn, Germany, www.brueggemann.com). Examples of ethylenically unsaturated monomers d) which can be copolymerized with the ethylenically unsaturated monomers having acid groups are acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, maleic acid and maleic anhydride.

As water-soluble polymers e) it is possible to use polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.

Usually, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75% by weight, more preferably from 45 to 70% by weight, most preferably from 50 to 65% by weight. It is also possible monomer suspensions, i. to use supersaturated monomer solutions. With increasing water content, the energy expenditure increases during the subsequent drying and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

The preferred polymerization inhibitors require dissolved oxygen for optimum performance. Therefore, the monomer solution can be freed of dissolved oxygen prior to the polymerization by inerting, ie by flowing with an inert gas, preferably nitrogen or carbon dioxide. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, most preferably less than 0.1 ppm by weight. The monomer mixture may contain other components. Examples of other components used in such monomer mixtures include chelating agents to keep metal ions in solution. This is known, all known chelating agents can be used. The chelating agents used most commonly are Aminocarboxisäuren and their salts, such as nitrilotriacetic acid (,, ΝΤΑ), ethylenediaminetetraacetic acid ( "EDTA") and comparison of similar structure compounds, but also polymers, such as the sodium salt of N-carboximethyliertem polyamine (tri- lon ^® P of BASF SE, Ludwigshafen, Germany); amides of polybasic carboxylic acids such as citric and malonic acid amides; acylated amino acids; hydroxycarboxylic acids and their salts such as lactic acid, glycolic acid, malic acid, glyceric acid, tartaric acid, citric acid, isocitronic acid and its salts, in particular sodium salts; diketones and their derivatives, tropolone and derivatives thereof; esters of phosphoric acid or phosphorous acid and salts thereof; chelating organic compounds of phosphonic acid; inorganic phosphates such as sodium tripolyphosphate; organic heterocyclic compounds such as phenanthroline, 2, 2'-bipyridine, terpyridine and their derivatives.

Further examples of further components used in such monomer mixtures are, for example, reducing agents (also called "antioxidants" or "stabilizers") which reduce the tendency of the finished product to yellow. Again, any known addition can be used. Examples of such reducing agents are phenols, phosphonic acid (HP (0) (OH) 2), phosphorous acid (H3PO3) and the salts and esters of these acids. Among the phenols, the sterically hindered phenols are preferred. Hindered phenols are understood as meaning phenols which have a single or double-branched substituent, preferably a double-branched substituent, at least in the 2-position and optionally also in the 6-position on the phenyl ring. Branched substituents are understood to mean substituents which bear at least two radicals other than hydrogen on the atom bonded to the phenyl ring of the phenol, apart from the C atom of the phenyl ring to which they are attached. However, sterically hindered phenols are also those which carry a sterically demanding unbranched substituent at least in the 2-position and optionally also in the 6-position. These are understood to mean substituents which comprise at least 6, preferably at least 8, and in a particularly preferred form at least 12 atoms other than hydrogen, but only one on the atom bound to the phenyl ring of the phenol other than the C atom of the phenyl ring to which they are attached other than hydrogen. The simplest examples of singly branched substituents are secondary alkyl radicals such as 2-propyl, 2-butyl, 2-pentyl, 3-pentyl, ethylhexyl or cycloalkyl radicals such as cyclobutyl, cyclopentyl, cyclohexyl or aromatic radicals such as phenyl. The simplest examples of di-branched substituents are tertiary alkyl radicals such as tert-butyl, tert-pentyl or norbornyl. The simplest examples of unbranched radicals are hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl, but also neo-pentyl, neo-hexyl or dodecylthiomethyl. All of these radicals can also be substituted or contain atoms other than carbon and hydrogen. The phenyl ring of the phenol may optionally carry further substituents in addition to the substituent in the 2-position and optionally in the 6-position. Examples of preferred sterically hindered phenols are 2-tert-butylphenol, 2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methyl-phenol (also referred to as 2,6-di-tert tert-butyl-para-cresol or 3,5-di-tert-butyl-4-hydroxytoluene), 3,5-di-tert-butyl-4-hydroxyphenylacetic acid, 3,5-di-tert-butyl 4-hydroxyphenylpropionic acid and the esters of these acids with alcohols and polyols, for example their simple or multiple esters with glycol, glycerol, 1, 2- or 1, 3-propanediol, trimethylolpropane or pentaerythritol, such as pentaerythritol tetrakis- (3 - (3,5-di-tert-butyl-4-hydroxyphenyl) propionate) or octadecyl (3,5-di-tert-butyl-4-hydroxyphenyl) propionate), 4,4-thio-bis (6 tert-butyl meta-cresol), 4,6-bis (dodecylthiomethyl) ortho-cresol, 3,3 ', 3 ", 5,5', 5" -hexa-tert-butyl-a, a ' , a "- (mesitylene-2,4,6-triyl) tri-para-cresol (another name for 2,4,6-tri [(4-hydroxy-3,5-di-tert-butylphenyl) methyl] mesitylene, CAS No. 1709-70-2, from BASF Switzerland AG, Basel, Switzerland, under the trademark Irganox ^® 1330), N, N-hexane-1,3-diylbis (3- (3,5-di-tert-butyl-4-hydroxyphenylpropionamide)), 2,2'-ethylidenebis [4,6-bis ( 1,1-dimethylethyl) phenol] and ethylene bis (oxyethylene) bis-3- (5-tert-butyl-4-hydroxy-m-tolyl) propionate) (CAS-No. 36443-68-2, from BASF Switzerland AG, Basel, Switzerland, under the trademark Irganox ^® 245). Further reducing agents are salts and esters of phosphonic acid (HP (0) (OH) 2) and phosphorous acid (H3PO3) as well as phosphonic acid itself. Phosphonic acid is tautomeric with phosphorous acid, the latter does not exist as free acid. Genuine derivatives of phosphorous acid are only their triesters, which are commonly referred to as phosphites. The tautomeric phosphonic acid derivatives are commonly referred to as phosphonates. For example, all primary and secondary phosphonates of the alkali metals, including ammonium, and the alkaline earth metals are suitable. Also suitable, for example, are aqueous solutions of phosphonic acid which contain primary and / or secondary phosphonations and at least one cation selected from sodium, potassium, calcium, strontium. Examples of suitable phosphites or phosphonates are calcium bis [monoethyl (3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate], tris (2,4-di-tert-butylphenyl) phosphite, 3,9-bis (octa -decyloxy) -2,4,8,10-tetraoxa-3,9-diphosphaspiro [5.5] undecane and bis (2,4-di-tert-butylphenol) pentaerythritol diphosphite. Stabilizers may simultaneously be phosphonates or phosphites and hindered phenols.

Suitable polymerization reactors are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel formed in the polymerization of an aqueous monomer solution or suspension is comminuted continuously by, for example, counter-rotating stirring shafts, as described in WO 2001/38402 A1. The polymerization on the belt is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928. Polymerization in a belt reactor produces a polymer gel which must be comminuted in a further process step, for example in a meat grinder, extruder or kneader. However, it is also possible to produce spherical superabsorbent particles by suspension, spray or drop polymerization processes. The use of the particularly preferred urea phosphate according to the invention is particularly advantageous in particular in polymerization processes such as, for example, a kneading reactor or a drop polymerization with a relatively short polymerization time.

The acid groups of the polymer gels obtained are usually partially neutralized. The neutralization is preferably carried out at the stage of the monomers, in other words salts of the acid group-carrying monomers or, strictly speaking, a mixture of acid group-carrying monomers and salts of the acid group-carrying monomers ("partially neutralized acid") are used as component a) in the polymerization. This is usually done by mixing the neutralizing agent as an aqueous solution or preferably also as a solid in the monomer mixture intended for the polymerization or preferably in the acid group-carrying monomer or a solution thereof The degree of neutralization is preferably from 25 to 95 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 65 to 72 mol%, it being possible to use the customary neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and mixtures thereof instead of alkali metal sal zen also ammonium salts can be used. Sodium and potassium are particularly preferred as alkali metal cations, but most preferred are sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof. But it is also possible to carry out the neutralization after the polymerization at the stage of Polymergeis formed during the polymerization. Furthermore, it is possible to neutralize up to 40 mol%, preferably 10 to 30 mol%, particularly preferably 15 to 25 mol%, of the acid groups before the polymerization by adding a part of the neutralizing agent already to the monomer solution and the desired final degree of neutralization is adjusted only after the polymerization at the stage of Polymergeis. If the polymer gel is at least partially neutralized after the polymerization, the polymer gel is preferably comminuted mechanically, for example by means of an extruder, wherein the neutralizing agent can be sprayed, sprinkled or poured on and then thoroughly mixed in. For this purpose, the gel mass obtained can be extruded several times for homogenization.

However, it is preferred to carry out the neutralization at the stage of the monomer. In other words: in a very particularly preferred embodiment, the monomer a) is a mixture of from 25 to 95 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 65 to 72 mol% salt of the acid group-carrying monomer and the remainder used to 100 mol% acid group-carrying monomer. This mixture is, for example, a mixture of sodium acrylate and acrylic acid or a mixture of potassium acrylate and acrylic acid.

In a preferred embodiment, a neutralizing agent is used for neutralization, the content of iron is generally below 10 ppm by weight, preferably below 2 ppm by weight, and more preferably below 1 ppm by weight. Similarly, a low content of chloride and anions of oxygen acids of the chlorine is desired. A suitable neutralizing agent is, for example, the 50% strength by weight sodium hydroxide solution or potassium hydroxide solution, which is usually sold as "membrane grade"; the 50% strength by weight sodium hydroxide solution usually sold as "amalgam grade" or "mercury process" is even purer, but also more expensive The polymer gel obtained from the aqueous solution polymerization and optionally subsequent neutralization is then preferably dried with a belt dryer until the residual moisture content is preferably 0.5 to 15% by weight, particularly preferably 1 to 10% by weight, very particularly preferably 2 If the residual moisture content is too high, the dried polymer gel has too low a glass transition temperature Tg and is difficult to process further dried polymer gel too brittle and fall in the subsequent crushing unwanted requires large amounts of polymer particles with too small particle size ("fines"). The solids content of the gel before drying is generally from 25 to 90% by weight, preferably from 30 to 80% by weight, particularly preferably from 35 to 70% by weight, very particularly preferably from 40 to 60% by weight. %. Alternatively, for drying but also a fluidized bed dryer or a heatable mixer with mechanical mixing element such as a paddle dryer or a similar dryer with be used differently shaped mixing tools. Optionally, the dryer may be operated under nitrogen or other non-oxidizing inert gas or at least a reduced partial pressure of oxygen to prevent oxidative yellowing. However, as a rule, sufficient ventilation and removal of the water vapor also leads to an acceptable product. Advantageous in terms of color and product quality is usually the shortest possible drying time. In the conventional belt dryers, a temperature of the gas used for the drying of at least 50 ° C, preferably at least 80 ° C and in a particularly preferred form of at least 100 ° C and generally of at most 250 ° C, preferably at most 200 ° C and in a particularly preferred form of not more than 180 ° C. Common belt dryers often have multiple chambers, the temperature in these chambers may be different. For each type of dryer, the operating conditions must be selected as a whole in a known manner so that the desired drying result is achieved. During drying, the residual monomer content in the polymer particles also decreases and the last residues of the initiator are destroyed.

The dried polymer gel is then ground and classified, wherein for grinding usually one- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibratory mills can be used. Oversized gel lumps, which are often not dried in the interior, are rubber-elastic, lead to grinding problems and are preferably separated before grinding, which can easily be achieved by air classification or a sieve ("protective sieve" for the mill) is to be chosen in view of the mill used so that as possible no interference from oversized, rubber-elastic particles occur.

Too large, insufficiently finely ground superabsorbent particles can be felt in their predominant use, in hygiene products such as diapers, as coarse particles, they also reduce the average swelling rate of the superabsorbent. Both are undesirable. Advantageously, coarse-grained polymer particles are therefore separated from the product. This is done by conventional classification methods, such as air classification or sieving through a sieve with a mesh size of at most 1000 μηη, preferably at most 900 μηη, more preferably at most 850 μηη and most preferably at most 800 μηη. For example, screens are used with 700 μηη, 650 μηη or 600 μηη mesh size. The separated coarse-grained polymer particles ("oversize") can be fed back to the grinding and screening circuit for cost optimization or further processed separately.

Particles that are too small in size reduce the permeability to liquids in the swollen gel. Advantageously, therefore, fine-particle polymer particles are also separated in this classification. This can, if sieved, conveniently by means of a sieve with a mesh size of at most 300 μηη, preferably at most 200 μηη, more preferably at most 150 μηη and most preferably not more than 100 μηη be achieved. The separated fine-grained polymer particles ("undersize" or "fines") can be fed back to the monomer stream, the polymerizing gel, or the polymerized gel before drying the gel for cost optimization. The mean particle size of the polymer particles separated off as product fraction is generally at least 200 μm, preferably at least 250 μm, and preferably at least 300 μm, and generally at most 600 μm, and more preferably at most 500 μm. The proportion of particles having a particle size of at least 150 μm is generally 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 μm, is generally at least 90% by weight, preferably at least 95% by weight and more preferably at least 98% by weight.

The polymer thus produced has superabsorbent properties and falls under the term "superabsorbent." Its CRC is typically comparatively high, but its AAP (0.7 psi) or its permeability to liquids in the swollen gel is comparatively low often called "base polymer" or "base polymer" from a surface postcrosslinked superabsorbent prepared therefrom.

The superabsorbent particles can be postcrosslinked on their surface to further improve the properties, in particular increase the AAP (0.7 psi) and permeability (with the CRC value decreases). Suitable postcrosslinkers are compounds which contain groups which can form bonds with at least two functional groups of the superabsorbent particles. Acrylic / sodium acrylate-based superabsorbents which are predominant in the market are suitable surface postcrosslinker compounds which contain groups which can form bonds with at least two carboxylate groups. Preferred postcrosslinkers are amide acetals or carbamates of the general formula (I)

wherein

R ¹ Ci-Ci2-alkyl, C ₂ -C ₂ hydroxyalkyl, C ₂ -C ₂ -alkenyl or C ₆ -C ₂ aryl, R ² X or OR ⁶ ' R ³ is hydrogen, Ci-Ci ₂ -alkyl, C ₂ -C ₂ hydroxyalkyl, C ₂ -Ci2 alkenyl or C ₆ -C ₂ aryl, or X,

R ⁴ Ci-Ci ₂ -alkyl, C ₂ -C ₂ hydroxyalkyl, C ₂ -C ₂ -alkenyl or C ₆ -C ₂ aryl, R ⁵ is hydrogen, Ci-Ci ₂ -alkyl, C ₂ -C ₂ - hydroxyalkyl, C ₂ -C ₂ -alkenyl, _Ci-Ci2 acyl, or C ₆ -C ₂ - aryl,

R ⁶ Ci-Ci ₂ -alkyl, C ₂ -C ₂ hydroxyalkyl, C ₂ -C ₂ -alkenyl or C ₆ -C ₂ aryl, and X is a for the radicals R ² and R ³ of common carbonyl oxygen, where R ¹ and R ⁴ and / or R ⁵ and R ^{6 may be} a bridged C ₂ -C ⁶ -alkanediyl, and wherein the abovementioned radicals R ¹ to R ^{6 may} still have a total of one to two free valencies and with these free valencies having at least one or polyhydric alcohols, wherein the polyhydric alcohol preferably has a molecular weight of less than 100 g / mol, preferably less than 90 g / mol, more preferably less than 80 g / mol, most preferably from less than 70 g / mol per hydroxy group and no vicinal, geminal, secondary or tertiary hydroxyl groups, and polyhydric alcohols either diols of general formula (IIa)

HO-R ⁷ - OH (IIa) where R ^{7 is} either an unbranched alkylene radical of the formula - (CH ₂ ) _n -, where n is an integer from 3 to 20, preferably 3 to 12, and both hydroxy groups are terminal, or R ^{7 is} an unbranched, branched or cyclic alkylene radical, or polyols of the general formula (IIb)

in which the radicals R ⁸ , R ⁹ , R ¹⁰ , R ¹¹ are each independently of one another hydrogen, hydroxyl, hydroxyethyl, hydroxyethyloxymethyl, 1-hydroxyprop-2-yloxymethyl, 2-hydroxypropyloxymethyl, methyl, ethyl, n-propyl, isopropyl, n- Butyl, n-pentyl, n-hexyl, 1, 2-dihydroxyethyl, 2-hydroxyethyl, 3-hydroxypropyl or 4-hydroxybutyl and a total of 2, 3, or 4, preferably 2 or 3, hydroxy groups are present, and not more than one of the radicals R ⁸ , R ⁹ , R ¹⁰ , or R ¹¹ is hydroxyl, are or cyclic carbonates of the general formula

wherein R ¹² , R ¹³ , R ¹⁴ , R ¹⁵ , R ¹⁶ and R ^{17 are} independently hydrogen, methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl or isobutyl, and n is either 0 or 1, or Bisoxazolines of the general formula (IV)

wherein R ¹⁸ , R ¹⁹ , R ²⁰ , R ²¹ , R ²² , R ²³ , R ²⁴ and R ^{25 are} independently hydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl or isobutyl, and R ^{26 represents} a single bond, a linear, branched or cyclic C 2 -C 12 -alkylene radical, or a polyalkoxydiyl radical which is composed of one to ten ethylene oxide and / or propylene oxide units, such as, for example, polyglycol dicarboxylic acids.

Preferred postcrosslinkers of the general formula (I) are 2-oxazolidones, such as 2-oxazolidone and N- (2-hydroxyethyl) -2-oxazolidone, N-methyl-2-oxazolidone, N-acyl-2-oxazolidones, such as N-acetyl 2-oxazolidone, 2-oxotetrahydro-1,3-oxazine, bicyclic amide acetals such as 5-methyl-1-aza-4,6-dioxa-bicyclo [3.3.0] octane, 1-aza-4,6-dioxa -bicyclo [3.3.0] octane and 5-isopropyl-1 -aza-4,6-dioxa-bicyclo [3.3.0] octane, bis-2-oxazolidones and poly-2-oxazolidones.

Particularly preferred postcrosslinkers of the general formula (I) are 2-oxazolidone, N-methyl-2-oxazolidone, N- (2-hydroxyethyl) -2-oxazolidone and N-hydroxypropyl-2-oxazolidone.

Preferred postcrosslinkers of the general formula (IIa) are 1, 3-propanediol, 1, 5-pentanediol, 1, 6-hexanediol and 1, 7-heptanediol. Further examples of postcrosslinkers of the formula (IIa) are 1, 3-butanediol, 1, 8-octanediol, 1, 9-nonanediol and 1, 10-decanediol. The diols are preferably water-soluble, wherein the diols of the general formula (IIa) at 23 ° C to at least 30 wt .-%, preferably at least 40 wt .-%, particularly preferably at least 50 wt .-%, most preferably at least 60 wt .-%, solve in water, such as 1, 3-propanediol and 1, 7-heptanediol. Even more preferred are those postcrosslinkers which are liquid at 25 ° C.

Preferred secondary crosslinkers of the general formula (IIb) are butane-1, 2,3-triol, butane-1, 2,4-triol, glycerol, trimethylolpropane, trimethylolethane, pentaerythritol, 1 to 3-fold ethoxylated glycerol per molecule, Trimethylolethane or trimethylolpropane and per molecule 1 to 3 times propoxylated glycerol, trimethylolethane or trimethylolpropane. Also preferred are 2-fold ethoxylated or propoxylated neopentyl glycol. Particularly preferred are 2-fold and 3-times ethoxylated glycerol, neopentyl glycol, 2-methyl-1, 3-propanediol and trimethylolpropane. Preferred polyhydric alcohols (IIa) and (IIb) have a viscosity at 23 ° C. of less than 3000 mPas, preferably less than 1500 mPas, preferably less than 1000 mPas, more preferably less than 500 mPas, very particularly preferably less than 300 mPas, on. Particularly preferred postcrosslinkers of the general formula (III) are ethylene carbonate and propylene carbonate.

A particularly preferred postcrosslinker of the general formula (IV) is 2,2'-bis (2-oxazoline). The preferred postcrosslinkers minimize side reactions and subsequent reactions which lead to volatile and thus malodorous compounds. The superabsorbers produced with the preferred postcrosslinkers are therefore odorless even when moistened.

It is possible to use a single postcrosslinker from the above selection or any mixtures of different postcrosslinkers.

The postcrosslinker is generally used in an amount of at least 0.001% by weight, preferably at least 0.02% by weight, more preferably at least 0.05% by weight, and generally at most 2% by weight, preferably at most 1% by weight, in particular preferred form at most 0.3% by weight, for example at most 0.15% by weight or at most 0.095% by weight, in each case based on the mass of the base polymer.

The postcrosslinking is usually carried out so that a solution of the postcrosslinker is sprayed onto the dried base polymer particles. Subsequent to the spraying, the polymer particles coated with postcrosslinker are thermally dried, wherein the postcrosslinking reaction can take place both before and during the drying. If surface postcrosslinkers with polymerizable groups are used, the topcoat Surface postcrosslinking also be carried out by free-radically induced polymerization of such groups by means of common radical formers or by means of high-energy radiation such as UV light. This can be done in parallel or instead of using postcrosslinkers that form covalent or ionic bonds to functional groups on the surface of the base polymer particles.

The spraying of Nachvernetzerlösung is preferably carried out in mixers with moving mixing tools, such as screw mixers, disc, paddle or paddle mixers or mixers with other mixing tools. However, vertical mixers are particularly preferred. However, it is also possible to spray the postcrosslinker solution in a fluidized bed. Suitable mixers are, for example, ^® as ploughshare mixers from Gebr Lödige Maschinenbau GmbH, Elsener Street. 7 - 9, 33102 Paderborn, Germany, or ^® as Schugi

Flexomix ^® mixer, Vrieco-Nauta ^® mixer or Turbulizer ^® mixer by Hosokawa Micron BV, Gildenstraat 26, 7000 AB Doetinchem, The Netherlands.

The applicable spray nozzles are subject to no restriction. Suitable nozzles and atomization systems are described, for example, in the following references: Atomization of Liquids, Expert-Verlag, Vol. 660, series Kontakt & Studium, Thomas Richter (2004) and in atomization technology, Springer-Verlag, VDI series, Günter Wozniak (2002 ). Applicable are mono- and polydisperse spray systems. Among the polydisperse systems are single-fluid pressure nozzles (jet or lamella-forming), rotary atomizers, two-component atomizers, ultrasonic atomizers and impact nozzles. In the two-component atomizers, the mixture of the liquid and the gas phase can take place both internally and externally. The spray pattern of the nozzles is not critical and can take any shape, such as omnidirectional, fan-beam, wide-angle omnidirectional or circular ring spray pattern. It is advantageous to use a non-oxidizing gas, if two-component atomizers are used, particularly preferably nitrogen, argon or carbon dioxide. Such nozzles, the liquid to be sprayed can be supplied under pressure. The division of the liquid to be sprayed can take place in that it is relaxed after reaching a certain minimum speed in the nozzle bore. Furthermore, single-substance nozzles, such as, for example, slot nozzles or twist chambers (full-cone nozzles) can also be used for the purpose according to the invention (for example, by Düsen-Schlick GmbH, DE, or by Spraying Systems Deutschland GmbH, DE). Such nozzles are also described in EP 0 534 228 A1 and EP 1 191 051 A2.

The postcrosslinkers are typically used as an aqueous solution. If only water is used as the solvent, the postcrosslinker solution or already the base polymer is advantageously added to a surfactant or Deagglomerisationshilfsmittel.

As a result, the wetting behavior is improved and the tendency to clog is reduced.

All anionic, cationic, nonionic and amphoteric surfactants are suitable as Deagglomera- tion auxiliary, but are preferred for skin compatibility reasons non-ionic and amphoteric surfactants. The surfactant may also contain nitrogen. For example, sorbitan monoesters, such as sorbitan monococoate and sorbitan monolaurate, or ethoxylated variants thereof, such as polysorbate ^20® , are added. Other suitable deagglomerating represent the ethoxylated and alkoxylated derivatives of 2-propylheptanol, which are marketed under the brand names Lutensol® XL ^® and Lutensol XP ^® (BASF SE, Carl-Bosch-Straße 38, 67056 Ludwigshafen, Germany).

The Deagglomerationshilfsmittel can be metered separately or the Nachvernetzerlosung be added. Preferably, the deagglomerating aid is simply added to the postcrosslinker solution.

The amount used of the deagglomerating assistant based on the base polymer is, for example, 0 to 0.1% by weight, preferably 0 to 0.01% by weight, particularly preferably 0 to 0.002% by weight. The deagglomerating assistant is preferably metered such that the surface tension of an aqueous extract of the swollen base polymer and / or the swollen postcrosslinked SAP at 23 ° C. is at least 0.060 N / m, preferably at least 0.062 N / m, particularly preferably at least 0.065 N / m, and advantageously at most 0.072 N / m. The aqueous Nachvernetzerlosung may also contain a cosolvent in addition to the at least one postcrosslinker. About the content of non-aqueous solvent or total solvent amount, the penetration depth of the postcrosslinker can be adjusted in the polymer particles. Technically suitable cosolvents are C 1 -C 6 -alcohols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol or 2-methyl-1-propanol, C 2 -C 5 -diols, such as Ethylene glycol, 1, 2-propylene glycol or 1, 4-butanediol, ketones, such as acetone, or carboxylic acid esters, such as ethyl acetate. A disadvantage of some of these cosolvents is that they have typical odors.

The co-solvent itself is ideally not a postcrosslinker under the reaction conditions. However, in the limiting case and depending on residence time and temperature, it may happen that the cosolvent partially contributes to crosslinking. This is particularly the case when the postcrosslinker is relatively inert and therefore can itself form its cosolvent, such as when using cyclic carbonates of the general formula (III), diols of the general formula (IIa) or polyols of the general formula (IIb) , Such postcrosslinkers can also be used as cosolvents in a mixture with more reactive postcrosslinkers, since the actual postcrosslinking reaction can then be carried out at lower temperatures and / or shorter residence times than in the absence of the more reactive crosslinker. Since co-solvent is used in relatively large amounts and also remains partially in the product, it must not be toxic.

In the process according to the invention, the diols of the general formula (IIa), the polyols of the general formula (IIb), and the cyclic carbonates of the general formula (III) are suitable. also as cosolvents. They fulfill this function in the presence of a reactive secondary network of the general formula (I) and / or (IV) and / or a di- or triglycidyl compound. However, preferred cosolvents in the process according to the invention are, in particular, the diols of the general formula (IIa), in particular if the hydroxyl groups are hindered sterically by neighboring groups on a reaction. Although such diols are in principle also suitable as postcrosslinkers, however, they require significantly higher reaction temperatures or optionally higher amounts of use than sterically unhindered diols.

Particularly preferred combinations of less reactive postcrosslinker as cosolvent and reactive postcrosslinker are combinations of preferred polyhydric alcohols, diols of general formula (IIa) and polyols of general formula (IIb), with amide acetals or carbamates of general formula (I).

Suitable combinations are, for example, 2-oxazolidone / 1, 2-propanediol and N- (2-hydroxyethyl) -2-oxazolidone / 1, 2-propanediol and ethylene glycol diglycidyl ether / 1, 2-propanediol.

Very particularly preferred combinations are 2-oxazolidone / 1,3-propanediol and N- (2-hydroxyethyl) -2-oxazolidone / 1,3-propanediol. Further preferred combinations are those with ethylene glycol diglycidyl ether or glycerol indi- or triglycidyl ethers with the following solvents, cosolvents or co-crosslinkers: isopropanol, 1,3-propanediol, 1,2-propylene glycol or mixtures thereof.

Further preferred combinations are those with 2-oxazolidone or (2-hydroxyethyl) -2-oxazolidone in the following solvents, cosolvents or co-crosslinkers: isopropanol, 1, 3-propanediol, 1, 2-propylene glycol, ethylene carbonate, propylene carbonate or mixtures thereof.

Frequently, the concentration of cosolvent in the aqueous postcrosslinker solution is from 15 to 50% by weight, preferably from 15 to 40% by weight, particularly preferably from 20 to 35% by weight, based on the postcrosslinker solution. For cosolvents which are only slightly miscible with water, it is advantageous to adjust the aqueous postcrosslinker solution so that only one phase is present, if appropriate by lowering the concentration of the cosolvent.

In a preferred embodiment, no cosolvent is used. The postcrosslinker is then used only as a solution in water, optionally with the addition of a Deagglomerati- onshilfsmittels.

The concentration of the at least one postcrosslinker in the aqueous postcrosslinker solution is typically from 1 to 20% by weight, preferably from 1 to 5% by weight, particularly preferably from 2 to 5% by weight, based on the postcrosslinker solution. The total amount of Nachvernetzerlösung based on the base polymer is usually from 0.3 to 15 wt .-%, preferably from 2 to 6 wt .-%.

The actual surface postcrosslinking by reaction of the surface postcrosslinker with functional groups on the surface of the base polymer particles is usually carried out by heating the base polymer wetted with surface postcrosslinker solution, commonly called "drying" (but not to be confused with the above-described drying of the polymer gel from the polymerization, typically Drying can be carried out in the mixer itself, by heating the jacket, by heat exchange surfaces or by blowing warm gases in. Simultaneous addition of the superabsorber with surface postcrosslinker and drying can take place, for example, in a fluidized bed dryer A downstream dryer, such as a hopper dryer, a rotary kiln, a paddle or disc dryer or a heated screw performed Suitable dryers are beispielswe ise as Solidair ^® or Torusdisc ^® -T Rockner from Bepex International LLC, 333 NE Taft Street, Minneapolis, MN 55413, USA, or as a paddle or paddle dryer or as a fluidized bed dryer of Nara Machinery Co., Ltd., branch Europa, Europa Allee 46, 50226 Frechen, Germany available. It is possible to heat the polymer particles for drying and performing the surface post-crosslinking via contact surfaces in a downstream dryer, or via supplied warm inert gas, or via a mixture of one or more inert gases with water vapor, or only with steam alone. When the heat is supplied via contact surfaces, it is possible to carry out the reaction under slight or complete underpressure under inert gas. When steam is used for directly heating the polymer particles, it is desirable according to the invention to operate the dryer at normal pressure or overpressure. In this case, it may be useful to split the postcrosslinking step into a heating step with steam and a reaction step under inert gas but without water vapor. This can be realized in one or more apparatuses. According to the invention, the polymer particles can already be heated in the post-crosslinking mixer with steam. The base polymer used may still have a temperature of 10 to 120 ° C from previous process steps, the postcrosslinker solution may have a temperature of 0 to 70 ° C. In particular, the postcrosslinker solution can be heated to reduce the viscosity. Preferred drying temperatures are in the range 100 to 250 ° C, preferably 120 to

220 ° C, more preferably 130 to 210 ° C, most preferably 150 to 200 ° C. The preferred residence time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and usually at most 60 minutes. Typically, the drying is conducted so that the superabsorber has a residual moisture content of generally at least 0.1% by weight, preferably at least 0.2% by weight and in a particularly preferred form at least 0.5% by weight and in the Generally not more than 15% by weight preferably at most 10% by weight and in a particularly preferred form at most 8% by weight.

Postcrosslinking can take place under normal atmospheric conditions. Normal atmospheric conditions means that no technical precautions are taken to reduce the partial pressure of oxidizing gases such as atmospheric oxygen in the apparatus in which the postcrosslinking reaction predominantly takes place (the "postcrosslinking reactor", typically the dryer) Oxidizing gases are substances which have a vapor pressure of at least 1013 mbar at 23 ° C. and act as oxidizing agents in combustion processes, for example oxygen, nitrogen oxide and nitrogen dioxide, in particular oxygen, and the partial pressure of oxidizing gases is preferably less as 140 mbar, preferably less than 100 mbar, particularly preferably less than 50 mbar, very particularly preferably less than 10 mbar If the thermal post-crosslinking is carried out at ambient pressure, ie at a total pressure of 1013 mbar rt, the total partial pressure of the oxidizing gases is determined by their volume fraction. The proportion of oxidizing gases is preferably less than 14% by volume, preferably less than 10% by volume, particularly preferably less than 5% by volume, very particularly preferably less than 1% by volume.

The post-crosslinking can be carried out under reduced pressure, ie at a total pressure of less than 1 .013 mbar. The total pressure is typically less than 670 mbar, preferably less than 480 mbar, more preferably less than 300 mbar, most preferably less than 200 mbar. If drying and post-crosslinking are carried out in air with an oxygen content of 20.8% by volume, the oxygen partial pressures corresponding to the abovementioned total pressures are 139 mbar (670 mbar), 100 mbar (480 mbar), 62 mbar (300 mbar ) and 42 mbar (200 mbar), the respective total pressures being in parentheses. Another way to reduce the partial pressure of oxidizing gases, the introduction of non-oxidizing gases, especially inert gases in the apparatus used for post-crosslinking. Suitable inert gases at the post-crosslinking temperature and given pressure in the post-crosslinking dryer are gaseous substances which do not oxidize under these conditions to the constituents of the drying polymer particles, for example nitrogen, carbon dioxide, argon, water vapor, nitrogen being preferred. The amount of inert gas is generally 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.

In the process according to the invention, the inert gas, if it does not contain water vapor, can be injected via nozzles into the postcrosslinking dryer, more preferably, however, the inert gas is already added to the polymer particle stream via nozzles in or just before the mixer by adding surface postcrosslinker to the superabsorber , Of course, vapors of cosolvents removed from the dryer can be condensed outside the dryer again and, if necessary, recycled.

In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the postcrosslinkers before, during or after the postcrosslinking. This is, in principle, further surface postcrosslinking by ionic, noncovalent bonds, but is sometimes referred to as "complexing" with the respective metal ions, or simply as "coating" with the subject substances (the "complexing agent").

This polyvalent cation is applied by spraying solutions of divalent or polyvalent cations, usually divalent, trivalent or tetravalent metal cations, but also polyvalent cations such as formally wholly or partially made of vinylamine monomers such as partially or completely hydrolyzed polyvinylamide (so-called "polyvinylamines"). min "), whose amine groups are always partially protonated to ammonium groups, even at very high pH values Examples of divalent metal cations which may be used are in particular the divalent cations of metals of groups 2 (especially Mg, Ca, Sr, Ba), 7 (in particular Mn), 8 (in particular Fe), 9 (in particular Co), 10 (in particular Ni), 1 1 (in particular Cu) and 12 (in particular Zn) of the Periodic Table of the Elements Examples of trivalent metal cations which may be used are in particular the trivalent cations of Group 3 metals including the lanthanides (especially Sc, Y, La, Ce), 8 (esp ere Fe), 1 1 (in particular Au), 13 (in particular Al) and 14 (in particular Bi) of the Periodic Table of the Elements. Examples of suitable tetravalent cations are, in particular, the tetravalent cations of metals of the lanthanides (in particular Ce) and of group 4 (in particular Ti, Zr, Hf) of the Periodic Table of the Elements. The metal cations can be used alone or mixed with each other. Particularly preferred is the use of trivalent metal cations. Very particularly preferred is the use of aluminum cations. Of the cited metal cations, all metal salts which have sufficient solubility in the solvent to be used are suitable. Particularly suitable are metal salts with weakly complexing anions such as chloride, nitrate and sulfate, hydrogen sulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogen phosphate, or dihydrogen phosphate. Preferred are salts of mono- and dicarboxylic acids, hydroxy acids, keto acids and amino acids or basic salts. Examples which may be mentioned are preferably acetates, propionates, tartrates, maleates, citrates, lactates, malates, succinates. Also preferred is the use of hydroxides. Particularly preferred is the use of 2-hydroxycarboxylic acid salts such as citrates and lactates. Examples of particularly preferred metal salts are alkali metal and alkaline earth metal aluminates and their hydrates, such as sodium aluminate and its hydrates, alkali metal and alkaline earth metal lactates and citrates and their hydrates, aluminum acetate, aluminum propionate, aluminum citrate and aluminum lactate. The cations and salts mentioned can be used in pure form or as a mixture of different cations or salts. The salts of the two and / or trivalent metal cation used may contain further secondary constituents such as unneutralized carboxylic acid and / or alkali metal salts of the neutralized carboxylic acid. Preferred alkali metal salts are those of sodium, potassium and ammonium. They are typically used as an aqueous solution, which is obtained by dissolving the solid salts in water, or is preferably produced directly as such, whereby optionally drying and cleaning steps are avoided. Advantageously, the hydrates of said salts can be used, which often dissolve faster in water than the anhydrous salts.

The amount of metal salt used is generally at least 0.001 wt .-%, preferably at least 0.01 wt .-% and in a particularly preferred form at least 0.1 wt .-%, for example at least 0.4 wt .-% and generally at most 5 wt .-%, preferably at most 2.5 wt .-% and in a particularly preferred form at most 1 wt .-%, for example at most 0.7 wt .-% in each case based on the mass of the base polymer.

The salt of the trivalent metal cation can be used as a solution or suspension. As solvents for the metal salts, water, alcohols, DMF, DMSO and mixtures of these components can be used. Particularly preferred are water and water / alcohol mixtures such as water / methanol, water / 1, 2-propanediol and water / 1, 3-propanediol.

The treatment of the base polymer with solution of a divalent or polyvalent cation is carried out in the same way as with surface postcrosslinkers, including the drying step. Surface postcrosslinker and polyvalent cation can be sprayed in a common solution or as separate solutions. The spraying of the metal salt solution onto the superabsorbent particles can be carried out both before and after the surface postcrosslinking. In a particularly preferred method, the spraying of the metal salt solution is carried out in the same step by spraying the crosslinker solution, wherein both solutions are sprayed separately successively or simultaneously via two nozzles, or crosslinker and metal salt solution can be sprayed together via a nozzle ,

In particular, when a trivalent or higher valent metal cation such as aluminum is used for complexation, optionally a basic salt of a divalent metal cation or a mixture of such salts is also added. Basic salts are salts which are suitable for increasing the pH of an acidic aqueous solution, preferably a 0.1 N hydrochloric acid. Basic salts are usually salts of a strong base with a weak acid.

The bivalent metal cation of the optional basic salt is preferably a metal cation of group 2 of the Periodic Table of the Elements, more preferably calcium or strontium, most preferably calcium. The basic salts of divalent metal cations are preferably salts of weak inorganic acids, weak organic acids and / or salts of amino acids, more preferably hydroxides, bicarbonates, carbonates, acetates, propionates, citrates, gluconates, lactates, tartrates, malates, succinates, maleates and / or fumarates, very particularly preferably hydroxides, bicarbonates, carbonates, propionates and / or lactates. The basic salt is preferably water-soluble. Water-soluble salts are salts which at 20 ° C a water solubility of at least 0.5 g of salt per liter of water, preferably at least 1 g of salt per liter of water, preferably at least 10 g of salt per liter of water, more preferably at least 100 g of salt per liter of water , very particularly preferably at least 200 g of salt per liter of water. However, salts which have this minimum solubility at the spray-on temperature of the spray solution can also be used according to the invention. Advantageously, the hydrates of said salts can be used, which often dissolve faster in water than the anhydrous salts. Suitable basic salts of divalent metal cations are, for example, calcium hydroxide, strontium hydroxide, calcium hydrogencarbonate, strontium hydrogencarbonate, calcium acetate, strontium acetate, calcium propionate, calcium lactate, strontium propionate, strontium lactate,

Zinc lactate, calcium carbonate and strontium carbonate. If the water solubility is insufficient to produce a spray solution of the desired concentration, dispersions of the solid salt in its saturated aqueous solution can also be used. For example, calcium carbonate, strontium carbonate, calcium sulfite, strontium sulfite, calcium phosphate and strontium phosphate can also be used as aqueous dispersions.

The amount of basic salt of the divalent metal cation, based on the mass of the base polymer, is typically from 0.001 to 5% by weight, preferably from 0.01 to 2.5% by weight, preferably 0.1 to 1.5% by weight .-%, more preferably from 0.1 to 1 wt .-%, most preferably from 0.4 to 0.7 wt .-%.

The basic salt of the divalent metal cation can be used as a solution or suspension. Examples of these are calcium lactate solutions or calcium hydroxide suspensions. Usually, the salts with an amount of water of not more than 15% by weight, preferably not more than 8% by weight, more preferably not more than 5% by weight, most preferably not more than 2% by weight. % sprayed on the superabsorber.

Preferably, an aqueous solution of the basic salt is sprayed onto the superabsorbent. Conveniently, the basic salt is added simultaneously with the surface postcrosslinker, the complexing agent, or as another component of the solutions of these agents. For these basic salts, addition in admixture with the complexing agent is preferred. If the solution of the basic salt is not compatible with the solution of Mixture is miscible without precipitation, the solutions can be sprayed separately in succession or simultaneously from two nozzles.

Optionally, a reducing compound is also added to the superabsorbent. Examples of reducing compounds are hypophosphites, sulfinates, sulfites, sulfonic acid derivatives or sulfinic acid derivatives as sulfonatoacetic acid, for example in the form of the disodium salt of 2-hydroxy-2 under the designation BLANCOLEN ^® HP or in the form of mixtures of the sodium salt of 2 hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite under the designations Brüggolit ^® FF6M or Brüggolit ^® FF7, alternatively Bruggolite ^® FF6M or Bruggolite ^® FF7 from L. Bruggemann KG (Salzstraße 131, 74076 Heilbronn, Germany, www.brueggemann.com) are available.

The addition of one or more reducing compounds to the superabsorber is carried out in the usual way by addition of the compound in bulk, as a solution or as a suspension in a solvent or suspending agent during or after the preparation of the superabsorber. Typically, a solution or suspension of the reducing compound in water or an organic solvent is used, for example in an alcohol or polyol or in mixtures thereof. Examples of suitable solvents or suspension agents are water, isopropanol / water, 1, 3-propanediol / water and propylene glycol / water, wherein the mixture mass ratio is preferably from 20:80 to 40:60. A surfactant may be added to the solution or suspension. If reducing compounds are added, they will generally be present in an amount of at least 0.0001% by weight, preferably at least 0.001% by weight, and more preferably at least 0.025% by weight, for example at least 0.1% by weight. % or at least 0.3 wt .-% and generally at most 3 wt .-%, more preferably at most 2.5 wt .-% and in a particularly preferred form at most 1, 5 wt .-%, for example at most 1 wt. -% or 0.7 wt .-% added, each based on the total weight of the superabsorbent. The reducing compound is generally mixed in exactly the same way with the per se known superabsorbent as the surface postcrosslinking agent applied to the superabsorbent, a surface postcrosslinker containing solution or suspension. The reducing compound can be applied as a constituent of the solution applied to the surface postcrosslinking or one of its components to a base polymer, that is to say added to the solution of the surface postcrosslinker or one of its components. The superabsorbent coated with surface postcrosslinker and reducing compound then passes through the further process steps required for surface postcrosslinking, for example a thermally induced reaction of the surface postcrosslinker with the superabsorber. This process is comparatively simple and economical. If the greatest stability against discoloration on prolonged storage is essential, the reducing compound is preferably applied after the surface postcrosslinking in a separate process step. If it is applied as a solution or suspension, it is applied to the already surface-postcrosslinked superabsorber in the same way as the application of the surface postcrosslinker to the base polymer. Most, but not necessarily, is then heated as well as in the surface postcrosslinking to rewet the superabsorber. However, the temperature set in this drying is then generally at most 1 10 ° C, preferably at most 100 ° C and most preferably at most 90 ° C to avoid undesirable reactions of the reducing compound. The temperature is adjusted so that, in view of the residence time in the drying unit, the desired water content of the superabsorber is achieved. It is also possible and convenient to add the reducing compound singly or together with other customary auxiliaries, for example dust binders, the pyrogenic alumina to be added according to the invention, other anti-caking agents or water for rewetting the superabsorber, as described below for these aids, for example in a downstream of the surface post-cooler. The temperature of the polymer particles in this case is between 0 ° C and 190 ° C, preferably less than 160 ° C, more preferably less than 130 ° C, even more preferably less than 100 ° C, and most preferably less than 70 ° C , The polymer particles are optionally rapidly cooled to temperatures below the decomposition temperature of the reducing compound after coating.

If a drying step is carried out after the surface postcrosslinking and / or treatment with complexing agent, it is advantageous, but not absolutely necessary, to cool the product after drying. The cooling can be continuous or discontinuous, conveniently the product is continuously conveyed to a dryer downstream cooler. For this purpose, it is possible to use any apparatus known for the removal of heat from pulverulent solids, in particular any apparatus mentioned above as a drying apparatus, unless it is supplied with a heating medium, but with a cooling medium, such as cooling water, so that over the walls and depending after construction, no heat is introduced into the superabsorber via the stirrers or other heat exchange surfaces, but is removed therefrom. Preference is given to the use of coolers in which the product is moved, ie cooled mixers, for example blade coolers, disk coolers or paddle coolers. The superabsorber can also be cooled in the fluidized bed by blowing in a cooled gas such as cold air. The conditions of the cooling are adjusted so that a superabsorbent is obtained with the temperature desired for further processing. Typically, an average residence time in the condenser of generally at least 1 minute, preferably at least 3 minutes and more preferably at least 5 minutes, and generally at most 6 hours, preferably at most 2 hours and most preferably at most 1

Adjusted hour and the cooling capacity so that the product obtained has a temperature of generally at least 0 ° C, preferably at least 10 ° C and in particular preferred form at least 20 ° C and generally at most 100 ° C, preferably at most 80 ° C and in a particularly preferred form at most 60 ° C.

The surface-postcrosslinked superabsorbent is optionally ground and / or sieved in the usual way. Milling is typically not required here, but most often, the setting of the desired particle size distribution of the product, the screening of formed agglomerates or fine grain is appropriate. Agglomerates and fines are either discarded or preferably recycled to the process in a known manner and at a suitable location; Agglomerates after comminution. The particle sizes desired for surface-postcrosslinked SAPs are the same as for base polymers.

To produce the superabsorber according to the invention, fumed alumina is added as an additive to the particulate superabsorbent in the process according to the invention. The addition is carried out according to at a time at which already particulate superabsorber is present, so at the earliest after the polymerization, preferably after drying and in a particularly preferred manner after the surface postcrosslinking. A particularly simple possibility is the addition of the pyrogenic alumina in the cooler, for example by spraying a dispersion or adding in finely divided solid form. Optionally, the superabsorbent, whether postcrosslinked or postcrosslinked, in the production process in any process step if required, all otherwise known coatings and other additives, such as film-forming polymers, thermoplastic polymers, dendrimers, polycationic polymers (such as polyvinylamine, polyethyleneimine or polyallylamine), water-insoluble polyvalent metal salts such as magnesium carbonate, magnesium oxide, magnesium hydroxide, calcium carbonate, calcium sulfate or calcium phosphate, all known to one skilled in the art of water-soluble mono- or polyvalent metal salts, such as aluminum sulfate, sodium, potassium, zirconium or iron salts, or other hydrophilic inorganic particles as pyrogenic Alumina, such as clay minerals, fumed silica, colloidal silica sols such as Levasil ^® , titanium dioxide, non-pyrogenic alumina and magnesium oxide are additionally applied. Examples of useful alkali metal salts are sodium and potassium sulfate, sodium and potassium lactates, citrates, sorbates. As a result, additional effects, for example a further reduced caking tendency of the end product or of the intermediate product in the respective process step of the production process, improved processing properties or a further increased liquid permeability in the swollen gel, can be achieved. If the additives are used and sprayed in the form of dispersions, then they are preferably used as aqueous dispersions, and it is preferably additionally applied a dedusting agent for fixing the additive on the surface of the superabsorbent. The dedusting agent is then added either directly to the dispersion of the inorganic powder additive, optionally it may also be added as a separate solution before, during, or after the inorganic powdery additive has been applied by spraying. Most preferred is the simultaneous spraying of postcrosslinking agent, dedusting agent and powdery inorganic additive in the postcrosslinking. In a further preferred variant of the method, however, the dedusting agent is added separately in the cooler, for example by spraying from above, below or from the side. Particularly suitable dedusting agents, which can also serve to fix powdery inorganic additives on the surface of the superabsorbent particles, are polyethylene glycols having a molecular weight of 400 to 20,000 g / mol, polyglycerol, 3 to 100-fold ethoxylated polyols, such as trimethylolpropane, glycerol, sorbitol and neopentylglycol. Particularly suitable are 7 to 20 times ethoxylated glycerol or trimethylolpropane, such as, for example, polyol TP ^70® (Perstorp, SE). The latter have the particular advantage that they only insignificantly reduce the surface tension of an aqueous extract of the superabsorbent particles.

It is also possible to adjust the superabsorber according to the invention by adding water to a desired water content.

The chelating agents mentioned above in the description of the composition of the monomer solution can be added at any point in the process for the preparation of the superabsorbent according to the invention.

Likewise, the reducing agents mentioned above in the course of the composition of the monomer solution can be added at any point in the process for preparing the superabsorber according to the invention. It is often even advantageous to add these additives together with the other additives to the finished superabsorber.

All coatings, solids, additives and auxiliaries can each be added in separate process steps, but most often the most convenient method is to add them to the superabsorber in the cooler, if not added during the displacement of the base polymer with surface postcrosslinker, such as by spraying a solution or Dispersion or addition in finely divided solid or in liquid form.

It is also possible to produce superabsorbents according to the invention by using a superabsorber with a high content of additives such as the pyrogenic aluminum oxide to be added according to the invention, but also other additives or only other additives with a superabsorber without such additives or with a relatively low content of such Additives mixed, so that the total of the superabsorbent with the desired additive content arises. This procedure is known as the "masterbatch" technique and is suitable wherever there are no devices for mixing the entire superabsorbent produced with the relatively small amounts of additives available, but devices for mixing the entire generated superabsorbent with those over pure additive amount significantly larger amounts of 'masterbatch' superabsorbent. Such a stepwise mixing of the additives into the total amount of superabsorber can be technically simpler overall. Incidentally, such superabsorbers with additives added after polymerization, drying or surface postcrosslinking are, in common usage, not only "superabsorber with additive" but also "superabsorber coated with the additive", "superabsorber containing the additive" or "composition of superabsorber and Additives' These are in practice synonyms.

The superabsorbent according to the invention generally has a centrifuge retention capacity (CRC) of at least 5 g / g, preferably of at least 10 g / g and in a particularly preferred form of at least 20 g / g. Further suitable minimum values of the CRC are, for example, 25 g / g or 30 g / g. Usually it is not above 40 g / g. A typical range of CRC for surface postcrosslinked superabsorbents is from 28 to 33 g / g.

The superabsorbent according to the invention, when surface postcrosslinked, typically has an absorbency under pressure (AAP (0.7 psi), see below) of at least 18 g / g, preferably at least 19 g / g, preferably at least 20 g / g and usually not over 30 g / g.

A further subject of the present invention are hygiene articles containing the inventive superabsorbent with pyrogenic alumina, preferably ultrathin threads containing an absorbent layer consisting of 50 to 100 wt .-%, preferably 60 to 100 wt .-%, preferably 70 to 100 Wt .-%, particularly preferably 80 to 100 wt .-%, most preferably 90 to 100 wt .-%, superabsorbent according to the invention, wherein the envelope of the absorbent layer is of course not taken into account. The superabsorbents according to the invention are also very particularly advantageous for the production of laminates and composite structures, as described, for example, in US 2003/0181115 and US 2004/0019342. In addition to the hot melt adhesives described in both documents for the production of such new absorbent structures and in particular the fibers of hot melt adhesives to which the superabsorbent particles are bonded, the superabsorbents according to the invention are also suitable for the preparation of completely analogous structures Use of UV-crosslinkable hotmelt adhesives, which are sold, for example, as AC- ^Resin® (BASF SE, Carl-Bosch-Str. 38, 67056 Ludwigshafen, Germany). These UV-crosslinkable hot-melt adhesives have the advantage of being processable at as low as 120 to 140 ° C, so they are better compatible with many thermoplastic substrates. Another significant advantage is that UV-crosslinkable hot melt adhesives are toxicologically very harmless and also cause no exhalations in the toiletries. A very important advantage, in connection with the superabsorbents according to the invention, is that the property of the UV-crosslinkable hotmelt adhesives does not tend to yellow during processing and crosslinking. This is particularly advantageous if ultrathin or partially transparent hygiene articles are to be produced. The combination of the superabsorbents according to the invention with UV-crosslinkable hotmelt adhesives is therefore particularly advantageous. Suitable UV-crosslinkable Hot melt adhesives are described, for example, in EP 0 377 199 A2, EP 0 445 641 A1, US Pat. No. 5,026,806, EP 0 655 465 A1 and EP 0 377 191 A2.

The superabsorber according to the invention can also be used in other fields of technology in which liquids, in particular water or aqueous solutions, are absorbed. These areas are for example storage, packaging, transport (as components of packaging material for water or moisture sensitive articles, such as flower transport, as well as protection against mechanical effects); Animal hygiene (in cat litter); Food packaging (transport of fish, fresh meat, absorption of water, blood in fresh fish or meat packaging); Medicine (wound plaster, water-absorbing material for burn dressings or for other weeping wounds), cosmetics (carrier material for pharmaceutical chemicals and medicaments, rheumatism plaster, ultrasound gel, cooling gel, cosmetic thickener, sunscreen); Thickener for oil / water or water / oil emulsions; Textiles (moisture regulation in textiles, shoe inserts, for evaporative cooling, for example in protective clothing, gloves, headbands); chemical-technical applications (as a catalyst for organic reactions, for the immobilization of large functional molecules such as enzymes, as adhesives in agglomerations, heat storage, filtration aids, hydrophilic component in polymer laminates, dispersants, condenser); as an aid in powder injection molding, in construction and construction (installation, in clay-based plasters, as vibration-inhibiting medium, aid in tunnel excavations in water-rich subsoil, cable sheathing); Water treatment, waste treatment, water separation (deicing, reusable sandbags); Cleaning; Agribusiness (irrigation, retention of meltwater and dew precipitation, addition of compost, protection of forests against fungal / insect attack, delayed release of active substances into plants); for fire fighting or fire protection; Coextrusion agents in thermoplastic polymers (eg for the hydrophilization of multilayer films); Production of films and thermoplastic moldings capable of absorbing water (eg rain and condensation water-retaining films for agriculture; superabsorbent-containing films for keeping fruit and vegetables fresh in moist films; superabsorbent polystyrene coextrudates, for example for food packaging such as meat , Fish, poultry, fruits and vegetables); or as a carrier substance in active ingredient formulations (pharmaceuticals, crop protection).

The liquid absorption articles according to the invention differ from those known in that they contain the superabsorber according to the invention.

A process has also been found for the preparation of articles for the absorption of liquid, in particular hygiene articles, which is characterized in that at least one superabsorber according to the invention is used in the production of the article in question. Otherwise, methods for producing such articles using superabsorbents are known. test methods

The superabsorbent is tested using the test methods described below.

The "WSP" standard test methods described below are described in: "Standard Test Methods for the Nonwovens Industry," Issue 201 1, published jointly by EDANA (European Disposables and Nonwovens Association, Avenue Eugene Plasky , 157, 1030 Brussels, Belgium, www.edana.org) and INDA (Association of the Nonwoven Fabrics Industry, 1 100 Crescent Green, Suite 1 15, Cary, North Carolina 27518, USA, www.inda.org). This publication is available from both EDANA and INDA.

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

Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity of the SAP is determined according to Standard Test Method No. WSP 241.3 (10) "Determination of Fluid Retention Capacity in Saline Solution by Gravimetry Measurement Following Centrifugation". Absorption under pressure (AAP (0.7 psi), "Absorption Against Pressure of 0.7 psi")

The absorption under a pressure of 4826 Pa (0.7 psi) of the superabsorbent is determined analogously to the standard test method no. WSP 242.3 (10) "Determination of the Absorption Against Pressure of Saline Solution by Gravimetric Measurement", but with a weight of 49 g / cm ² (results in a pressure of 4826 Pa = 0.7 psi) instead of a weight of 21 g / cm ² (resulting in a pressure of 2068 Pa = 0.3 psi).

Vortex Test To a 100 ml beaker containing a 30 mm x 6 mm magnetic stir bar is added 50.0 ml ± 1.0 ml of 0.9 wt% aqueous saline. The temperature of the saline solution is 23 ° C ± 0.5 ° C. Using a magnetic stirrer, the saline solution is stirred at 600 rpm. Then, as quickly as possible, 2.000 g ± 0.010 g of superabsorber granules (either a fraction obtained by sieving with particle sizes of 300 to 400 μm or without sieving (ie the entire grain spectrum of the superabsorbent to be subjected to the vortex test without sieving a specific grain fraction ), as indicated below), and measuring the time that elapses until the agitating Because of the absorption of the saline by the superabsorbent granules disappears. The entire contents of the beaker may still rotate as a unitary gel mass, but the surface of the gelled saline may no longer show any individual turbulence. The time required is reported as "Vortex".

Anti-caking test

5.0 ± 0.01 g of superabsorbent granules are weighed in an aluminum dish of 57 mm diameter, 1, 5 mm height and the previously determined weight Wd. By gently tapping the aluminum shell, the superabsorbent granules are evenly distributed. The aluminum tray with the superabsorbent granules is placed in a climate cabinet at a temperature of 30 ° C and a relative humidity of 80%. After 1 or 3 hours, the aluminum shell with the superabsorbent granules is removed from the climatic chamber and weighed; the weight is noted as WHYD. Then a sieve with a diameter of 76,2 mm (= 3 inches), a height of 22 mm and a mesh size of 1, 7 mm and a matching sieve bottom, the weight of which has been previously determined and recorded as WPAN, slipped over the aluminum tray with the superabsorbent granules and the whole arrangement is carefully turned upside down, so that the sieve bottom is now down and the aluminum tray is on top. A suitable sieve cover is placed on the sieve containing the aluminum shell with the superabsorbent granules and the entire arrangement is clamped in a screening machine (Retsch AS 200 control, available from Retsch GmbH, Rheinische Strasse 36, 42781 Haan, Germany). The sieving is done for 1 minute at a set amplitude of 0.20 mm. The assembly is removed from the screening machine, the sieve tray carefully removed and weighed; the weight is recorded as WUNC. The proportion of baked superabsorbent granules is calculated according to:

Caking [%] = 100 - ((WUNC - W _PA N) / (WHYD - W _d ) ^* 100) Examples

example 1

From commercially available superabsorbent (HySorb ^® B 7015, BASF SE, Carl-Bosch-Straße 38, 67056 Ludwigshafen, Germany) with a diameter greater than 600 μηη by sieving particles are removed. In each case 100 g of the sieved material were placed in a PE sample bottle (500 ml capacity) and mixed with 0.25 g each of the substances indicated in Table 1. The contents of the bottle were thoroughly mixed for 8 minutes with the aid of a tumble mixer (type T2C, Willy A. Bachofen AG Maschinenfabrik, Basel, Switzerland). The test results of the superabsorbers thus produced are shown in Table 1. Table 1: Characterization of the superabsorbents obtained in Example 1. Comparative tests are marked with (V).

Aeroxide ^® Alu C: fumed alumina having a BET surface area of 100 m ² / g Aeroxide ^® Alu 65: fumed alumina having a BET surface area of 65 m ² / g, Aeroxide ^® Alu 130: fumed alumina having a BET surface area of 130 m ² / g Aerosil ^® 200: a hydrophilic fumed silica having a BET surface area of 200 m ² / g

Sipernat ^® 22S: hydrophilic precipitated silica with a BET surface area of 200 m ² / g Sipernat ^® D17: hydrophobized precipitated silica with a BET surface area of 100 m ² / g Aerosil ^® R106: hydrophobic fumed silica having a BET surface area of 250 m ² / g

The substances marked Aeroxide ^®, Aerosil ^® or Sipernat ^® are Evonik Industries AG, Inorganic Materials, Rodenbach Chaussee 4, 63457 Hanau-Wolfgang, Germany, available.

Disperal ^®: dispersible, colloidal boehmite with a BET surface area of 180 m ² / g

Pural ^® SB: high-purity boehmite with a BET surface area of 250 m ² / g

Pural ^® 200: high-purity boehmite with a BET surface area of 100 m ² / g

Catapal ^® C1: high-purity boehmite with a BET surface area of 230 m ² / g

Puralox ^® SCFa140: high purity alumina having a BET surface area of 140 m ² / g The substances designated with Disperal ^®, Pural ^®, Catapal ^® are Puralox ^® or from Sasol Germany GmbH, Anckelmannsplatz 1, 20537 Hamburg, Germany, available.

alumina,

activated, neutral,

Brockmann I: alumina with a BET surface area of 150 m ² / g

alumina,

activated, acidic,

Brockmann I: alumina with a BET surface area of 150 m ² / g

The activated aluminas are available from Sigma-Aldrich Labora- chemicals GmbH, Wunstorferstrasse 40, 30926 Seelze, Germany.

The comparative values in Table 1 show that precipitated aluminas have virtually no effect as anti-caking agents, but fumed aluminas can achieve the same effect as anti-caking agents such as fumed silica, but do not deteriorate the absorption properties, in particular the AAP (0.7 psi), to the same extent as these , In addition, they lead to a desirably fast water absorption (shorter "vortex" time).

Example 2

326.7 g of 50% strength by weight sodium hydroxide solution and 675 g of frozen, deionized water were placed in a 2 l stainless steel beaker. With stirring, 392.0 g of acrylic acid was added, the rate of addition being adjusted so that the temperature did not exceed 35 ° C. The mixture was then cooled with stirring by means of a cooling bath. When the temperature of the mixture had dropped to 20 ° C, 0.90 g Laromer ^® LR were

9015X (trimethylolpropane-15EO triacrylate from BASF SE, Ludwigshafen, Germany), 0.037 g 2-hydroxy-2-methyl-1-phenylpropan-1-one (DAROCUR® 1 173 from BASF SE, Ludwigshafen, Germany), and 0.018 g of 2,2-dimethoxy-1, 2-diphenylethane-1 added -one (IRGACURE ^® 651 from BASF SE, Ludwigshafen, Germany). It was further cooled, and upon reaching 15 ° C, the mixture was deoxygenated by passing nitrogen through a glass frit. Upon reaching 0 ° C, 0.45 g of sodium persulfate (dissolved in 5 ml of water) and 0.06 g of hydrogen peroxide (dissolved in 6 ml of water) were added and the monomer solution was transferred to a glass dish. The glass dish was dimensioned such that a layer thickness of the monomer solution of 5 cm was established. Subsequently, 0.047 g of Bruggolite ^® FF7 (dissolved in 5 ml water) was added from L. Bruggemann KG, Salzstraße 131, 74076 Heilbronn, Germany) and the monomer solution stirred briefly with a glass rod. The glass dish with the monomer solution was placed under a UV lamp (UV intensity = 20 mW / cm ² ), the polymerization started. After 16 minutes, the gel obtained was wound three times with the aid of a standard meat grinder with a 6 mm perforated disc and dried in a laboratory drying oven at 160 ° C. for one hour. The product was then ground and sieved to yield the sieve cut of 150 to 600 μηη.

The polymer thus prepared was used for surface postcrosslinking in a ploughshare ^® - mixer with heating jacket (manufacturer: Gebr. Lödige Maschinenbau GmbH, Elsener-7 - 9, 33102 Paderborn, Germany; Type M5) at room temperature and a shaft speed of 250 revolutions per minute by means of a two-component spray nozzle coated with a solution of the following composition, wherein the weight fractions are each based on the coated polymer:

0.20% by weight of HEONON (= 2-hydroxyethyloxazolidinone) / 1,3-propanediol mixture (1: 1), 1, 80% by weight of 1,2-propanediol,

0.5% by weight of water, and

 2.0% by weight aqueous aluminum trilactate solution (22% by weight).

After spraying, the product temperature was raised to 170 ° C and the reaction mixture was maintained at that temperature for 90 minutes at a shaft speed of 60 revolutions per minute. The product obtained was allowed to cool back to room temperature and sieved, the sieve cut of 150 to 600 μm being recovered. The superabsorbent had the following properties:

CRC 37.2 g / g

 AAP (0.7 psi) 14.8 g / g

 Vortex (without sieving) 58 s

Example 3

Example 2 was repeated, but in the surface post-crosslinking instead of 2.0 wt .-% aqueous aluminum trilactate solution (22 wt .-%) 0.5 wt .-% aqueous Aluminiumdihydro- xymonoacetat solution (20 wt .-% strength ) were used based on polymer. The superabsorber had the following properties:

CRC = 37.4 g / g

AAP (0.7 psi) = 14.2 g / g

 Vortex (without sieving) = 52 s

Example 4 Example 2 was repeated except that the 2.0 wt.% Aqueous aluminum trilactate solution (22 wt.%) Was omitted in the surface postcrosslinking.

The superabsorbent had the following properties: CRC = 37.8 g / g

 AAP (0.7 psi) = 13.9 g / g

 Vortex (without sieving) = 64 s

Example 5

A mixture of 1000 g of the polymer obtained in Example 2 and different amounts of Aeroxide ^® Alu 130 were in Pflugschar ^® mixer type M5 at room temperature and a shaft speed of 250 revolutions per minute using a two-component spray nozzle with 1, 3 wt .-% , based on the mixture of a 7.5 wt .-% aqueous solution of disodium salt of 2-hydroxy-2-sulfonatoacetic acid coated (Blancolen ^® HP, L. Brüggemann KG, Salzstraße 131, 74076 Heilbronn, Germany). After spraying, the shaft speed was reduced to 60 revolutions per minute and remixed for another 10 minutes. The product obtained was sieved, the sieve cut of 150 to 600 μm being obtained. The product obtained in each case had the following properties:

Volume Aeroxide ^® CRC AAP (0.7 psi) Vortex (un- caking 3 h

Alu 130 sieves)

 [g / g] [g / g] [%]

 [g] [s]

 0.5 38.1 14.3 51 10

1, 0 37.8 14.1 50 1

1, 5 38.0 14.0 48 0 Example 6

Example 5 was repeated with the polymer obtained in Example 3. The product obtained in each case had the following properties:

Example 7

Example 5 was repeated with the polymer obtained in Example 4. The product obtained in each case had the following properties:

Volume Aeroxide ^® CRC AAP (0.7 psi) Vortex (un- caking 3 h Alu 130 sieved)

 [g / g] [g / g] [%]

 [g] [s]

 0.5 38.5 13.4 56 84

1, 0 38.4 13.0 53 45

1, 5 38.9 12.3 54 21

Claims

claims

1 . Superabsorber with pyrogenic alumina.

2. Superabsorber according to claim 1, characterized in that it is at least 0.01

 Wt .-% and at most 6.0 wt .-% pyrogenic alumina.

3. Superabsorber according to claim 1 or 2, characterized in that the fumed alumina has a BET surface area of at least 20 and at most 200 m ² / g.

4. Superabsorber according to one of claims 1 to 3, characterized in that it is surface postcrosslinked.

5. Superabsorber according to one of claims 1 to 4, characterized in that it is coated with at least one salt of a polyvalent cation.

Superabsorber according to claim 5, characterized in that it is coated with an aluminum salt.

Superabsorber according to claim 5 or 6, characterized in that it is coated with a salt of a polyvalent cation with a hydroxycarboxylic acid.

Process for the preparation of a superabsorbent as defined in claims 1 to 7 by polymerization of a monomer mixture, drying of the polymer and optional surface postcrosslinking of the dried polymer and coating with a salt of a polyvalent cation, characterized in that the superabsorber is dried after drying and / or after surface postcrosslinking and optional coating with a salt of a polyvalent cation fumed alumina added.

Process according to Claim 8, characterized in that fumed aluminum oxide is added to the superabsorber after surface postcrosslinking and selective coating with a salt of a polyvalent cation.

A method according to claim 8 or 9, characterized in that polymerizing an aqueous solution of a monomer mixture containing:

 a) at least one ethylenically unsaturated, acid group-carrying monomer which is optionally present at least partially as a salt,

 b) at least one crosslinker,

 c) at least one initiator,

d) optionally one or more ethylenically unsaturated monomers which can be copolymerized with the monomers mentioned under a), and e) optionally one or more water-soluble polymers. An article for the absorption of liquids, comprising one of the superabsorbents defined in claims 1 to 7.