Absorbent polymer structure provided with an improved retention capacity and permeability

The invention relates to a method for producing an absorbent polymer structure, an absorbent polymer structure obtainable by this method, an absorbent polymer structure:

Superabsorbers are water-insoluble, crosslinked polymers which are able to absorb large amounts of aqueous liquids, in particular body fluids, preferably urine or blood, with the swelling and formation of hydrogels and to retain them under a certain pressure. Due to these characteristic properties, these polymers are mainly used for incorporation into sanitary articles, such as baby diapers, incontinence products or sanitary napkins.

The superabsorbers currently commercially available are essentially crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers in which the carboxyl groups are partially neutralized with sodium hydroxide solution or potassium hydroxide solution.

For aesthetic and environmental reasons, there is an increasing tendency to make sanitary articles smaller and thinner. In order to ensure a constant overall retention capacity of the sanitary articles, this requirement can only be met by reducing the proportion of large-volume fluff. As a result, the superabsorbent has further tasks with regard to the transport and distribution of liquid, which can be summarized as permeability properties.

In the case of superabsorbent materials, permeability means the ability to transport added liquids in the swollen state and to distribute them three-dimensionally. This process takes place in the swollen superabsorbent gel via capillary transport through spaces between the gel particles. Liquid transport through swollen superabsorbent particles themselves follows the laws of diffusion and is a very slow process, which plays no role in the distribution of the liquid in the use situation of the sanitary article. In the case of superabsorbent materials which cannot achieve capillary transport due to a lack of gel stability, embedding these materials in a fiber matrix ensures that the particles are separated from one another while avoiding the gel blocking phenomenon. In new generation diaper constructions there is little or no fiber material in the absorber layer to support the liquid transport. The superabsorbers used here must therefore have a sufficiently high stability in the swollen state so that the swollen gel still has a sufficient amount of capillary spaces through which liquid can be transported.

In order to obtain superabsorbent materials with high gel stability, on the one hand the degree of crosslinking of the polymer can be increased, which inevitably leads to a reduction in the swellability and the retention capacity. An optimized combination of different crosslinkers and comonomers, as in<patcit id="pcit0001" dnum="DE19646484"><text>DE 196 46 484</text></patcit> described may improve the permeability properties, but not to a level that allows, for example, the incorporation of a layer, which may consist only of superabsorbers, into a diaper construction.

Methods for post-treating the surface of polymer particles to improve the superabsorbent properties can also be used. Post-crosslinking of the absorbent polymer structure on the surface, contacting the surface with inorganic compounds or post-crosslinking of the surface in the presence of inorganic compounds are known from the prior art as surface treatment.

Describe like this <patcit id="pcit0002" dnum="EP0450923A"><text>EP-A-0 450 923</text></patcit>, <patcit id="pcit0003" dnum="EP0450922A"><text>EP-A-0 450 922</text></patcit>, <patcit id="pcit0004" dnum="DE3523617A"><text>DE-A-35 23 617</text></patcit>, <patcit id="pcit0005" dnum="US5140076A"><text>US 5,140,076</text></patcit> and <patcit id="pcit0006" dnum="US4734478A"><text>US 4,734,478</text></patcit> treating the surface of absorbent polymers by contacting the surface with inorganic compounds, such as finely divided silica, during or after the post-crosslinking of the surface. In addition to an increased absorption rate under pressure, this type of surface treatment also achieves an increased permeability of the absorbent polymers.

The <patcit id="pcit0007" dnum="DE3503458"><text>DE 35 03 458</text></patcit> describes a process for producing an improved absorbent resin in which a water-absorbent resin containing units of a monomer having a carboxyl group in the form of the free acid or a salt as a constituent component thereof absorb a crosslinking agent and water in the presence of a powder of a fine metal oxide leaves and the resulting mixture is heated with stirring, to cause resin crosslinking and water removal. Here absorbent resins with a good water absorption capacity are obtained, which at the same time have a good absorption rate.

<patcit id="pcit0008" dnum="US4535098A"><text>US 4,535,098</text></patcit> describes a process for increasing the gel strength of non-post-crosslinked superabsorbers by swelling absorbent polymers in the presence of a colloidally disperse, inorganic compound, such as a silica sol, or by producing an absorbent polymer in the presence of a colloidally disperse, inorganic compound.

<patcit id="pcit0009" dnum="DE19805447"><text>DE 198 05 447</text></patcit> discloses a method for post-crosslinking polyacrylonitrile hydrolyzates with bifunctional compounds and a simultaneous immobilization of silica in the surface structure of the superabsorbent polymer. The silica was brought into contact with the surface together with the crosslinking agent in a water / alcohol mixture. Immobilization of the silica is said to improve the<i>Absorbency under load</i> as well as a reduction in gel blocking.

<patcit id="pcit0010" dnum="DE19854575"><text>DE 198 54 575</text></patcit> describes the addition of alkali salts of silica before, during or after the polymerization or for the partial neutralization of the superabsorbent. This surface treatment improves permeability, which is mainly due to the reduced renting of the polymers due to the non-swellable additive.

<patcit id="pcit0011" dnum="US5147921A"><text>US 5,147,921</text></patcit> discloses the addition of a silica sol as an inert filler that can be dispersed in the monomer solution to be polymerized.

<patcit id="pcit0012" dnum="EP1211266A1"><text>EP 1211266 A1</text></patcit> describes a process for the preparation of superabsorbent polymers, in which crosslinked and / or uncrosslinked aqueous polyacrylonitrile emulsions based on homo- and / or copolymers by reaction with an alkali metal hydroxide solution in highly concentrated reaction mixtures adiabatically without the addition of mechanical or thermal energy at an initial temperature of 10 ° C to 40 ° C to react. In<patcit id="pcit0013" dnum="EP1211266A1"><text>EP 1211266 A1</text></patcit> there is also an optional surface modification to improve the anti-gel blocking properties. The modification in question provides that the ground and classified particles are treated in a water / alcohol mixture with an aldehyde crosslinking agent and in the presence of silica. An extremely large excess of methanol is used for this. Polymers based on polyacrylic acid are described in<patcit id="pcit0014" dnum="EP1211266A"><text>EP 1211266</text></patcit> however, it is not described and the use of silica sol is also not described.

<patcit id="pcit0015" dnum="WO0113841A1"><text>WO 01/13841 A1</text></patcit> describes an absorbent, crosslinked polymer for water or aqueous body fluids, based on optionally partially neutralized, monoethylenically unsaturated acid groups bearing monomers, the polymer containing cyclodextrin or cyclodextrin derivatives and silicon-rich zeolites at least partially covalently, ionically bound or enclosed therein. Flavith S 108 is used as the silicon-rich zeolite. In contrast, the use of silica sol is not intended.

<patcit id="pcit0016" dnum="JP6016822A"><text>JP 1994-16822</text></patcit> describes the post-treatment of the surface of absorbent polymers with an inorganic sol. In order to enable improved processability of the mixture which tends to form agglomerates, an organic solvent component is additionally added. Mono- and dimethyl ethers of diols or diols themselves are mentioned as organic solvent components. After drying, the absorbent polymers should have a higher gel stability, a lower tendency to gel blocking and an improved permeability for water in simple tests without pressure loading of the superabsorbers.

The prior art describes processes in which inorganic particles are either mixed dry with the superabsorbent or are introduced into the post-crosslinking process with the aid of large, in part organic, amounts of solvent in order to prevent agglomeration of the superabsorbent particles. However, these processes have the disadvantage that either large amounts of solvent have to be handled, which is undesirable for both economic and ecological reasons. In addition, superabsorbent polymers tend to agglomerate when mixed with large amounts of liquid, which can severely impair the processability within a continuous manufacturing process. A simple mixing with inorganic, finely divided substances, however, has disadvantages such as segregation or dusting. The addition of inorganic additives in aqueous solutions for post-crosslinking itself is difficult because the inorganic particles quickly settle again. In addition, inorganic dispersions are difficult to dose.

The presence of the finely divided, inorganic substances disclosed in the prior art results in an inhomogeneous distribution of the chemical postcrosslinker on the surface of the absorbent polymers and accordingly inhomogeneous postcrosslinking. This in turn means that superabsorbent polymers with an unsatisfactory overall performance are obtained, especially with regard to retention and permeability. A homogeneous distribution in the processes for surface treatment described in the prior art is at best possible by using large amounts of an aqueous or alcoholic solution containing the chemical crosslinking agent.

In general, the invention is based on the object of overcoming the disadvantages arising from the prior art.

Furthermore, it is an object of the invention to provide superabsorbent polymers which, as a combination of properties, combine not only a high absorption capacity under pressure, but also the usually opposing properties of high retention capacity and good permeability, in order to meet the requirements of modern hygiene articles, in particular diapers , Incontinence products or sanitary napkins to cater to absorbent polymers. In particular, these polymers should contain the smallest possible amounts of toxic monomers, such as acrylamide or acrylonitrile, which wash out when the superabsorbent polymers come into contact with body fluids and, in this way, for example when the superabsorbent polymers are used in diapers, come into contact with the skin of the diaper wearer can kick.

A further object on which the present invention is based was to provide hygiene articles, such as, for example, diapers, which, in comparison to the hygiene articles known from the prior art, are better able to retain body fluids taken up, take up liquids under pressure and when taking up liquids distribute them as quickly and evenly as possible in the hygiene article.

In addition, another object of the invention is to provide a method with which such absorbent polymers can be produced in a simple, continuous manner as small as possible amounts of organic solvents. In this manufacturing process, added inorganic auxiliaries should detach from the superabsorbent polymer in small amounts at most, which do not adversely affect the polymer properties. The solution used in this method for treating the surface of the absorbent polymer should be able to be handled like a single-phase system and should be able to be metered uniformly. The coated superabsorber should form only a minor amount of agglomerates in the course of the process and should be able to be fed to a continuously operating annealing step in a simple manner.

The above objects are achieved by a method for producing an absorbent polymer structure (Pa) by treating the outside of an untreated absorbent polymer structure (Pu1) which is based at least to 50% by weight on acrylic acid which is neutralized to at least 20 mol%, comprising the Steps:<ul id="ul0001" list-style="dash" compact="compact"><li>bringing the outer area of ​​the untreated, absorbent polymer structure (Pu1) into contact with an aqueous solution which is based on water and contains at least one, at least 50% by weight, based on the total amount of all components which are liquid in the aqueous solution and are liquid at room temperature chemical crosslinkers and - silica sol;</li><li>Heating the absorbent polymer structure, the outer region of which has been brought into contact with the aqueous solution, to a temperature in the range from 40 to 300 ° C., so that the outer region of the absorbent polymer structure is more cross-linked in comparison to the inner region and the inorganic compound in the outer region of the absorbent Polymer structure is at least partially immobilized.</li></ul>

The above objects are also achieved by a method for producing an absorbent polymer structure (Pa) by treating the outer region of an absorbent polymer structure (Pu2) which has not been treated with an inorganic compound in colloidally dispersed form and which is based at least 50% by weight on acrylic acid, which is too at least 20 mol% is neutralized, comprising the steps:<ul id="ul0002" list-style="dash" compact="compact"><li>bringing the outer region of the absorbent polymer structure (Pu2) into contact with an aqueous solution which is based on water to at least 50% by weight, based on the total amount of all components which are liquid in the aqueous solution and are at room temperature, containing at least one chemical crosslinking agent and silica sol;</li><li>Heating the absorbent polymer structure, the outer region of which has been brought into contact with the aqueous solution, to a temperature in the range from 40 to 300 ° C., so that the outer region of the absorbent polymer structure is more crosslinked in comparison to the inner region and the inorganic compound in the outer region of the absorbent Polymer structure is at least partially immobilized.</li></ul>

Absorbent polymer structures (Pa) according to the invention are fibers, foams or particles, fibers and particles being preferred and particles being particularly preferred. Absorbent polymer structures (Pa) in these forms are obtained by using fibers, foams or particles in a corresponding manner as the absorbent polymer structures (Pu1) or (Pu2).

Absorbent polymer fibers preferred according to the invention are dimensioned such that they can be incorporated in or as yarns for textiles and also directly in textiles. It is preferred according to the invention that the absorbent polymer fibers have a length in the range from 1 to 500, preferably 2 to 500 and particularly preferably 5 to 100 mm and a diameter in the range from 1 to 200, preferably 3 to 100 and particularly preferably 5 to 60 denier have.

Absorbent polymer particles which are particularly preferred according to the invention are dimensioned such that they have an average particle size in accordance with ERT 420.1-99 in the range from 10 to 3000, preferably 20 to 2000 and particularly preferably 150 to 850 μm.

The absorbent polymer structure (Pu1) or (Pu2) used in the method according to the invention is preferably a polymer structure which is based on<ul id="ul0003" list-style="none" compact="compact"><li>(α1 55 to 98.99 wt .-% and particularly preferably 70 to 98.79 wt .-% polymerized, ethylenically unsaturated, acid group-containing monomers or their salts or polymerized, ethylenically unsaturated monomers containing a protonated or quaternized nitrogen, or mixtures thereof , mixtures containing at least ethylenically unsaturated monomers containing acid groups, preferably acrylic acid, being particularly preferred,</li><li>(α2) 0-80% by weight, preferably 0-44.99% by weight and particularly preferably 0.1-44.89% by weight, of polymerized, monoethylenically unsaturated monomers copolymerizable with (α1),</li><li>(α3) 0.001-5% by weight, preferably 0.01-3% by weight and particularly preferably 0.01-2.5% by weight of one or more crosslinking agents,</li><li>(α4) 0-30% by weight, preferably 0-5% by weight and particularly preferably 0.1-5% by weight of a water-soluble polymer, and</li><li>(α5) 0-20% by weight, preferably 0 to 10% by weight and particularly preferably 0.1-8% by weight, of one or more auxiliaries, the sum of the amounts by weight (α1) to (α5) 100 % By weight.</li></ul>

The monoethylenically unsaturated monomers (α1) containing acid groups are partially neutralized. The monoethylenically unsaturated monomers containing acid groups are preferably neutralized to at least 25 mol%, particularly preferably to at least 50 mol% and moreover preferably to 50-80 mol%. In this regard, is on<patcit id="pcit0017" dnum="DE19529348"><text>DE 195 29 348</text></patcit> referred. Some or all of the neutralization can also be carried out after the polymerization. Neutralization can also be carried out using alkali metal hydroxides, alkaline earth metal hydroxides, ammonia and carbonates and bicarbonates. In addition, any other base is conceivable that forms a water-soluble salt with the acid. Mixed neutralization with different bases is also conceivable. Neutralization with ammonia and alkali metal hydroxides is preferred, particularly preferably with sodium hydroxide and with ammonia.

Furthermore, the free acid groups can predominate in a polymer, so that this polymer has a pH value in the acidic range. This acidic water-absorbing polymer can be at least partially neutralized by a polymer with free basic groups, preferably amine groups, which is basic in comparison to the acidic polymer. These polymers are known in the literature as<i>"Mixed-Bed Ion-Exchange Absorbent Polymers"</i> (MBIEA polymers) and are among others in the <patcit id="pcit0018" dnum="WO9934843A"><text>WO 99/34843</text></patcit> disclosed. At the revelation of the<patcit id="pcit0019" dnum="WO9934843A"><text>WO 99/34843</text></patcit> is hereby referred. In general, MBIEA polymers are a composition which contains, on the one hand, basic polymers which are able to exchange anions and, on the other hand, a polymer which is acidic in comparison with the basic polymer and which is able to exchange cations. The basic polymer has basic groups and is typically obtained by polymerizing monomers that carry basic groups or groups that can be converted to basic groups. These monomers are above all those which have primary, secondary or tertiary amines or the corresponding phosphines or at least two of the above functional groups. This group of monomers includes in particular ethylene amine, allylamine, diallylamine, 4-aminobutene, alkyloxycycline, vinylformamide, 5-aminopentene, carbodiimide, formaldacin, melamine and the like, and also their secondary or tertiary amine derivatives.

The monoethylenically unsaturated monomers (α1) containing acid groups are partially neutralized. The monoethylenically unsaturated monomers containing acid groups are preferably neutralized to at least 25 mol%, particularly preferably to at least 50 mol% and moreover preferably to 50-90 mol%. The monomers (α1) can also be neutralized before the polymerization. Neutralization can also be carried out using alkali metal hydroxides, alkaline earth metal hydroxides, ammonia and carbonates and bicarbonates. In addition, any other base is conceivable that forms a water-soluble salt with the acid. Mixed neutralization with different bases is also conceivable. Neutralization with ammonia or with alkali metal hydroxides is preferred, particularly preferably with sodium hydroxide or with ammonia.

Preferred monoethylenically unsaturated monomers (α1) containing acid groups are acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid_, α-cyanoacrylic acid, β-methylacrylic acid (crotonic acid), α-phenylacrylic acid, β-acryloxypropionic acid, sorbic acid, α-chlorosorbic acid, 2'-methylisocrotonic acid, cinnamic acid, p-chlorocinnamic acid, β-stearyl acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid Maleic acid, fumaric acid, tricarboxyethylene and maleic anhydride, acrylic acid and methacrylic acid being particularly preferred and acrylic acid being preferred.

In addition to these monomers containing carboxylate groups, preferred monoethylenically unsaturated, acid group-containing monomers (α1) are furthermore ethylenically unsaturated sulfonic acid monomers or ethylenically unsaturated phosphonic acid monomers.

Ethylenically unsaturated sulfonic acid monomers are preferred allylsulfonic acid or aliphatic or aromatic vinylsulfonic acids or acrylic or methacrylic sulfonic acids. Vinyl sulfonic acid, 4-vinylbenzyl sulfonic acid, vinyl toluenesulfonic acid and styrene sulfonic acid are preferred as aliphatic or aromatic vinyl sulfonic acids. As an acrylic or Methacrylic sulfonic acids are preferred sulfoethyl (meth) acrylate, sulfopropyl (meth) acrylate, 2-hydroxy-3-methacryloxypropylsulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid.

Also preferred are ethylenically unsaturated phosphonic acid monomers, such as vinylphosphonic acid, allylphosphonic acid, vinylbenzylphosphonic acid, (meth) acrylamidoalkylphosphonic acids, acrylamidoalkyldiphosphonic acids, phosphonomethylated vinylamines and (meth) acrylicphosphonic acid derivatives.

Preferred ethylenically unsaturated monomers (α1) containing a protonated nitrogen are dialkylaminoalkyl (meth) acrylates in protonated form, for example dimethylaminoethyl (meth) acrylate hydrochloride or dimethylaminoethyl (meth) acrylate hydrosulfate, and dialkylaminoalkyl (meth) acrylamides in protonated form, for example, dimethylaminoethyl (meth) acrylamide hydrochloride or dimethylaminoethyl (meth) acrylamide hydrosulfate is preferred.

Dialkylammoniumalkyl (meth) acrylates in quaternized form, for example trimethylammoniumethyl (meth) acrylate methosulfate or dimethylethylammoniumethyl (meth) acrylate ethosulfate and (meth) acrylamidoalkyl dialkylamines in quartemized form, for example, are ethylenically unsaturated monomers (α1) containing a quaternized nitrogen ) acrylamidopropyltrimethylammonium chloride and (meth) acrylamidopropyltrimethylammonium sulfate are preferred.

According to the invention, component (α1) consists of at least 50% by weight, preferably at least 70% by weight and moreover preferably at least 90% by weight, of monomers containing carboxylate groups. According to the invention, component (α1) consists of at least 50% by weight, preferably at least 70% by weight, of acrylic acid, which is neutralized to at least 20 mol%, particularly preferably to at least 50 mol%.

Acrylamides and methacrylamides are preferred as monoethylenically unsaturated monomers (α2) copolymerizable with (α1).

In addition to acrylamide and methacrylamide, possible (meth) acrylamides are alkyl-substituted (meth) acrylamides or aminoalkyl-substituted derivatives of (meth) acrylamide, such as N-methylol (meth) acrylamide, N, N-dimethylamino (meth) acrylamide, dimethyl (meth) acrylamide or diethyl (meth) acrylamide. Possible vinylamides are, for example, N-vinylamides, N-vinylformamides, N-vinylacetamides, N-vinyl-N-methylacetamides, N-vinyl-N-methylformamides, vinylpyrrolidone. Among these monomers, acrylamide is particularly preferred.

Furthermore, preferred monoethylenically unsaturated monomers (α2) which are copolymerizable with (α1) are water-dispersible monomers. Preferred water-dispersible monomers are acrylic acid esters and methacrylic acid esters, such as methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate or butyl (meth) acrylate, as well as methyl polyethylene glycol (meth) acrylate, methyl polyethylene glycol allyl ether, vinyl acetate, styrene and isobutylene .

Crosslinkers (α3) preferred according to the invention are compounds which have at least two ethylenically unsaturated groups within one molecule (crosslinker class I), compounds which have at least two functional groups which react with functional groups of the monomers (α1) or (α2) in a condensation reaction ( = Condensation crosslinker), can react in an addition reaction or in a ring opening reaction (crosslinker class II), compounds, which have at least one ethylenically unsaturated group and at least one functional group which can react with functional groups of the monomers (α1) or (α2) in a condensation reaction, in an addition reaction or in a ring-opening reaction (crosslinking class III), or polyvalent metal cations (crosslinking class IV). The compounds of crosslinking class I achieve a crosslinking of the polymers through the radical polymerization of the ethylenically unsaturated groups of the crosslinking molecule with the monoethylenically unsaturated monomers (α1) or (α2), while for the compounds of crosslinking class II and the polyvalent metal cations of crosslinking class IV crosslinking of the polymers by condensation reaction of the functional groups (crosslinker class II) or is achieved by electrostatic interaction of the polyvalent metal cation (crosslinker class IV) with the functional groups of the monomers (α1) or (α2). In the case of the compounds of crosslinking class III, the polymer is accordingly crosslinked both by radical polymerization of the ethylenically unsaturated group and by a condensation reaction between the functional group of the crosslinking agent and the functional groups of the monomers (α1) or (α2).

Preferred compounds of crosslinker class I are poly (meth) acrylic esters or poly (meth) acrylamides, which, for example, by the reaction of a polyol, such as ethylene glycol, propylene glycol, trimethylol propane, 1,6-hexanediol, glycerol, pentaerythritol, polyethylene glycol or polypropylene glycol, of an amino alcohol a polyalkylene polyamine, such as, for example, diethylene triamine or triethylene tetraamine, or an alkoxylated polyol with acrylic acid or methacrylic acid. Polyvinyl compounds, poly (meth) allyl compounds, (meth) acrylic acid esters of a monovinyl compound or (meth) acrylic acid esters of a mono (meth) allyl compound, preferably the mono (meth) allyl compounds of a polyol or an amino alcohol, are furthermore preferred as compounds of crosslinker class I. In this regard, is on<patcit id="pcit0020" dnum="DE19543366"><text>DE 195 43 366</text></patcit> and <patcit id="pcit0021" dnum="DE19543368"><text>DE 195 43 368</text></patcit> referred.

Examples of compounds of crosslinking class I are alkenyldi (meth) acrylates, for example ethylene glycol di (meth) acrylate, 1,3-propylene glycol di (meth) acrylate, 1,4-butylene glycol di (meth) acrylate, 1,3-butylene glycol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, 1,10-decanediol di (meth) acrylate, 1,12-dodecanediol di (meth) acrylate, 1,18-octadecanediol di (meth) acrylate, cyclopentanediol di (meth) acrylate, neopentyl glycol di ( meth) acrylate, methylene di (meth) acrylate or pentaerythritol di (meth) acrylate, Alkenyldi (meth) acrylamides, for example N-methyldi (meth) acrylamide, N, N'-3-methylbutylidenebis (meth) acrylamide, N, N '- (1,2-di-hydroxyethylene) bis (meth) acrylamide, N, N'-hexamethylene-bis (meth) acrylamide or N, N'-methylenebis (meth) acrylamide, polyalkoxydi (meth) acrylates, for example diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, dipropylene glycol di (meth ) acrylate, tripropylene glycol di (meth) acrylate or tetrapropylene glycol di (meth) acrylate, Bisphenol-A-di (meth) acrylate, ethoxylated bisphenol-A-di (meth) acrylate, benzylidinedi (meth) acrylate, 1,3-di (meth) acryloyloxypropanol-2, hydroquinone di (meth) acrylate, di (meth ) acrylate esters of trimethylolpropane, preferably ethoxylated, preferably ethoxylated, with 1 to 30 mol of alkylene oxide per hydroxyl group, thioethylene glycol di (meth) acrylate, thiopropylene glycol di (meth) acrylate, thiopolyethylene glycol di (meth) acrylate, thiopolypropylene glycol di (meth) acrylate, divinyl ether, for example 1,4-butanediol divinyl ether, divinyl esters, for example divinyl adipate, alkanedienes, for example butadiene or 1,6-hexadiene, divinylbenzene, di (meth) allyl compounds, for example di (meth) allyl phthalate or di (meth) allylsuccinate, homo- and copolymers of Di (meth) allyldimethylammonium chloride and homo- and copolymers of diethyl (meth) allylaminomethyl (meth) acrylate ammonium chloride, vinyl (meth) acrylic compounds, for example vinyl (meth) acrylate, (Meth) allyl (meth) acrylic compounds, for example (meth) allyl (meth) acrylate, ethoxylated (meth) allyl (meth) acrylate with 1 to 30 moles of ethylene oxide per hydroxyl group, di (meth) allyl esters of polycarboxylic acids, for example di (meth) allyl maleate, di (meth) allyl fumarate, di (meth) allyl succinate or di (meth) allyl terephthalate, compounds with 3 or more ethylenically unsaturated, free-radically polymerizable groups such as, for example, glycerol tri (meth) acrylate, (Meth) acrylate ester of glycerol oxyethylated with preferably 1 to 30 mol of ethylene oxide per hydroxyl group, trimethylolpropane tri (meth) acrylate, tri (meth) acrylate ester of the preferably oxyalkylated, preferably ethoxylated trimethylolpropane, trimethacrylamide, (meth) allylidenedi with 1 to 30 mol of alkylene oxide per hydroxyl group (meth) acrylate, 3-allyloxy-1,2-propanediol di (meth) acrylate, tri (meth) allyl cyanurate, tri (meth) allyl isocyanurate, pentaerythritol tetra (meth) acrylate, Pentaerythritol tri (meth) acrylate, (meth) acrylic acid ester of pentaerythritol oxyethylated with preferably 1 to 30 mol ethylene oxide per hydroxyl group, tris (2-hydroxyethyl) isocyanurate tri (meth) acrylate, trivinyl trimellitate, tri (meth) allylamine, di (meth) allyl alkyl amines, for example Di (meth) allylmethylamine, tri (meth) allylphosphate tetra (meth) allylethylenediamine, poly (meth) allyl ester, tetra (meth) allyloxiethan or tetra (meth) allylammonium halide.

Preferred compounds of crosslinker class II are compounds which have at least two functional groups which in a condensation reaction (= condensation crosslinker), in an addition reaction or in a ring opening reaction with the functional groups of the monomers (α1) or (α2), preferably with acid groups, of the monomers (α1) can react. These functional groups of the compounds of crosslinker class II are preferably alcohol, amine, aldehyde, glycidyl, isocyanate, carbonate or epichloride functions.

Examples of compounds of crosslinking class II include polyols, for example ethylene glycol, polyethylene glycols such as diethylene glycol, triethylene glycol and tetraethylene glycol, propylene glycol, polypropylene glycols such as dipropylene glycol, tripropylene glycol or tetrapropylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 2,4-pentanediol, 1,6-hexanediol, 2,5-hexanediol, glycerol, polyglycerol, trimethylolpropane, polyoxypropylene, oxyethylene-oxypropylene block copolymers, Sorbitan fatty acid esters, polyoxyethylene sorbitan, pentaerythritol, polyvinyl alcohol and sorbitol, aminoalcohols, for example ethanolamine, diethanolamine, triethanolamine or propanolamine, polyamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine or pentaethylenehexamine, polyglycidyl ether compounds such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol polyglycidyl ether, Pentareritritpolyglycidylether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, Hexandiolglycidylether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, diglycidyl phthalate, Adipinsäurediglycidylether, 1,4-phenylene-bis (2-oxazoline), glycidol, polyisocyanates, preferably diisocyanates, such as 2,4-toluene diisocyanate and hexamethylene diisocyanate, polyaziridine compounds such as 2,2-bishydroxymethylbutanol-tris [3- (1-aziridinyl) propionate], 1,6-hexamethylene diethylene urea and diphenylmethane-bis-4,4'-N, N'-diethylene urea, halogen epoxides, for example epichloro- and epibromohydrin and α-methylepichlorohydrin, alkylene carbonates such as 1,3-dioxolan-2-one (ethylene carbonate), 4- Methyl-1,3-dioxolan-2-one (propylene carbonate), 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl -1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4 , 6-dimethyl-1,3-dioxan-2-one, 1,3-dioxolan-2-one, Poly-1,3-dioxolan-2-one, polyquaternary amines such as condensation products of dimethylamines and epichlorohydrin. Further compounds of crosslinker class II are polyoxazolines such as 1,2-ethylene bisoxazoline, crosslinkers with silane groups such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltrimethoxysilane, oxazolidinones such as 2-oxazolidinone, bis- and poly-2-oxazolidinones and diglycol silicates.

Preferred compounds of class III are hydroxyl- or amino group-containing esters of (meth) acrylic acid, such as 2-hydroxyethyl (meth) acrylate, as well as hydroxyl- or amino group-containing (meth) acrylamides, or mono (meth) allyl compounds of diols.

The polyvalent metal cations of crosslinking class IV are preferably derived from mono- or polyvalent cations, the monovalent in particular from alkali metals such as potassium, sodium, lithium, lithium being preferred. Preferred divalent cations are derived from zinc, beryllium, alkaline earth metals, such as magnesium, calcium, strontium, magnesium being preferred. Other higher-value cations which can be used according to the invention are cations of aluminum, iron, chromium, manganese, titanium, zirconium and other transition metals, and also double salts of such cations or mixtures of the salts mentioned. Aluminum salts and alums and their different hydrates such as, for. B. AlCl₃ × 6H₂O, NaAl (SO₄)₂ × 12 H₂O, KAl (SO₄)₂ × 12 H₂O or Al₂(SO₄)₃ × 14-18 h₂O used.

Al are particularly preferred₂(SO₄)₃ and uses its hydrates as crosslinking class IV.

Preferred absorbent polymer structures (Pu1) or (Pu2) are polymer structures which are crosslinked by crosslinking agents of the following crosslinking classes or by crosslinking agents of the following combinations of crosslinking classes: I, II, III, IV, I II, I III, I IV, I II III, I II IV, I III IV, II III IV, II IV or III IV. The above combinations of crosslinking classes each represent a preferred embodiment of crosslinking agents of a polymer.

Further preferred embodiments of the absorbent polymer structures (Pu1) or (Pu2) are polymer structures which are crosslinked by any of the crosslinking agents of crosslinking class I mentioned above. Among them, water-soluble crosslinkers are preferred. In this context, N, N'-methylenebisacrylamide, polyethylene glycol di (meth) acrylates, triallylmethylammonium chloride, tetraallylammonium chloride and allylnonaethylene glycol acrylate prepared with 9 moles of ethylene oxide per mole of acrylic acid are particularly preferred.

As water-soluble polymers (α4), water-soluble polymers, such as partially or fully hydrolyzed polyvinyl alcohol, polyvinylpyrrolidone, starch or starch derivatives, polyglycols or polyacrylic acid can preferably be copolymerized in the absorbent polymer structures (Pu1) or (Pu2) according to the invention. The molecular weight of these polymers is not critical as long as they are water soluble. Preferred water-soluble polymers are starch or starch derivatives or polyvinyl alcohol. The water-soluble polymers, preferably synthetic such as polyvinyl alcohol, can also serve as a graft base for the monomers to be polymerized.

Auxiliaries (.alpha.5) can preferably be used in the absorbent polymer structures (Pu1) or (Pu2) used in the process according to the invention to include adjusting agents, surface-active agents, odor binders, fillers or antioxidants.

It is particularly preferred according to the invention that the absorbent polymer structure (Pu1) or (Pu2) is a crosslinked polyacrylate in particulate form, which is obtained by polymerizing an acrylic acid and optionally one of the above-mentioned crosslinking agents in aqueous solution, containing the acrylic acid in an amount in a range from 5 to 80% by weight, preferably 10 to 70% by weight and particularly preferably 20 to 50% by weight, based on the weight of the aqueous solution, and subsequent comminution of the polymer gel obtained, drying of the comminuted gel and optionally further grinding of the dried polymer gel was obtained. The absorbent polymer structures obtained in this way are preferably characterized by a water content of 0.5 to 25% by weight, preferably 1 to 10% by weight.

In the processes according to the invention, the absorbent polymer structures (Pu1) or (Pu2) are based to at least 50% by weight, preferably to at least 75% by weight and moreover preferably to at least 90% by weight, on acrylic acid, which is at least 20 mol -%, particularly preferably at least 50 mol% is neutralized.

It is further preferred that the absorbent polymer structure (Pu1) or (Pu2) is not based on polyacrylonitrile emulsions. It is preferred that the absorbent polymer structures (Pu1) or (Pu2) less than 37 mol%, particularly preferably less than 20 mol%, more preferably less than 10 mol% and moreover even more preferably less than 5 mol% based on acrylamide and / or acrylonitrile monomers. In this connection it is further preferred that the absorbent polymer structure (Pu1) or (Pu2) has a proportion of soluble monomers or polymers based on acrylonitrile and / or acrylamide monomers of less than 1,000 ppm, particularly preferably less than 500 ppm, moreover preferably have less than 100 ppm and moreover even less preferably less than 10 ppm.

The absorbent polymer structure (Pu1) or (Pu2) can be produced from the aforementioned monomers and crosslinkers by various polymerization methods. For example, in this context, bulk polymerization, which is preferably carried out in kneading reactors such as extruders, solution polymerization, spray polymerization, inverse emulsion polymerization and inverse suspension polymerization. The solution polymerization is preferably carried out in water as the solvent. The solution polymerization can be carried out continuously or batchwise. A wide range of possible variations with regard to reaction conditions such as temperatures, type and amount of the initiators and also of the reaction solution can be found in the prior art. Typical processes are described in the following patents:<patcit id="pcit0022" dnum="US4286082A"><text>US 4,286,082</text></patcit>, <patcit id="pcit0023" dnum="DE2706135"><text>DE 27 06 135</text></patcit>, <patcit id="pcit0024" dnum="US4076663A"><text>US 4,076,663</text></patcit>, <patcit id="pcit0025" dnum="DE3503458"><text>DE 35 03 458</text></patcit>, <patcit id="pcit0026" dnum="DE4020780"><text>DE 40 20 780</text></patcit>, <patcit id="pcit0027" dnum="DE4244548"><text>DE 42 44 548</text></patcit>, <patcit id="pcit0028" dnum="DE4323001"><text>DE 43 23 001</text></patcit>, <patcit id="pcit0029" dnum="DE4333056"><text>DE 43 33 056</text></patcit>, <patcit id="pcit0030" dnum="DE4418818"><text>DE 44 18 818</text></patcit>.

Another possibility for producing the absorbent polymer structures (Pu1) or (Pu2) is to first produce uncrosslinked, in particular linear polymers, preferably by radical means, from the aforementioned monoethylenically unsaturated monomers (α1) or (α2) and then crosslinking them Reagents (α3), preferably those of classes II and IV to implement. This variant is preferably used when the polymer structures are first to be processed in shaping processes, for example into fibers, foils or other flat structures, such as woven fabrics, knitted fabrics, spunbond or nonwovens, and are to be crosslinked in this form.

The polymerization is initiated by an initiator, as is generally the case. All initiators which form free radicals under the polymerization conditions and are customarily used in the production of superabsorbers can be used as initiators for initiating the polymerization. It is also possible to initiate the polymerization by the action of electron beams on the polymerizable, aqueous mixture. However, the polymerization can also be initiated in the absence of initiators of the type mentioned above by exposure to high-energy radiation in the presence of photoinitiators. Polymerization initiators can be dissolved or dispersed in a solution of monomers according to the invention. All radical-decomposing compounds known to those skilled in the art are suitable as initiators. This includes in particular peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds and the so-called redox catalysts. The use of water-soluble catalysts is preferred. In some cases it is advantageous to use mixtures of different polymerization initiators. Among these mixtures, those of hydrogen peroxide and sodium or potassium peroxodisulfate are preferred, which can be used in any conceivable quantitative ratio. Suitable organic peroxides are preferably acetylacetone peroxide, methyl ethyl ketone peroxide, t-butyl hydroperoxide, cumene hydroperoxide, t-amyl perpivalate, t-butyl perpivalate, t-Butylpemeohexonat, t-butyl isobutyrate, t-butyl per-2-ethylhexenoat, t-butyl perisononanoate, t-butyl permaleate, t-butyl perbenzoate , t-butyl 3,5,5-tri-methyl hexanoate and amyl pemeodecanoate. Also preferred as polymerization initiators are: azo compounds such as 2,2'-azobis- (2-amidinopropane) dihydrochloride, azo-bis-amidinopropane dihydrochloride, 2,2'-azobis- (N, N-dimethylene) isobutyramidine dihydrochloride , 2- (carbamoylazo) isobutyronitrile and 4,4'-azobis (4-cyanovaleric acid). The compounds mentioned are used in customary amounts, preferably in a range from 0.01 to 5, preferably from 0.1 to 2 mol%, in each case based on the amount of the monomers to be polymerized.

The redox catalysts contain as oxidic component at least one of the above per-compounds and as reducing component preferably ascorbic acid, glucose, sorbose, mannose, ammonium or alkali metal hydrogen sulfite, sulfate, thiosulfate, hyposulfite or sulfide, metal salts such as iron-II -ions or silver ions or sodium hydroxymethyl sulfoxylate. Ascorbic acid or sodium pyrosulfite is preferably used as the reducing component of the redox catalyst. Based on the amount of monomers used in the polymerization, 1 × 10^-5 to 1 mol% of the reducing component of the redox catalyst and 1 × 10^-5 up to 5 mol% of the oxidizing component of the redox catalyst used. Instead of, or in addition to, the oxidizing component of the redox catalyst, one or more, preferably water-soluble, azo compounds can be used.

If the polymerization is triggered by exposure to high-energy radiation, so-called photoinitiators are usually used as initiators. These can be, for example, so-called α splitters, H-abstracting systems or also azides. Examples of such initiators are benzophenone derivatives such as Michlers ketone, phenanthrene derivatives, fluorene derivatives, anthraquinone derivatives, thioxanone derivatives, coumarin derivatives, benzoin ethers and their derivatives, azo compounds such as the radical formers mentioned above, substituted hexaarylbisimidazoles or acylphosphine oxides. Examples of azides are: 2- (N, N-dimethylamino) ethyl 4-azidocinnamate, 2- (N, N-dimethylamino) ethyl 4-azidonaphthyl ketone, 2- (N, N-dimethylamino) ethyl 4 -azidobenzoate, 5-azido-1-naphthyl-2 '- (N, N-dimethylamino) ethylsulfone, N- (4-sulfonylazidophenyl) maleimide, N-acetyl-4-sulfonylazidoaniline, 4-sulfonylazidoaniline, 4-azidoaniline, 4- Azidophenacyl bromide, p-azidobenzoic acid, 2,6-bis (p-azidobenzylidene) cyclohexanone and 2,6-bis (p-azidobenzylidene) -4-methylcyclohexanone. If used, the photoinitiators are usually used in amounts of from 0.01 to 5% by weight, based on the monomers to be polymerized.

A redox system consisting of hydrogen peroxide, sodium peroxodisulfate and ascorbic acid is preferably used according to the invention. In general, azo compounds are preferred as initiators according to the invention, with azo-bis-amidinopropane dihydrochloride being particularly preferred. As a rule, the polymerization is initiated with the initiators in a temperature range from 30 to 90 ° C.

The polymer gel is dried to a water content of 0.5 to 25% by weight, preferably 1 to 10% by weight, at temperatures which are usually in the range from 100 to 200.degree.

In a preferred embodiment, the absorbent polymer structure (Pu1) or (Pu2) used in the process according to the invention exhibits at least one of the following properties (ERT = <i>EDANA Recommended Test):</i><ol id="ol0001" compact="compact" ol-style=""><li>(A) the maximum absorption of 0.9% by weight of NaCl solution according to ERT 440.1-99 is in a range from at least 10 to 1000, preferably from 15 to 500 and particularly preferably from 20 to 300 g / g,</li><li>(B) the proportion extractable according to ERT 470.1-99 with 0.9% by weight of aqueous NaCl solution is less than 30, preferably less than 20 and particularly preferably less than 10% by weight, based on the absorbent polymer structure ( Pu1) or (Pu2),</li><li>(C) the bulk density according to ERT 460.1-99 is in the range from 300 to 1000, preferably 310 to 800 and particularly preferably 320 to 700 g / l,</li><li>(D) the pH according to ERT 400.1-99 of 1 g of the absorbent polymer structure (Pu1) or (Pu2) in 11 water is in the range from 4 to 10, preferably from 5 to 9 and particularly preferably from 5.5 to 7 5</li><li>(E) the CRC value according to ERT 441.1-99 is in the range from 10 to 100, preferably 15 to 80 and particularly preferably 20 to 60 g / g.</li></ol>

The property combinations of two or more of these properties resulting from the above properties each represent preferred embodiments of the method according to the invention. Also particularly preferred as embodiments according to the invention are methods in which the absorbent polymer structure (Pu1) or (Pu2) shows the properties or property combinations shown below as letters or combinations of letters: A, B, C, D, E, AB, AC, AD, AE , ABC, ABD, ABE, ACD, ACE, ADE, ABCD, ABCE, ABDE, ACDE, ABCDE.

The contacting of the absorbent polymer structure (Pu1) or (Pu2) with the aqueous solution is preferably carried out in the process according to the invention by thorough mixing of the aqueous solution with the absorbent polymer structure (Pu1) or (Pu2). The aqueous solution is preferably essentially free of organic solvents, in particular free of polyhydric alcohols and polyalkylene glycol ethers, particularly preferably free of diethylene glycol monomethyl ether and 1,3-butanediol. An aqueous solution is understood to mean a solution which comprises at least 50% by weight, particularly preferably at least 60% by weight, more preferably at least 70% by weight and moreover still more preferably at least 90% by weight. %, based in each case on the total amount of all components present in the aqueous solution which are liquid at room temperature, based on water.

The chemical crosslinker is contained in the aqueous solution containing the silica sol from the outset. However, it is also possible for the chemical crosslinker and the silica sol to be brought into contact separately, but preferably at the same time with the absorbent polymer structure (Pu1) or (Pu2). In this case, preferably two separate solutions, one containing the chemical crosslinker and the other the silica sol, are preferably mixed simultaneously with the absorbent polymer structure (Pu1) or (Pu2), but ensuring a homogeneous distribution of the chemical crosslinker and the silica sol have to be.

Suitable mixing units for applying the components are e.g. B. the Patterson-Kelley mixer, DRAIS turbulence mixer, Lödige mixer, Ruberg mixer, screw mixer, plate mixer and fluidized bed mixer, as well as continuously operating vertical mixers in which the polymer structure is mixed at high frequency by means of rotating knives (Schugi mixer). The absorbent polymer structure (Pu1) or (Pu2) is preferably in the process according to the invention preferably at most 20% by weight, particularly preferably at most 15% by weight, furthermore preferably at most 10% by weight, and even more preferably with at most 5% by weight of water and most preferably with less than 3% by weight, in each case based on the weight of the absorbent polymer structure (Pu1) or (Pu2).

If absorbent polymer structures (Pu1) or (Pu2) are used in the form of preferably spherical particles, it is further preferred according to the invention that this is brought into contact in such a way that only the outer region, but not the inner region, of the particulate absorbent polymer structure with the Silica sol are brought into contact. In this context, the outer region of the polymer structures is preferably understood to be that region which is characterized in that the distance of each spatial point lying in this region from the center of the particle is at least 50%, particularly preferably at least 75%, more preferably at least 90% and above more preferably at least 95% of the radius of the particulate absorbent polymer structures. The inhomogeneous immobilization of the silica sol on the polymer structures achieved in this way is achieved according to the invention in that dry polymer structures are brought into contact with the aqueous solution and, in addition, only small amounts of water are used such that absorption occurs only in the outer region of the absorbent polymer structures the aqueous liquid comes.

It is further preferred in the process according to the invention that at least 30% by weight, particularly preferably at least 60% by weight and moreover preferably at least 90% by weight of the colloidally disperse inorganic compound have a particle size in the range from 1 to 100, preferably from 5 to 80 and more preferably from 6 to 50 nm.

According to the method according to the invention, the inorganic compound is preferably used in an amount of 0.001 to 10% by weight, particularly preferably 0.01 to 5% by weight and more preferably 0.05 to 1.5% by weight, based on the absorbent polymer structure (Pu1) or (Pu2), brought into contact with the absorbent polymer structure (Pu1) or (Pu2).

Silica sol is used as the inorganic compound, and all water-insoluble, inorganic compounds can be used, from which stable, colloidally disperse, preferably single-phase, aqueous solutions can be obtained which at 20 ° C. and normal pressure over a period of at least 6 hours, preferably at least 24 hours and particularly preferably at least 72 hours up to 6 months, show no phase separation, such as the settling of a solid, inorganic precipitate.

A colloidally disperse solution is preferably understood to mean a solution which contains particles with a particle diameter in a range from 100-1000 Å (10th^-4 until 10^-5 cm) contains. These solutions have the property of scattering a light beam sent through the solution in all directions so that the path of the light beam can be followed through the colloidally dispersed solution (Tyndall effect, see here Hollemann-Wiberg,<i>Textbook of inorganic chemistry,</i> 91.-100. Edition, de Gruyter-Verlag, page 765).

Particles containing polysilicic acid in the form of silica sol are used as the colloidally disperse inorganic compound in the process according to the invention. A colloidally disperse solution containing such particles (silica sol) can be obtained, for example, by carefully acidifying sodium silicate solutions which react alkaline as a result of hydrolysis, or by dissolving molecular silica in water and possibly subsequently stabilizing the resulting colloidally dispersed solution. The exact preparation of such silica sols is known to the person skilled in the art and is described, for example, in Jander-Blasius,<i>"Textbook of analytical and preparative inorganic chemistry"</i> S. Hirzel Verlag, Stuttgart.

In addition to the colloidally disperse silica, iron (III) oxide hydrate brine, tin (IV) oxide hydrate brine or brine based on silver halides, in particular silver chloride, can also be used with particular preference as the colloidally disperse inorganic compound.

Chemical crosslinkers which are contained in the aqueous solution in the process according to the invention are preferably understood to mean compounds which have at least two functional groups which can react with functional groups of a polymer in a condensation reaction (= condensation crosslinker), in an addition reaction or in a ring opening reaction or polyvalent metal cations, which enable crosslinking of the polymer by means of electrostatic interaction between the polyvalent metal cation and the functional groups of a polymer. Preferred chemical crosslinkers for postcrosslinking the outer region of the absorbent polymer structure (Pu1) or (Pu2) - also called "postcrosslinker" - are those which were mentioned in connection with the crosslinkers (α3) as crosslinkers of crosslinking classes II and IV.

Among these compounds are useful as postcrosslinkers particularly preferred condensation cross such as diethylene glycol, triethylene glycol, polyethylene glycol, glycerol, polyglycerol, propylene glycol, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymers, sorbitan fatty acid esters, polyoxyethylene sorbitan, trimethylolpropane, pentaerythritol, polyvinyl alcohol, sorbitol, 1.3 -Dioxolan-2-one (ethylene carbonate), 4-methyl-1,3-dioxolan-2-one (propylene carbonate), 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl 1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, 1,3-dioxolan-2-one, poly-1,3-dioxolan-2-one.

Ethylene carbonate is particularly preferably used as postcrosslinker.

The postcrosslinker is preferably used in the process according to the invention in an amount in the range from 0.01 to 30, particularly preferably 0.1 to 20 and moreover preferably from 0.3 to 5% by weight, based on the absorbent polymer structure (Pu1) or (Pu2) used.

After the chemical crosslinker and the aqueous solution containing silica sol have been brought into contact with the absorbent polymer structure (Pu1) or (Pu2), the postcrosslinking reaction takes place in the process according to the invention by heating the absorbent polymer structure to temperatures in the range from 40 to 300 ° C., preferably from 80 to 250 ° C and particularly preferably from 150 to 220 ° C. The optimal duration of the reheating can easily be determined for the individual types of crosslinkers and colloidally disperse inorganic compounds. It is limited if the desired property profile of the super absorber is destroyed again as a result of heat damage. The thermal treatment can be carried out in conventional dryers or ovens, examples being rotary tube ovens, fluidized bed dryers, plate dryers, paddle dryers or infrared dryers.

It is preferred according to the invention that, as a result of the thermal treatment, the outer region of the absorbent polymer structure is crosslinked more than the inner region and that the inorganic compound is at least partially immobilized in the outer region by the thermal treatment. In this context, it is further preferred that the radius of the outer region is smaller than three times the value of the radius of the inner region.

In another embodiment of the method according to the invention, the outer region of the absorbent polymer structures is brought into contact before or after, preferably after, the contact with the aqueous solution containing the chemical crosslinking agent and the silica sol with a compound containing Al³⁺-Ions contacted. It is preferred that the compound containing Al³⁺Ions in an amount in a range from 0.01 to 30% by weight, particularly preferably in an amount in a range from 0.1 to 20% by weight and moreover preferably in an amount in a range from 0, 3 to 5 wt .-%, each based on the weight of the absorbent polymer structures, is brought into contact with the polymer structures.

Contacting the outer area of ​​the absorbent polymer structures with the Al³⁺-Ion-containing compound is preferably carried out by mixing the absorbent polymer structure (Pa) with the compound under dry conditions or else by having the absorbent polymer structure (Pa) with a fluid comprising a solvent, preferably water, water-miscible organic solvents such as about methanol or ethanol or mixtures of at least two of them, and the Al³⁺-Ion-containing compound are brought into contact, the contacting preferably being carried out by spraying the polymer particles with the fluid and mixing. In this context, it is further preferred that the contacting of the absorbent polymer structure (Pa) with the fluid containing the Al³⁺-Ion-containing compound is carried out in a two-step process. The two-stage process comprises a first mixing process, in which a plurality of absorbent polymer structures are mixed with the fluid, and a second mixing process, in which the fluid is homogenized inside the polymer particles, the polymer particles being mixed at a rate in the first mixing process that the kinetic energy of the individual polymer particles is on average greater than the adhesive energy between the individual polymer particles, and the polymer particles in the second mixing process are mixed at a slower rate than in the first mixing process.

By treating the absorbent polymer structures (Pa) with the fluid containing the Al³⁺-Ion-containing compound by the two-step process described above, absorbent polymer structures with improved absorption properties can be obtained.

Preferably, the Al³⁺Compound containing ions without taking crystal water into account in an amount in a range from 0.1 to 50% by weight, particularly preferably in an amount in a range from 1 to 30% by weight, in each case based on the total weight of the fluid, contained in the fluid. It is further preferred that the fluid in an amount in a range from 0.01 to 15 wt .-%, particularly preferably in an amount in a range from 0.05 to 6 wt .-%, each based on the weight of the absorbent polymer structure (Pa) is brought into contact with the absorbent polymer structure (Pa).

Preferably Al³⁺Compounds containing ions are AlCl₃ × 6H₂O, NaAl (SO₄)₂ × 12 H₂O, KAl (SO₄)₂ × 12 H₂O or Al₂(SO₄)₃× 14-18 h₂O.

The present invention further relates to absorbent polymer structures (Pa) which can be obtained by the inventive method described above.

In addition, the invention relates to an absorbent polymer structure (Pa) comprising an inner region and an outer region surrounding the inner region, the outer region being more cross-linked than the inner region, in the outer region, preferably only in the outer region and not in the inner region, an inorganic compound being at least partially immobilized and wherein the absorbent polymer structure (Pa) has at least one of the following properties:<ul id="ul0004" list-style="none" compact="compact"><li>(β1) with a CRC according to ERT 441.1-99 &lt;26 g / g an SFC of at least 80 · 10^-7 , preferably of at least 100 · 10^-7 and particularly preferably of at least 120 · 10^-7 cm³· S · g^-1,</li><li>(β2) with a CRC according to ERT 441.1-99 in the range ≥26 to &lt;27 g / g an SFC of at least 70 · 10^-7, preferably of at least 90 · 10^-7 and particularly preferably of at least 110 · 10^-7 cm³· S · g^-1,</li><li>(β3) with a CRC according to ERT 441.1-99 in the range ≥27 to &lt;28 g / g an SFC of at least 60 · 10^-7, preferably of at least 80 · 10^-7 and particularly preferably of at least 100 · 10^-7 cm³· S · g^-1,</li><li>(β4) with a CRC according to ERT 441.1-99 in the range ≥28 to &lt;29 g / g an SFC of at least 45 · 10^-7, preferably of at least 65 · 10^-7 and particularly preferably of at least 85 · 10^-7 cm³· S · g^-1,</li><li>(β5) with a CRC according to ERT 441.1-99 in the range ≥29 to &lt;30 an SFC of at least 30 · 10^-7, preferably of at least 50 · 10^-7 and particularly preferably of at least 70 · 10^-7 cm³· S · g^-1,</li><li>(β6) with a CRC according to ERT 441.1-99 in the range ≥30 to &lt;31 an SFC of at least 20 · 10^-7, preferably of at least 40 · 10^-7 and particularly preferably of at least 60 · 10^-7 cm³· S · g^-1,</li><li>(β7) with a CRC according to ERT 441.1-99 in the range ≥31 an SFC of at least 10 · 10^-7, preferably of at least 20 · 10^-7 and particularly preferably of at least 30 · 10^-7 cm³· S · g^-1.</li></ul>

The property combinations of two or more of these properties resulting from the above properties each represent preferred embodiments of the absorbent polymer structure (Pa) according to the invention. Also particularly preferred as embodiments of the invention is an absorbent polymer structure (Pa) which shows the properties or combinations of properties shown below as letters or letter combinations: β1, β2, β3, β4, β5, β6, β7, where β2, β3, β4, β5 and β6 are particularly preferred.

It is further preferred according to the invention that the absorbent polymer structure (Pa) has a<i>Absorbency against Pressure</i> (AAP) according to ERT 442.1-99 at a pressure of 50 g / cm² of at least 18 g / g, particularly preferably at least 20 g / g and furthermore particularly preferably of at least 22 g / g.

It is further preferred in the inventive absorbent polymer structure that the radius of the outer region is smaller than twice the value of the radius of the inner region.

In a particularly preferred embodiment of the absorbent polymer structures (Pa) according to the invention, the outer region of the polymer structures is preferably understood to be that region which is characterized in that the distance of each spatial point in this region from the center of the particle is at least 50%, particularly preferably at least 75% , more preferably at least 90% and more preferably at least 95% of the radius of the particulate absorbent polymer structures.

The inorganic compound, which is at least partially immobilized in the outer region of the absorbent polymer structure (Pa) according to the invention, can be any water-insoluble, inorganic compound from which stable, colloidally disperse aqueous solutions can be obtained.

A particularly preferred inorganic compound which is at least partially immobilized in the outer region of the absorbent polymer structure (Pa) according to the invention is a condensate of polysilicic acids.

It is further preferred that the above-mentioned features of the absorbent polymer structures (Pa) according to the invention also apply to the absorbent polymer structures (Pa) obtainable by the method according to the invention mentioned at the beginning.

According to an embodiment of the method according to the invention and of the absorbent polymer structures (Pa) according to the invention, it is preferred that the values ​​of features according to the invention given only with a lower limit have an upper limit that is 20 times, preferably 10 times and particularly preferably 5th -fold the most preferred lower limit value.

The invention will now be explained in more detail on the basis of non-limiting examples.

EXAMPLES

PRODUCTION OF THE UNTREATED ABSORBENT POLYMER FABRIC (PU1)

Powder A

A monomer solution consisting of 280 g of acrylic acid, 70 mol% of which was neutralized with sodium hydroxide solution, 466.8 g of water, 1.4 g of polyethylene glycol 300 diacrylate and 1.68 g of allyloxypolyethylene glycol acrylic acid ester is freed from the dissolved oxygen by flushing with nitrogen and dissolved the start temperature cooled from 4 ° C. After the start temperature had been reached, the initiator solution (0.1 g of 2,2'-azobis-2-amidinepropane dihydrochloride in 10 g of H₂0.3 g of sodium peroxydisulfate in 10 g of H₂0.07 g of 30% ge hydrogen peroxide solution in 1 g of H₂O and 0.015 g ascorbic acid in 2 g H₂O) added. After the final temperature of about 100 ° C was reached, the resulting gel was crushed and dried at 150 ° C for 90 minutes. The dried polymer was roughly crushed, ground and sieved onto a powder with a particle size of 150 to 850 μm.

Powder A has a retention capacity of 28.8 g / g.

Powder B

A monomer solution consisting of 280 g of acrylic acid, 70 mol% of which was neutralized with sodium hydroxide solution, 467.6 g of water, 0.98 g of polyethylene glycol 300 diacrylate and 1.26 g of allyloxypolyethylene glycol acrylic acid ester is freed from the dissolved oxygen by flushing with nitrogen and dissolved the start temperature cooled from 4 ° C. After the start temperature had been reached, the initiator solution (0.1 g of 2,2'-azobis-2-amidinepropane dihydrochloride in 10 g of H₂0.3 g of sodium peroxydisulfate in 10 g of H₂O 0.07 g 30% ge hydrogen peroxide solution in 1 g H₂O and 0.015 g ascorbic acid in 2 g H₂O) added. After the final temperature of about 100 ° C was reached, the resulting gel was crushed and dried at 150 ° C for 90 minutes. The dried polymer was roughly crushed, ground and sieved onto a powder with a particle size of 150 to 850 μm.

Powder B has a retention capacity of 31.2 g / g.

Powder C

A monomer solution consisting of 280 g of acrylic acid, 70% of which was neutralized with sodium hydroxide solution, 468.6 g of water, 0.42 g of polyethylene glycol 300 diacrylate and 0.84 g of allyloxypolyethylene glycol acrylic acid ester is freed from the dissolved oxygen by flushing with nitrogen and dissolved the start temperature cooled from 4 ° C. After the start temperature had been reached, the initiator solution (0.1 g of 2,2'-azobis-2-amidinepropane dihydrochloride in 10 g of H₂0.3 g of sodium peroxydisulfate in 10 g of H₂0.07 g of 30% ge hydrogen peroxide solution in 1 g of H₂O and 0.015 g ascorbic acid in 2 g H₂O) added. After the final temperature of about 100 ° C was reached, the resulting gel was crushed and dried at 150 ° C for 90 minutes. The dried polymer was roughly crushed, ground and sieved onto a powder with a particle size of 150 to 850 μm.

Powder C has a retention capacity of 37.1 g / g.

The amounts given in the examples below, in which the individual components, such as the postcrosslinker, the water or the silica sol, are used in the treatment of the outer area of ​​the untreated, absorbent polymer structure (Pu1), are amounts based on the weight of the untreated to understand absorbent polymer structure (Pu1).

INFLUENCE OF THE TREATMENT OF THE EXTERIOR OF THE TREATED ABSORBENT POLYMER FABRIC (PU1) ON RETENTION, PERMEABILITY AND ABSORPTION UNDER PRESSURE

Example 1:

50 g Powder A is mixed using a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate, 0.42 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 1.08 g of water mixed with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Example 2:

50 g Powder A is mixed using a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate, 0.84 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 0.66 g of water mixed with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Example 3:

50 g Powder B is mixed with a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate, 0.42 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 1.08 g of water mixed with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Example 4:

50 g Powder B is mixed with a solution of 0.5 g ethylene carbonate, 0.84 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 0.66 g of water mixed with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Example 5:

50 g powder C is mixed with a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate, 0.42 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 1.08 g of water mixed with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Comparative Example 1:

50 g powder A is mixed using a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate and 1.5 g water with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Comparative Example 2:

50 g powder B is mixed using a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate and 1.5 g water with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Comparative Example 3:

The post-crosslinked polymer structure obtained in Comparative Example 2 is mixed with 0.84 g of silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 0.16 g of water mixed with vigorous stirring. The product is then not subjected to an annealing step.

Comparative Example 4:

The post-crosslinked polymer structure obtained in Comparative Example 2 is mixed with 0.84 g of silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 0.16 g of water mixed with vigorous stirring and then for 60 min. heated in an oven heated to 100 ° C.

Comparative Example 5:

50 g Powder B is made using a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate, 0.125 g Aerosil^® (pyrogenic silica from Degussa AG) and 2 g of water mixed with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C. To make the suspension of Aerosil^® increased amounts of water were required in water. However, no easily metered suspension could be obtained because the Aerosil^® settles very quickly and a homogeneous dosage to powder B is not possible. The coated polymer tends to form lumps and is inhomogeneous.

Comparative Example 6:

50 g powder C is mixed using a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate and 1.5 g water with vigorous stirring and then for 30 min. heated in an oven heated to 180 ° C.

Comparative Example 7:

50 g powder B is mixed with a solution of 0.25 g diethylene glycol monomethyl ether, 0.25 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 1.25 g of water mixed with vigorous stirring and then for 3 min. heated in an oven heated to 120 ° C. This treatment corresponds to the treatment according to Example 1 of the<patcit id="pcit0031" dnum="JP6016822A"><text>JP 1994/16822</text></patcit>.

Comparative Example 8:

50 g Powder B is mixed using a Krups kitchen mixer with a solution of 0.25 g 1,3-butanediol, 0.25 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 1.25 g of water mixed with vigorous stirring and then for 3 min. heated in an oven heated to 120 ° C. This treatment corresponds to the treatment according to Example 2 of the<patcit id="pcit0032" dnum="JP6016822A"><text>JP 1994/16822</text></patcit>.

The properties of the absorbent polymer structures obtained in Examples 1 to 4 and in Comparative Examples 1 to 8 are summarized in Table 1 below.

The absorbent polymer structures produced according to the invention show a significant increase in permeability (SFC) with constant or even increased retention compared to products whose outer region has been crosslinked in the absence of a silica sol (Examples 1 to 4, Comparative Examples 1 and 2). An aftertreatment of the already crosslinked polymer structures with silica sol does not lead to the desired result regardless of the subsequent thermal treatment (comparative example 3, 4).

The addition of Aerosil 200^® the post-crosslinking does not lead to comparably good superabsorber characteristics (comparative example 5). There are also increased amounts of Aerosil 200^® can no longer be dispersed in an acceptable amount of water and are therefore no longer dispersible.

Comparative examples 7 and 8 show that in the examples according to the invention the untested <patcit id="pcit0033" dnum="JP6016822A"><text>JP 1994/16822</text></patcit> the polymers cannot perform well in terms of their permeability and retention.<tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="89mm" /><colspec colnum="2" colname="col2" colwidth="33mm" /><colspec colnum="3" colname="col3" colwidth="28mm" /><colspec colnum="4" colname="col4" colwidth="18mm" /><thead><row><entry align="center" valign="top" /><entry align="center" valign="top">SFC (10th^-7·cm³· S · g^-1)</entry><entry align="center" valign="top">AAP at 50 g / cm² (g / g)</entry><entry align="center" valign="top">CRC (g / g)</entry></row></thead><tbody><row><entry align="center">example 1</entry><entry align="center">140</entry><entry align="center">23,5</entry><entry align="center">27</entry></row><row><entry align="center">Example 2</entry><entry align="center">150</entry><entry align="center">23,5</entry><entry align="center">27,2</entry></row><row><entry align="center">Example 3</entry><entry align="center">100</entry><entry align="center">24</entry><entry align="center">29</entry></row><row><entry align="center">Example 4</entry><entry align="center">110</entry><entry align="center">24</entry><entry align="center">29</entry></row><row><entry align="center">Comparative Example 1 (without silica sol)</entry><entry align="center">50</entry><entry align="center">24,5</entry><entry align="center">26,4</entry></row><row><entry align="center">Comparative Example 2 (without silica sol)</entry><entry align="center">30</entry><entry align="center">25</entry><entry align="center">27,8</entry></row><row><entry align="center">Comparative Example 3 (after post-crosslinking with silica sol)</entry><entry align="center">25</entry><entry align="center">24</entry><entry align="center">28,1</entry></row><row><entry align="center">Comparative Example 4 (after post-crosslinking with silica sol and heating)</entry><entry align="center">30</entry><entry align="center">24</entry><entry align="center">28,7</entry></row><row><entry align="center">Comparative Example 5 (Fumed Silica)</entry><entry align="center">55</entry><entry align="center">23</entry><entry align="center">29</entry></row><row><entry align="center">Comparative Example 6 (without silica sol)</entry><entry align="center">17</entry><entry align="center">25</entry><entry align="center">31,6</entry></row><row><entry align="center">Comparative Example 7 (<patcit id="pcit0034" dnum="jp6016822a"><text>JP 1994/16822</text></patcit>)</entry><entry align="center">0</entry><entry align="center">9</entry><entry align="center">31,3</entry></row><row><entry align="center">Comparative Example 8 (<patcit id="pcit0035" dnum="jp6016822a"><text>JP 1994/16822</text></patcit>)</entry><entry align="center">0</entry><entry align="center">9</entry><entry align="center">31,2</entry></row></tbody></tgroup></table></tables>

INFLUENCE OF THE TREATMENT OF THE EXTERIOR OF THE UNTREATED ABSORBENT POLYMER FABRICS (PU1) ON THE AGGLOMERATION TENDENCY OF THE POLYMER FIELDS.

Example 6:

50 g Powder B is mixed with a solution of 0.5 g ethylene carbonate, 0.125 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 1.38 g of water mixed with vigorous stirring. A compact is then produced from the absorbent polymer structure brought into contact with the aqueous solution, and its density and the pressure to be used to destroy the compact are determined.

Example 7:

50 g Powder B is mixed with a solution of 0.5 g ethylene carbonate, 0.125 g silica sol (product Levasil^® 200 from Bayer AG, solids content approx. 30% by weight) and 1.25 g of water mixed with vigorous stirring. A compact is then produced from the absorbent polymer structure brought into contact with the aqueous solution, and its density and the pressure to be used to destroy the compact are determined.

Comparative Example 9:

50 g Powder B is mixed using a Krups kitchen mixer with a solution of 0.5 g ethylene carbonate and 1.5 g water with vigorous stirring. A compact is then produced from the absorbent polymer structure brought into contact with the aqueous solution, and its density and the pressure to be used to destroy the compact are determined.

The properties of the absorbent polymer structures brought into contact with the aqueous solution in Examples 5 and 6 and in Comparative Example 9 are summarized in Table 2 below:<tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="33mm" /><colspec colnum="2" colname="col2" colwidth="46mm" /><colspec colnum="3" colname="col3" colwidth="51mm" /><thead><row><entry align="center" valign="top" /><entry align="center" valign="top">Density of the compact (kg / cm³)</entry><entry align="center" valign="top">Pressure to be applied (Pascal)</entry></row></thead><tbody><row><entry align="center">Example 6</entry><entry align="center">486</entry><entry align="center">8.795</entry></row><row><entry align="center">Example 7</entry><entry align="center">474</entry><entry align="center">4.575</entry></row><row><entry align="center">Comparative Example 9</entry><entry align="center">410</entry><entry align="center">16.295</entry></row></tbody></tgroup></table></tables>

The results show that the formation of stable agglomerates is significantly suppressed by the addition of silica sol. This addition means that the untreated, absorbent polymer structure (Pu1) can be charged with increased amounts of liquid without the processability being impaired by clumping.

TEST METHODS

PERMEABILITY IN THE SWELLED CONDITION (SFC TEST)

Determination of permeability in the swollen state <i>(Saline Flow Conductivity =</i> SFC) takes place after an in <patcit id="pcit0036" dnum="WO9522356A"><text>WO 95/22356</text></patcit> described method. Approx. 0.9 g superabsorbent material is weighed into a cylinder with a sieve bottom and carefully distributed on the sieve surface. The superabsorbent material is left in JAYCO synthetic urine for 1 hour against a pressure of 20 g / cm² swell. After determining the swelling height of the superabsorbent, 0.118 M NaCl solution is run through the swollen gel layer from a leveled storage vessel at constant hydrostatic pressure. The swollen gel layer is covered during the measurement with a special sieve cylinder, which ensures a uniform distribution of the 0.118 M NaCl solution above the gel and constant conditions (measuring temperature 20-25 ° C) during the measurement with regard to the condition of the gel bed. The pressure on the swollen superabsorbent is still 20 g / cm². With the help of a computer and a balance, the amount of liquid that passes through the gel layer as a function of time is recorded at intervals of 20 seconds within a period of 10 minutes. The flow rate g / s through the swollen gel layer is determined by means of regression analysis with extrapolation of the gradient and determination of the center point at the time t = 0 of the flow quantity within minutes 2-10. The SFC value (K) was in cm³· S · g^-1 specified and calculated as follows: <maths id="math0001" num=""><math display="block"><mi>K</mi><mo>=</mo><mfrac><mrow><msub><mi>F</mi><mi>s</mi></msub><mfenced separators=""><mi>t</mi><mo>=</mo><mn>0</mn></mfenced><mo>⋅</mo><msub><mi>L</mi><mi>O</mi></msub></mrow><mrow><mi>r</mi><mo>⋅</mo><mi>A</mi><mo>⋅</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>P</mi><mn>1</mn></msub></mrow></mfrac><mo>=</mo><mfrac><mrow><msub><mi>F</mi><mi>s</mi></msub><mfenced separators=""><mi>t</mi><mo>=</mo><mn>0</mn></mfenced><mo>⋅</mo><msub><mi>L</mi><mi>O</mi></msub></mrow><mn>139506</mn></mfrac></math><img file="EP1563002B1_D0001.tif" /></maths> in which<dl id="dl0001" compact="compact"><dt>F_s(t = 0)</dt><dd>the flow rate in g / s,</dd><dt>L_O</dt><dd>the thickness of the gel layer in cm,</dd><dt>r</dt><dd>the density of the NaCl solution (1.003 g / cm³),</dd><dt>A</dt><dd>the area of ​​the top of the gel layer in the measuring cylinder (28.27 cm²),</dd><dt>ΔP</dt><dd>the hydrostatic pressure on the gel layer (4,920 dyne / cm²), and</dd><dt>K</dt><dd>is the SFC value.</dd></dl>

DETERMINATION OF AGGLOMERATION TENDENCY

The tendency of liquid-coated superabsorbents to form agglomerates is shown by a <i>Indicator</i> the company JR Johanson Inc. determined. For this purpose, the superabsorbent is coated with the postcrosslinker solution to be investigated and then 50 g of the powder are fed to the investigation. The device produces a compact with a height of approximately 2 cm with a defined pressure of 160,000 Pascals using a press ram in a hollow metal cylinder with an inner diameter of 5.23 cm. This compact is then destroyed again by passing through a second cylinder, which has a diameter of 4.2 cm, the force applied for this being measured.