CN1897988A - Swellable hydrogel-forming polymers having a low fine dust concentration - Google Patents

Abstract

Disclosed are a swellable hydrogel-forming polymer containing at least one hydrophilic polymer with a dendritic structure, a method for producing said swellable hydrogel-forming polymer, and the use thereof in hygiene articles.

Description

Swellable hydrogel-forming polymers with low fines content

The present invention relates to swellable hydrogel-forming polymers having a low fines content, a process for the preparation of swellable hydrogel-forming polymers having a low fines content and their use.

Swellable hydrogel-forming polymers are known from the prior art as superabsorbent polymers (SAP) or superabsorbents for short.

Swellable hydrogel-forming polymers are especially polymers such as (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable graft matrix , cross-linked cellulose ethers, cross-linked starch ethers, cross-linked carboxymethyl cellulose, partially cross-linked polyoxyalkylenes or natural products that swell in aqueous fluids such as guar derivatives. Such hydrogels are used as products that absorb aqueous solutions for the preparation of diapers, tampons, sanitary napkins and other hygiene products, as water retaining agents for market gardens or for thickening various wastes, especially medical waste.

The swellable hydrogel-forming polymers are preferably capable of absorbing at least 10 times their own weight, preferably 20 times their own weight, based on the polymer used, of a 0.9% by weight sodium chloride solution. Preferably such absorption is obtainable even at a pressure of 0.7 psi, for example.

Swellable hydrogel-forming polymers are often surface or post-gel crosslinked to improve their performance characteristics. Such postcrosslinking is known per se to the person skilled in the art and is preferably carried out in the aqueous gel phase or, for example, by postcrosslinking the surfaces of substrates and classified polymer particles.

Superabsorbents undergo abrasive handling during preparation and processing and cause knock-off particles in the corners, which create fine powders or fines. The resulting fines cause filter clogging, sticky sedimentation, caking and the obvious problems with transporting superabsorbents. The large surface area of the powder leads to rapid absorption of moisture from the environment and makes the precipitated powder sticky, which causes contamination of the production equipment. Furthermore, hard agglomerates of the powder are formed in the pneumatic conveying system. Furthermore, fine grounds are highly undesirable for hygienists and medical practitioners.

Unless the fines are specifically separated off, it will cause problems when making superabsorbents or before introducing polymers into hygiene articles, as well as greatly reducing the saline flow conductivity (SFC) of the gel in use. This then causes the leakage rate to increase.

Accordingly, methods for dedusting superabsorbents have been proposed in patent applications WO-A-92/13912, WO-A-94/22940 and EP-A-0 679 678.

WO-A-92/13912 and WO-A-94/22940 describe the dedusting of superabsorbents by surface coating with polyethylene glycol. The disadvantage of polyethylene glycols is that they are linear water soluble polymers and they greatly increase the viscosity of the solution surrounding the gel particles, thus reducing the fluidity of the solution. This leads to poor saline flow conductivity (SFC) in the swollen gel when used in hygiene articles.

EP-A-0 679 678 describes a process in which the powder content of pulverulent superabsorbents is reduced by aftertreatment with polysiloxanes. This reference, for example, also suggests the use of additional dedusting agents such as polyethylene glycols and polyethylene glycol ethers. Silicones have an undesirable hydrophobic effect on superabsorbents, which reduces their swelling rate.

EP-A-0 755 964 describes the surface coating of superabsorbents with insoluble waxes. However, the waxes used have in some cases a hydrophobic effect on the superabsorbents and are difficult to disperse without auxiliaries. Conventional dispersants, which are usually used as auxiliaries for this purpose, however, have surface-active properties and reduce the surface tension of liquids falling into the hygiene article, which in turn can lead to leakage.

US 5,641,561 and US 5,589,256 describe the use of suitable polymeric and non-polymeric binders to bond superabsorbent particles to fibers. In hygiene articles, the binder should be able to bind the superabsorbent particles to the fibers via hydrogen bonding. There is no mention of binders, particularly with regard to the possible adverse effects of the disclosed polymeric binders on the saline conductivity of the swollen gel, or the surface tension of liquids falling into the hygienic article, nor is any disclosure of these problems disclosed. solution. Furthermore, it must be emphasized with regard to non-polymeric binders that the use of these as solvents in hygiene articles is not very desirable because of their outgassing during use and their effect on the skin. There is no published teaching of minimizing these solvents while removing dust and sticking the particles to the fibers.

It is therefore an object of the present invention to provide a process for the preparation of swellable hydrogel-forming polymers known as superabsorbents, wherein superabsorbents with a low fines content are obtained and compared with untreated superabsorbent Neither the swelling nor the saline conductivity was worsened compared with the agent.

Another object of the present invention is to provide a process for the preparation of swellable hydrogel-forming polymers in which the fines content of the resulting superabsorbent increases only slightly if fully exposed to mechanical stress. Fine powder refers to particles with a diameter of less than 10 μm.

A further object of the present invention is to provide a process for the production of superabsorbents in which the superabsorbents are post-treated with pulverulent and/or pulverulent additives and in which the superabsorbents have a low fines content.

Another object of the present invention is to provide a process for the preparation of superabsorbents in which superabsorbents with optimum transport properties are obtained. The superabsorbents obtained here should have a certain viscosity, for example, so that they can be metered in easily with a conveying screw, especially at high relative humidity, without increasing any tendency to lump.

We have found that these objects are surprisingly achieved by the use of dendritic-structured hydrophilic polymers in the preparation of swellable hydrogel-forming polymers in which superabsorbents, especially after exposure to mechanical stress, have a low Fines content, improved ability to bind powdered and/or powdered additives, high swelling rate, high saline conductivity and optimum flow characteristics.

Dendrimers are defined in Rmpp, Lexikon-Chemie, Georg Thieme Verlag, Stuttgart, 10th edition, p. 898 as a stepwise attachment of two or more A monomer, so that the number of monomer end groups increases exponentially in each step, and finally produces a synthetic macromolecule constructed of a spherical tree structure.

Useful hydrophilic polymers of dendritic structure for the purposes of the present invention are polyols having 8 or more, preferably 16 or more, more preferably 32 or more hydroxyl groups, and preferably already branched 14x or more, more preferably 30x or more nonlinear backbone.

Hydrophilic polymers of dendritic structure include, for example, those obtained by esterifying polyols with C ₃ -C ₂₀ hydroxycarboxylic acids, preferably with C ₄ -C ₁₂ hydroxycarboxylic acids, more preferably with C ₅ -C ₈ hydroxycarboxylic acids. A polyester wherein the hydroxycarboxylic acid comprises at least two hydroxyl groups, preferably two hydroxyl groups and/or at least two carboxyl groups. Particular preference is given to hydroxycarboxylic acids having two hydroxyl groups and one carboxyl group, especially 2,2-dimethylolpropionic acid. Polyols are compounds having at least two hydroxyl groups, examples being ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, butylene glycol, 1,3 - Propylene glycol, 1,4-butanediol, bisphenol A, glycerol, trimethylolpropane, pentaerythritol and/or sorbitol. Dendritic polyesters are preferred, Boltorn ^(R) 20, Boltorn ^(R) 30, Boltorn ^(R) 40 and Boltorn ^(R) 310 (Perstorp Specialty Chemicals AB, Sweden) are particularly preferred.

Useful hydrophilic polymers of dendritic structure for the purposes of the present invention also include polymers obtainable by condensation of polyols having at least three hydroxyl groups and subsequent alkoxylation. Examples of such polymers are branched polyethylene glycols obtainable by condensation of glycerol molecules followed by ethoxylation.

Useful hydrophilic polymers of dendritic structure for the purposes of the present invention also include all polymers obtainable by polyaddition of monomers having at least one hydroxyl group and subsequent alkoxylation. The polyaddition is preferably carried out in the presence of crosslinkers. This results in polymer particles with a hydrophilic surface due to the multitude of hydroxyl groups on the surface. For example, so-called star polyethylene glycols are obtainable according to Makromol. Chem. 189, 2885 (1988) by free-radical polymerization of p-hydroxyethylstyrene and subsequent alkoxylation.

Further examples of polymers useful according to the invention are the highly branched polymers of HYBRANE ^(R) and AstramolDendrimers ^(R ) (DSM NV, The Netherlands). They include, for example, highly branched polymethylethyleneimines obtainable by repeated multiple Michael additions of butanediamine with acrylonitrile and hydrogenation, star polycaprolactone, star nylon-6, e.g. based on molar ratios A highly branched polyesteramide that is a 1:1 addition product of succinic anhydride and diethanolamine. The process according to the invention can also be carried out using so-called PAMAM dendrimers based on polyamidoamines, which are obtainable, for example, by repeated multiple reactions of ammonia with methyl acrylate and ethylenediamine.

Polyglycerols, star polyethylene glycols, and other hydrophilic compounds may be used, but spherical or cumulus-shaped non-linear molecular geometry polyols are preferred.

Preference is given to hydrophilic polymers of dendritic structure, for example with a glass transition temperature Tg of 20-100° C., more preferably 25-50° C. and/or an average molecular weight of 1000-10000 g/mol, more preferably 2000-6000 g/mol.

In the process of the invention, the amount of the hydrophilic polymer of the dendritic structure is preferably 0.005-10% by weight, more preferably 0.01-5% by weight, even more preferably 0.05-1% by weight, based on the swellable hydrogel-forming polymer %, especially 0.10-0.80% by weight.

Preferably, the dendritic structured hydrophilic polymer is mixed with the dried water-absorbing hydrogel. Dry means preferably a water content of less than 20% by weight, more preferably less than 10% by weight. However, it is also possible to add the hydrophilic polymer of the dendritic structure to the swellable hydrogel-forming polymer before, during and/or after the surface postcrosslinking operation, but preferably during the surface postcrosslinking operation .

The manner of mixing is not subject to any restrictions, but preference is given to using reaction mixers or mixing and drying cabinets, such as Lödige ^(R) mixers, BEPEX ^(R) mixers, NAUTA ^(R) mixers, SCHUGGI ^(R) mixers, NARA ^(R) dryers and PROCESSALL ^① . In addition, fluidized bed dryers can also be used. Advantageously, mixing is performed using a residence time of 1-180 minutes, preferably 5-20 minutes, and a speed of 25-375 rpm, preferably 100-150 rpm.

When used with a surface postcrosslinking solution, the surface postcrosslinker can be used together with the dendrimer in solution; or a separate liquid stream can be sprayed from a separate nozzle into the postcrosslinker mixer. When applying the additive in powdered or powdered form, the dendrimer may also be dissolved in a solvent in which the powdered or powdered additive may also be dispersed. Such mixtures may also optionally include surface postcrosslinkers.

Useful solvents include all customary solvents for surface postcrosslinking. Particular preference is given to water, also to 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, isopropanol, ethanol, methanol, ethylene carbonate, propylene carbonate, glycerol and mixtures thereof. Particular preference is given to mixtures of water and one or more of the aforementioned organic solvents. However, the choice of solvent is determined by the requirements leading to efficient preparation of the solution and is not limited to the solvents mentioned above.

One or more surface active substances or dispersants may optionally be added to the solvent. Non-ionic surfactants such as sorbitan monolaurate (Span 20), sorbitan monolaurate, sorbitan monohexadecanoate, sorbitan Monostearate, sorbitan monooleate, sorbitan sesquioleate, sorbitan trioleate, which can be traded as Span40, Span60, Span80, Span83, Span85 get.

Preferably, however, no surface-active substances are added to the solution as dispersing aids.

may optionally include one or more dispersed powdered or powdered additives, may optionally include a dispersing aid, may optionally include aluminum sulfate or some other soluble metal salt of a 3- or 4-valent metal, and may optionally include The dendrimer solution or dispersion comprising at least one surface postcrosslinker is preferably selected by melting the dendrimer (if desired) and injecting the melt into a solvent or injecting part of the solvent and subsequently diluting it with another part while preparing. Preferably the operation is accompanied by thorough stirring, more preferably turbulent stirring, eg using Ultraturax. Alternatively, the dendritic polymer can be melted directly in a hot solvent or partially hot solvent. Furthermore, it is also possible, for example, to disperse the dendrimers using ultrasound or suitable nozzles.

When also dispersing pulverulent or pulverulent additives, it is particularly advisable to use suitable continuous mixers or batch mixers for preparing the dispersions. Particular preference is given to mixers designated MHD 2000/4, MHD 2000/5 and CMS 2000/4 from IKA-Werke GmbH & CoKG. Mixers of similar construction as well as other constructions can of course also be used. The dispersion can also be prepared using a grinding pump as described in DE10131606, eg NEWX80-50-315 from Wernert-Pumpen GmbH.

In specific cases, dispersions can also be prepared by ultrasonication in batch or continuous operation. Dispersions can also be prepared using usual conventional wet milling methods. Also particularly preferred is wet chemical precipitation of fine particles of chemical reactions between soluble components by stirring with or without heating. This usually produces a particularly finely divided precipitate.

The present invention further provides swellable hydrogel-forming polymers obtainable by the process according to the invention, especially swellable polymers comprising less than 100 wt. ppm, preferably less than 50 wt. ppm, more preferably less than 10 wt. Hydrogel-forming polymers and their use in the absorption of blood and/or body fluids, especially urine.

The present invention further provides swellable hydrogel-forming polymers obtainable by the process according to the invention, especially swellable polymers comprising less than 100 wt. ppm, preferably less than 50 wt. ppm, more preferably less than 10 wt. A hydrogel-forming polymer, wherein the swellable hydrogel-forming polymer comprises at least one pulverulent and/or pulverulent additive, for example metal salts such as aluminum sulfate and/or magnesium sulfate, fumed silica Such as Aerosil ^(R) 200, polysaccharides and their derivatives, nonionic surfactants, waxes, diatomaceous earth and/or hollow microspheres, and their use in the absorption of blood and/or body fluids, especially urine.

Hollow microspheres are described in Chem. Ing. Techn. 75, 669 (2003). Hollow microspheres are gas-filled or evacuated spherical solid particles with a diameter of 1-1000 μm. Their wall thickness is usually 1-10% of the diameter. There are no restrictions on the wall material. Wall materials may be glasses, ceramic-forming oxides or interoxides, silicates, aluminosilicates, polymers, polycondensates and metals.

The saline flow conductivity (SFC) of the swellable hydrogel-forming polymers of the present invention is generally at least 20×10 ⁻⁷ cm ³ s/g, preferably at least 40×10 ⁻⁷ cm ³ s/g, more preferably at least 60×10 ⁻⁷ cm ³ s/g, even more preferably at least 150×10 ⁻⁷ cm ³ s/g, most preferably at least 300×10 ⁻⁷ cm ³ s/g. Very particular preference is given to hydrogel-forming polymers obtained according to the invention having SFC values of from 500 to 2000×10 ⁻⁷ cm ³ s/g.

Preferably the powdered additive has an average particle size of less than 2000 μm, more preferably less than 400 μm.

The average particle size of the pulverulent additive is less than 200 μm, preferably less than 50 μm, more preferably less than 10 μm.

The invention further provides hygiene articles comprising superabsorbents produced according to the invention.

Swellable hydrogel-forming polymers useful in the method of the invention are especially cross-linked (co)polymerized polymers of hydrophilic monomers, polyaspartic acid, one or more hydrophilic monomers in Graft (co)polymers on suitable graft bases, crosslinked cellulose ethers, crosslinked starch ethers or natural products swellable in aqueous fluids such as guar gum derivatives. Preferably, the polymer to be crosslinked is a polymer comprising a structural unit derived from acrylic acid or an ester thereof, or a polymer obtained by graft-copolymerizing acrylic acid or an ester thereof on a water-soluble polymer matrix. Such hydrogels will be known to those skilled in the art and are described, for example, in US-4,286,082, DE-C-27 06 135, US-A-4,340,706, DE-C-37 13 601, DE-C-28 40 010, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-35 33 337 , DE-A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261, DE-A-35 11 086, DE -A-31 18 172, DE-A-30 28 043, DE-A-44 18 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A -0 312 952, EP-A-0 205 874, EP-A-0 499 774, DE-A-26 12 846, DE-A-40 20 780, EP-A-0 205 674, US 5,145,906, EP- A-0 530 438, EP-A-0 670 073, US 4,057,521, US 4,062,817, US 4,525,527, US 4,295,987, US 5,011,892, US 4,076,663 or US 4,931,497. As long as the content of the above-mentioned patent documents refers to the type and preparation of these hydrogels, it is an obvious part of the present disclosure.

Examples of suitable hydrophilic monomers for the preparation of these swellable hydrogel-forming polymers are addition polymerizable acids such as acrylic acid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, maleic acid, maleic anhydride , fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-methylpropanephosphonic acid and their amides, hydroxyalkyl esters and amino or Esters and amides of amine groups and alkali metal and/or ammonium salts of acid functional monomers. Water-soluble N-vinylamides such as N-vinylformamide or diallyldimethylammonium chloride may also be used. Preferred hydrophilic monomers are compounds of general formula I:

in

^R is hydrogen, C ₁ -C ₄ alkyl such as methyl or ethyl, or carboxyl,

R ² is -COOR ⁴ , hydroxysulfonyl or phosphono, phosphono esterified with C ₁ -C ₄ alkanol or a group of formula II:

R ³ is hydrogen, C ₁ -C ₄ alkyl such as methyl or ethyl,

R is hydrogen, C ₁ -C ₄ aminoalkyl ^, C ₁ -C ₄ hydroxyalkyl, alkali metal ion or ammonium ion, and

R ⁵ is sulfonyl, phosphono or carboxyl or the respective alkali metal or ammonium salts.

Examples of C ₁ -C ₄ alkanols are methanol, ethanol, n-propanol, isopropanol or n-butanol.

Particularly preferred hydrophilic monomers are acrylic acid and methacrylic acid and their alkali metal or ammonium salts, such as sodium acrylate, potassium acrylate or ammonium acrylate.

Suitable graft matrices for the hydrophilic hydrogels obtainable by graft copolymerization of ethylenically unsaturated acids or their alkali metal or ammonium salts may be of natural or synthetic origin. Examples are starch, cellulose or cellulose derivatives and other polysaccharides and oligosaccharides, polyoxyalkylenes, especially polyoxyethylene and polyoxypropylene, and hydrophilic polyesters.

Suitable polyoxyalkylenes are, for example, formula III

in

R ⁶ , R ⁷ are independently hydrogen, C ₁ -C ₁₂ alkyl such as methyl, ethyl, n-propyl or isopropyl, C ₂ -C ₁₂ alkenyl such as vinyl, n-propenyl or isopropenyl base, C ₇ -C ₂₀ aralkyl such as benzyl, 1-phenethyl or 2-phenethyl, or aryl such as 2-methylphenyl, 4-methylphenyl or 4-ethylbenzene base, R ⁸ is hydrogen or methyl, and

n is an integer of 1-10000.

R ⁶ and R ⁷ are each preferably hydrogen, C ₁ -C ₄ alkyl, C ₂ -C ₆ alkenyl or phenyl.

Preferred hydrogels are especially polyacrylates, polymethacrylates and graft polymers of US-4,931,497, US-5,011,892 and US-5,041,496.

Preferably the swellable hydrogel-forming polymers have been crosslinked, ie they comprise compounds having at least two double bonds which have been polymerized into a polymeric network. Suitable crosslinkers are especially N,N'-methylenebisacrylamide and N,N'-methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids of polyols, such as diacrylates or triacrylates such as butanediol diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate or ethylene glycol dimethacrylate and trimethylolpropane triacrylate, and allyl Compounds such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl ester, tetraallyloxyethane, triallylamine, tetraallylethylene Diamines, allyl esters of phosphonic acids and vinylphosphonic acid derivatives, as described, for example, in EP-A-0 343 427. The method of the present invention can also take advantage of the use of polyallyl ether as a crosslinking agent and the preparation of hydrogels by acidic homopolymerization of acrylic acid. Suitable crosslinkers are pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol diallyl ether, glycerol triallyl Ethyl ethers, polyallyl ethers based on sorbitol and their ethoxylated products.

A preferred method of preparing base polymers useful in the process of the invention is described in "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pp. 77-84. Particular preference is given to base polymers prepared in kneaders, e.g. as described in WO-A-01/38402, or in belt reactors, e.g. as described in EP-A-0 955 086 .

The water-absorbing polymer is preferably polyacrylic acid or polyacrylate. Such water-absorbing polymers can be prepared by methods known from the literature. Preference is given to polymers which comprise 0.001-10 mol %, preferably 0.01-1 mol %, of crosslinking comonomers, but very particular preference is given to polymers obtained by free-radical polymerization and in which polyfunctional olefinic compounds which additionally have at least one free hydroxyl group are used Unsaturated free-radical crosslinkers (eg pentaerythritol triallyl ether or trimethylolpropane diallyl ether).

The swellable hydrogel-forming polymers are prepared by polyaddition methods known per se. Polyaddition carried out as gel polymerization in aqueous solution is preferred. It involves, for example, making 15-50% by weight of an aqueous solution of one or more hydrophilic monomers and, if appropriate, a suitable graft base, in the presence of a free-radical initiator by using Trommsdorff-Norrish (Tromsdorff - Nolish) effect (Makromol. Chem. 1, 169 (1947)) polyaddition, preferably without mechanical mixing. The polyaddition reaction can be carried out at a temperature of 0-150°C, preferably 10-100°C, under atmospheric pressure or superatmospheric pressure or reduced pressure. In general, the polyaddition can also be carried out under a protective gas atmosphere, preferably under nitrogen. Polyaddition can be initiated using high-energy electromagnetic rays or conventional chemical addition polymerization initiators such as organic peroxides such as benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide, cumene hydroperoxide, Azo compounds such as azobisisobutyronitrile and inorganic peroxides such as (NH ₄ ) ₂ S ₂ O ₈ or K ₂ S ₂ O ₈ or H ₂ O ₂ . They can be used, if appropriate, in combination with reducing agents such as sodium bisulfite and iron(II) sulfate, or redox systems comprising reducing components of aliphatic and aromatic sulfinic acids such as Benzenesulfinic and toluenesulfinic acids or derivatives of these acids such as Mannich adducts of sulfinic acids, aldehydes and amino compounds are described, for example, in DE-A-13 01 566 . The performance characteristics of the polymers can be further improved by post-heating the polymer gel at a temperature of 50-130°C, preferably 70-100°C, for several hours.

Based on the monomers used, the resulting gel can be neutralized, for example, to 0-100 mol%, preferably 5-90 mol%, especially 25-80 mol%, very preferably 30-55 mol% and 70-75 mol%, which can be used for neutralization Conventional neutralizing agents are preferably alkali metal hydroxides or alkali metal oxides, but more preferably sodium hydroxide, sodium carbonate and sodium bicarbonate.

Neutralization is usually achieved by mixing the neutralizing agent into the gel either as an aqueous solution or preferably as a solid. For this purpose, the gel is comminuted mechanically, for example with the aid of a meat grinder, and the neutralizing agent is sprayed, spread or poured on it and mixed in carefully. The resulting gel mass can then be homogenized by passing it repeatedly through a meat grinder. The neutralized gel mass is then dried with a belt drier or can drier until the residual moisture content is preferably below 10% by weight, in particular below 5% by weight. The dried hydrogel is then ground and sieved, typically using a roll mill, pin mill or vibratory mill. The particle size of the sieved hydrogel is preferably 45-1000 μm, more preferably 45-850 μm, even more preferably 100-800 μm, still more preferably 100-700 μm. Further preferred particle sizes are 100-500 μm, 300-600 μm, less than 400 μm, more preferably less than 300 μm, most preferably less than 150 μm. At least 80%, preferably at least 90%, of all particles are within this range.

The postcrosslinking of the swellable hydrogel-forming polymer is usually carried out by spraying a solution of the surface postcrosslinker onto the dry base polymer powder.

After spraying, the polymer powder is dried thermally, and crosslinking reactions can take place not only before drying but also during drying.

The spraying of the crosslinker solution is preferably carried out in reaction mixers or mixing and drying cabinets, such as Lödige ^(R) mixers, BEPEX ^(R) mixers, NAUTA ^(R) mixers, SCHUGGI ^(R) mixers, NARA ^(R) dryers and PROCESSALL( ^R) . Fluid bed dryers can also be used.

Drying can be carried out within the mixer itself by heating the jacket or introducing a stream of hot air. It is likewise possible to use co-current dryers such as tray dryers, rotary tube ovens and heatable screws. However, it is also possible, for example, to use azeotropic distillation as drying method.

The preferred drying temperature is 50-250°C, preferably 60-200°C, more preferably 70-180°C. Preferably the residence time at this temperature in the reaction mixer or dryer is less than 60 minutes, preferably less than 30 minutes, more preferably less than 10 minutes.

Surface postcrosslinkers can be used alone or in combination with other surface postcrosslinkers such as ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether Ether, dipropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycerol diglycidyl ether, polyglyceryl diglycidyl ether, epichlorohydrin, ethylenediamine, ethylene glycol, diethylene glycol, triethylene glycol , polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, butylene glycol, 1,3-propanediol, 1,4-butanediol, bisphenol A, glycerin, trimethylolpropane, pentaerythritol , sorbitol, diethanolamine, triethanolamine, ethylenediamine, ethylene carbonate, propylene carbonate, 2-oxazolidinones such as 2-oxazolidinone or N-hydroxyethyl-2-oxazolidinone , 2,3-morpholine dione such as N-2-hydroxyethyl-2,3-morpholine dione, N-methyl-2,3-morpholine dione, N-ethyl-2,3- Morpholindione and/or N-tert-butyl-2,3-morpholinedione, 2-oxotetrahydro-1,3-oxazine, N-acyl-2-oxazolidinone such as N-acetyl -2-oxazolidinone, bicyclic amide acetal such as 5-methyl-1-aza-4,6-dioxabicyclo[3.3.0]octane, 1-oxa-4,6-diox Heterobicyclo[3.3.0]octane and/or 5-isopropyl-1-aza-4,6-dioxabicyclo[3.3.0]octane, and/or di-2-oxazolidinone and poly 2-oxazolidinones.

The surface postcrosslinker is preferably dissolved in a non-self-reactive solvent, preferably in a lower alcohol such as methanol, ethanol, isopropanol, propylene glycol, ethylene glycol, preferably isopropanol, most preferably in such a suitable In the aqueous solution of alcohol, the alcohol content of the solution at this time is 10-90% by weight, more preferably 25-70% by weight, especially 30-50% by weight.

The surface postcrosslinkers are used in an amount of 0.01 to 1% by weight, based on the polymer used, and the crosslinker solution itself is used in an amount of 1 to 20% by weight, preferably 3 to 15% by weight, based on the polymer used.

The AUL 0.7psi value [g/g] of the swellable hydrogel-forming polymer of the present invention can be measured by the method reported in DE-A-199 09 653 and is preferably greater than 10, especially greater than 15, more preferably greater than 20, especially greater than 25, especially preferably greater than 30.

The swellable hydrogel-forming polymers of the present invention are used in hygienic articles such as incontinence articles, diapers, tampons, pads to absorb blood and/or body fluids. To this end, the swellable hydrogel-forming polymers of the present invention can be treated with fibers such as cellulose and fibrous webs to form absorbent composites.

The dendrimers used in the process of the invention are hydrophilic due to their non-linear structure, but their special geometry significantly reduces any undesired tendency for thermal post-crosslinking, allowing the dendrimers to be formed on the surface added during postprocessing operations. No additional mixing steps are required. With regard to the viscosity of the aqueous solution of the incipient or fully swollen superabsorbent, a spherical shape is particularly advantageous here. Thus, high saline conductivity is maintained even at high polymer dosages.

Dendritic polymers are excellent in their powder binding ability, especially in powder form or powder form additives. After harsh mechanical exposure and directly after application, there were practically no detectable fines in the product.

The transport properties of the final product are also influenced by the solvent used during surface postcrosslinking. Propylene glycol/water is significantly better than isopropanol/water. On the other hand, unconverted propylene glycol (unlike unconverted isopropanol) is difficult to remove and remains in the final product. When propylene glycol is used, the alcohol content of the dried final product is typically 5000-15000 ppm by weight, but when the preferred isopropanol is used, the alcohol content is less than 1000 ppm by weight, preferably less than 500 ppm by weight, more preferably less than 100 ppm by weight.

In the process according to the invention, the addition of dendrimers makes it possible to use isopropanol/water (30% by weight isopropanol in water) as solvent in the surface postcrosslinking to obtain the (30% by weight propylene glycol in water) to obtain superabsorbents with transport properties.

To determine the quality of the post-treatment of the present invention, the dried hydrogels were tested by the following test method described here:

method:

Unless otherwise stated, measurements should be performed at an ambient temperature of 23±2°C and a relative humidity of 50±10%. The swellable hydrogel-forming polymer was thoroughly mixed for measurement.

Centrifuge retention capacity (CRC)

This method measures the free swelling properties of hydrogels in tea bags. 0.2000±0.0050 g of dried hydrogel (particle fraction 106-850 μm) was weighed and then filled into tea bags with dimensions 60×85 mm. The tea bags were placed in an excess of 0.9% by weight sodium chloride solution (at least 0.83L sodium chloride solution per 1 g polymer powder) for 30 minutes. The tea bags were then centrifuged at 250G for 3 minutes. The amount of liquid retained by the hydrogel was determined by weighing the teabag after centrifugation.

The centrifuge retention capacity can also be determined by the centrifuge retention capacity test method No. 441.2-02 recommended by EDANA (European Disposables and Nonwovens Association).

Absorbency under load (AUL) 0.7psi (4830Pa)

The measuring cell used to measure the AUL 0.7psi value is a Plexiglas cylinder with an inner diameter of 60mm and a height of 50mm. Adhered below it is a stainless steel sieve bottom with a mesh size of 36 μm. The measuring cell also includes a plastic disc with a diameter of 59 mm and a weight that can be placed in the measuring cell together with the plastic disc. The measuring cell and weights weigh a total of 1344g. AUL 0.7 psi was determined by measuring the weight of the empty Plexiglas cylinder and the weight of the plastic pan and recording this as W ₀ . 0.900±0.005 g of swellable hydrogel-forming polymer (particle size distribution 150-800 μm) was then weighed and placed in a Plexiglas cylinder and dispersed very evenly on the bottom of the stainless steel screen. The plastic pan was then carefully placed inside the Plexiglas cylinder, the entire unit was weighed and the weight recorded as W _a . The weights are then placed on the plastic pan inside the Plexiglas cylinder. A ceramic filter plate (Duran from Schott) with a diameter of 120 mm, a height of 10 mm and a porosity of 0 is then placed in the middle of a petri dish with a diameter of 200 mm and a height of 30 mm and sufficient 0.9 wt% chloride For sodium solution, the liquid level is flush with the surface of the filter plate without wetting the surface of the filter plate. A round filter paper with a diameter of 90 mm and a pore size of less than 20 μm (S & S 589 Schwarzband from Schleicher & Schüll) was then placed on the ceramic plate. The Plexiglas cylinder containing the swellable hydrogel-forming polymer and the plastic disc and weights were then placed on top of the filter paper and left for 60 minutes. At the end of this process, the entire unit was removed from the filter paper of the Petri dish, and the weights were removed from the Plexiglas cylinder. The Plexiglas cylinder containing the swollen hydrogel was weighed together with the plastic pan and weights and the weight was recorded as _Wb .

The Absorbent Capacity Under Load (AUL) is calculated as follows:

AUL 0.7psi[g/g]＝[W _b -W _a ]/[W _a -W ₀ ]

The absorbency under load can also be determined by the test method No. 442.2-02 for absorption under pressure recommended by EDANA (European Disposable and Nonwovens Association).

Saline Flow Capacity (SFC)

Determination of the saline conductivity of the swollen gel layer at a confining pressure of 0.3 psi (2070 Pa) as described in EP-A-0 640 330, the gel layer permeability of the swollen gel layer as superabsorbent polymer, Although the device described on page 19 and Fig. 8 of the previously cited patent application is changed mainly to no longer use glass frit (40), the piston (39) is made of the same plastic material as the cylinder (37) and It now contains 21 holes of the same size evenly distributed over the entire contact surface. Compared to EP-A-0 640 330, the procedure and evaluation of measurements remain unchanged. Automatically records flow rate.

Saline flow conductivity (SFC) is calculated as follows:

SFC[cm ³ s/g]=(F _g (t=0)×L ₀ )/(d×A×WP), where F _g (t=0) is the F _g (t) data measured from the flow rate by The flow rate (g/s) of the NaCl solution obtained by extrapolating to t=0 linear regression analysis; L ₀ is the thickness of the gel (cm); d is the density of the NaCl solution (g/cm ³ ); A is the gel layer area (cm ² ); WP is the hydrostatic pressure on the gel layer (dyn/cm ² ).

Flow rate (FLR)

This method measures the rate at which a swellable hydrogel-forming polymer flows through a funnel. Weigh 100 ± 0.01 g of dried hydrogel and place into a sealable metal funnel. Record the weight of the swellable hydrogel-forming polymer as W ₁ . The funnel corresponds to the German Industrial Specification DIN 53492. The height of the outlet pipe of the funnel is 145.0±0.5mm, and the inner diameter is 10.00±0.01mm. The inclination of the walls of the funnel to the horizontal is 20°. Ground the metal funnel. The funnel was then opened and the time required for the funnel to empty was measured. Denote the time as t.

Take two measurements. The difference between the two measurements obtained must not exceed 5%.

Calculate the flow rate (FLR) as follows:

FLR[g/s]=W ₁ /t

The flow rate can also be determined by the flow rate test method No. 450.2-02 recommended by EDANA (European Disposable and Nonwovens Association).

Pour-out weight (ASG)

This method measures the density of a swellable hydrogel-forming polymer after pouring. The measurement is carried out according to DIN 53466 with a cylindrical pycnometer. The volume (V) of the pycnometer is 100.0±0.5ml, the inner diameter is 45.0±0.1mm, and the height is 63.1±0.1mm. Weigh the empty weight of the pycnometer. Let the weight be W ₁ . Approximately 100 g of dried hydrogel was weighed and placed into a sealable metal funnel. Record this weight as W ₁ . The funnel corresponds to the German Industrial Specification DIN 53492. The height of the outlet pipe of the funnel is 145.0±0.5mm, and the inner diameter is 10.00±0.01mm. The inclination of the walls of the funnel to the horizontal is 20°. Ground the metal funnel and pycnometer. The funnel was then emptied into the pycnometer and excess swellable hydrogel-forming polymer overflowed. The spilled swellable hydrogel-forming polymer was scraped off with a spatula. Weigh the filled pycnometer and record the weight as _W2 .

Take two measurements. The difference between the two measurements obtained must not exceed 5%.

The poured weight (ASG) is calculated as follows:

ASG [g/ml] = [W ₂ -W ₁ ]/V

Poured weight can also be determined by EDANA (European Disposable and Nonwovens Association) recommended density test method No. 460.2-02.

fragility test

The friability test was used to determine the behavior of swellable hydrogel-forming polymers exposed to mechanical stress. Test it with a sealable china cup. The diameter of the porcelain cup is 85.7mm (3 3/8 inches), the height is 111.1mm (4 3/8 inches), and the volume is 379cm ³ (0.1 gallons). The diameter of the opening is 31.8 mm (1 1/4 inches). Height with lid is 139.7mm (5 1/2 inches). A porcelain cup was charged with 50 g of swellable hydrogel-forming polymer and 127 g of cylindrical porcelain milling media. The porcelain grinding media had a diameter of 12.7 mm (1/2 inch) and a height of 12.7 mm (1/2 inch), each weighing approximately 5.3 grams. Porcelain cups were filled and spun on a roller mill (eg from US Stoneware, USA) at 180 rpm for 15 minutes.

pink

The powder fraction can be determined by the powder test method No. 490.2-02 recommended by EDANA (European Disposable and Nonwovens Association).

Anti-caking test

30 g of the swellable hydrogel-forming polymer were weighed into a 100 ml glass beaker. The beakers were then stored at 40° C. and 95% relative humidity for 2 hours. After storage, the swellable hydrogel-forming polymer is decanted. The pouring behavior was assessed qualitatively on a scale where "very poor" means that a stable skin has formed on the surface of the swellable hydrogel-forming polymer and the swellable hydrogel-forming polymer remains in the beaker, "Very good" means that the swellable hydrogel-forming polymer can be poured off completely, and a value in the middle means that a ring of swellable hydrogel-forming polymer remains on the beaker wall.

Example

Examples 1 and 2:

The SAP 500 Z base polymer was sprayed in a Lödige laboratory mixer by spraying with 3.16 wt. % isopropanol/water (30:70) and 0.085 wt. , and then post-crosslinked by heating to 175° C. for 120 minutes. The dendrimers are added as appropriate during operation. The resulting polymer was subsequently sieved at 850 μm and delumped. A fine powder (<10 μm) of the product was tested and analyzed by the laser expelling tube method. The same product was mechanically disintegrated by means of a roller mill and the powder was retested.

Table 1: operate additive CRC[g/] AUL 0.7[g/g] SFC[10 ⁷ cm ³ s/g] FLR[g/s] ASG[g/ml] Powder before grinding [ppm] Grinding powder [ppm] Anti-caking test 1 30.9 23.3 45 10.4 0.68 2 26 very bad 2 0.5 wt% Boltorn ^(R) H40 29.7 22.3 33 9.4 0.64 1 <10 not detected

Claims (16)

CLAIMS 1. A swellable hydrogel-forming polymer comprising at least one dendritic structured hydrophilic polymer. 2. The polymer according to claim 1, wherein said hydrophilic polymer of dendritic structure is a polyester formed from polyol and 2,2-dimethylolpropionic acid. 3. The polymer according to claim 1, wherein said hydrophilic polymer of dendritic structure is polymethylethyleneimine, polyamidoamine or polyesteramide. 4. The polymer according to any one of claims 1-3, further comprising pulverulent and/or pulverulent additives. 5. The polymer according to claim 4, wherein the additive is a metal salt, fumed silica, polysaccharide, nonionic surfactant, wax and/or diatomaceous earth. 6. A polymer according to claim 4 or 5, wherein the additive is present in the form of hollow microspheres with a diameter of 1-1000 [mu]m and a wall thickness of 1-10% of said diameter. 7. A polymer according to any one of claims 1 to 6 comprising less than 50 ppm by weight of particles having a diameter of less than 10 μm. 8. A polymer according to any one of claims 1 to 7 comprising less than 50 ppm by weight of particles having a diameter of less than 10 μm after exposure to mechanical stress. 9. A method of preparing a swellable hydrogel-forming polymer comprising post-treating the hydrogel with a dendritic structured hydrophilic polymer. 10. The method according to claim 9, wherein the hydrophilic polymer of the dendritic structure is a polyester formed from a polyol and 2,2-dimethylolpropionic acid. 11. The method according to claim 9, wherein the hydrophilic polymer of the dendritic structure is polymethylethyleneimine, polyamidoamine or polyesteramide. 12. The method according to any one of claims 9-11, wherein said post-treatment is carried out together with a surface post-crosslinking operation. 13. The method according to claim 12, wherein the solvent comprising at least one surface postcrosslinker is a mixture of isopropanol and water. 14. Use of a polymer according to any one of claims 1-8 for the absorption of blood and/or body fluids. 15. Use according to claim 14 for absorbing urine. 16. Hygienic article comprising a polymer according to any one of claims 1-8.