RU2471812C2 - Method of producing mechanically stable water-absorbent polymer particles - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: present invention relates to a method of producing water-absorbent polymer particles. Described is a method of producing water-absorbent polymer particles which contain at least one cavity inside the particle through polymerisation of droplets of a monomer solution containing: a) at least one ethylenically unsaturated monomer having at least one acid group, b) at least one crosslinker, c) at least one initiator, d) water, in the gaseous phase surrounding the droplets, the obtained polymer particles being re-crosslinked further, said re-crosslinked polymer particles being at least covered by water, an aqueous solution, alkanolamine, a polymer and/or wax. The invention also describes water-absorbent particles obtained using the method given above. The invention describes hygiene items containing said polymer particles.

EFFECT: high mechanical stability of polymer particles containing at least one cavity inside.

12 cl, 11 dwg, 1 tbl, 5 ex

Description

The invention relates to a method for producing water-absorbing polymer particles by polymerizing droplets of a monomer solution in the surrounding gas phase and second crosslinking of the polymer particles, where the second crosslinked polymer particles are at least partially coated.

The production of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", F. L. Buchholz und AT. Graham Wiley-VCH, 1998, pp. 71-103.

Water-absorbing polymers are used as absorbent aqueous solutions of products for the production of diapers, tampons, women's pads and other hygiene products, as well as as a water-holding agent in agricultural gardening.

The properties of water-absorbing polymers can be determined through the degree of crosslinking. With an increasing degree of crosslinking, the density of the gel increases and the absorption capacity decreases.

To improve application properties, such as fluid transfer in the gel layer (SFC) of the diaper and pressure absorption (AUP), the water-absorbing polymer particles are typically crosslinked. As a result, the degree of crosslinking increases only on the surface of the particles, so that the absorption under pressure (AUP) and holding capacity during centrifugation (CRC) can at least partially disconnect. This secondary crosslinking can be carried out in the aqueous helium phase. Advantageously, however, dry, crushed and sieved polymer particles (base polymer) are coated over the surface with a crosslinking agent, thermally crosslinked and dried. Suitable crosslinking agents are compounds containing groups that can form covalent bonds with the carboxylate groups of hydrophilic polymers.

By spray polymerization, process steps such as polymerization and drying can be combined. In addition, particle sizes can be set within certain limits by suitable process control.

The production of water-absorbing polymer particles through the polymerization of droplets of a monomer solution is described, for example, in European application EP 348180 A1, in international application WO 96/40427 A1, US patent US 5,269,980, German patent applications DE 10314466 A1, DE 10340253 A1, DE 102004024437 A1 and DE 102005002412 A1, as well as in the older German application with registration number 102006001596.7.

German patent applications DE 102004042946 A1, DE 102004042948 A1, DE 102004042955 A1 and DE 102005019398 A1 describe the production of a thickener using spray polymerization.

International patent application WO 2006/079631 A1 describes a drip polymerization process, the polymer particles being dried and selectively second crosslinked in a vortex layer.

EP 703265 A1 describes a method for improving the wear resistance of water-absorbing polymer particles by applying a layer of film-forming polymers.

European patent EP 755964 A2 also discloses a method for improving the wear resistance of water-absorbing polymer particles, the particles being coated with waxes.

The objective of the invention is the provision of an improved method for the production of water-absorbing polymer particles by polymerizing droplets of a solution of monomer in the surrounding gas phase droplets.

The problem was solved through a method of producing water-absorbing polymer particles by polymerizing drops of a monomer solution containing:

a) at least one ethylene unsaturated monomer,

b) at least one crosslinking agent,

c) at least one initiator;

d) water

in the gaseous phase surrounding the droplet, wherein the obtained polymer particles are secondarily crosslinked, characterized in that the secondly crosslinked polymer particles are at least partially coated.

Through the polymerization of droplets of a monomer solution in the gas phase surrounding the droplet (droplet polymerization), round polymer particles with a higher average sphericity (mSPHT) are obtained. Median sphericity is a measure of the roundness of polymer particles and can be determined, for example, by the Camsizer® image processing system (Retsch Technolgy GmbH; DE).

The presented invention is the basis of the scientific conclusion that polymer particles obtained by drop polymerization are hollow spheres and that hollow spheres can be damaged by mechanical loading.

Due to the coating, in accordance with this invention, the mechanical stability of the hollow spheres clearly increases. Suitable coating means are, for example, impurities that increase the elasticity of the outer surface of the particles, for example, by lowering the glass transition temperature and / or hardening the outer surface of the particles.

The coating can be obtained due to the fact that through the drop polymerization of the obtained water-absorbing polymer particles are coated in a well-known manner - by introducing into the mixture a coating agent in the desired mass ratio. This coating is preferably carried out in mixers with a movable mixing mechanism, such as a screw mixer, paddle mixer, disk mixer, ploughshare mixer, blade mixer. Vertical mixers are particularly preferred, and ploughshare and paddle mixers are highly preferred. Suitable mixers are Lödige, Bepex, Nauta, Processall and Schugi. The use of high-speed mixers, such as Schugi-Flexomix or Turbolizer types, is highly preferred. But vortex mixers are also preferred.

The second-crosslinked water-absorbing polymer particles are coated predominantly with water, an aqueous solution, alkanolamine, a polymer and / or wax.

The amount of added water or aqueous solution during the coating according to the invention is 1-30% by weight, particularly preferably 2-20% by weight, highly preferably 3-15% by weight.

In a preferred embodiment, when a layer of water or an aqueous solution is applied, a surfactant is added as an aid to agglomerate breakdown, for example sorbitan monoester, such as sorbitan monococoate and sorbitan monolaurate or their ethoxylated variants. Another very suitable aid for breaking down agglomerates is ethoxylated and alkoxylated derivatives of 2-propylheptanol, which are sold under the trademarks Lutensol® XL and Lutensol® XP (BASF Aktiengesellschaft, DE).

The amount of aid used to break down agglomerates relative to water-absorbing polymer particles is, for example, 0.01-1% by weight, preferably 0.05-0.5% by weight, particularly preferably 0.1-0.2% by weight.

The amount of alkanolamine used in the coating according to the invention is preferably 0.5-30% by weight, particularly preferably 1-20% by weight, highly preferably 2-15% by weight, and alkanolamines can also be used as a solution in a suitable solvent, for example water.

Suitable alkanolamines are mono-, di- and trialkanolamines, such as ethanolamine, diethanolamine and triethanolamine. Tertiary alkanolamines such as triethanolamine, methyldiethanolamine, dimethylaminodiglycol, dimethylethanolamine and N, N, N ′, N′-tetra- (hydroxyethyl) ethylenediamine are preferred. Triethanolamine is particularly preferred.

The amount of polymer and / or waxes is preferably 0.005-10% by weight, particularly preferably 0.05-5% by weight, highly preferably 0.1-2% by weight, relative to the polymer particles.

The polymer can also be applied to the surface of the particles in such a way that the initial components of this polymer are first applied to the surface of the particle, which are converted on the surface of the particle into the desired polymer, for example, through the interaction of polyols with polyepoxides.

The polymers, their starting components or waxes can be applied to the surface of the particles as aqueous dispersions, emulsions and / or suspensions of polymers.

The polymers, their starting components or waxes can also be used in the form of a solution in an organic solvent or in a mixture of water and an organic water-miscible solvent. Also, the aforementioned aqueous dispersions, emulsions and suspensions may contain a portion of an organic solvent selectively miscible with water.

Suitable organic solvents are, for example, aliphatic and aromatic hydrocarbons such as n-hexane, cyclohexane, toluene and xylene, alcohols such as methanol, ethanol, isopropanol, ethylene glycol, propylene glycol, glycerin and polyethylene glycols with an average molecular weight of 200-10000, simple ether, such as diethyl ether, ester, such as ethyl acetate and n-butyl acetate and ketone, such as acetone and 2-butanone.

Suitable water-miscible organic solvents are, for example, aliphatic alcohols having 1-4 carbon atoms, such as methanol, isopropanol, tert-butanol, ethylene glycol, propylene glycol, glycerin and polyethylene glycol with an average molecular weight of 200-10000, ether and ketone such as acetone and 2-butanone.

Polymers and / or waxes can also be dosed in the form of a melt.

The polymers and / or waxes used in the method according to this invention are not active, which means that they do not have reactive groups that react with groups on the outer surface of the polymer particle.

Preferred polymers and / or waxes are, first of all, those which are not prone to sticking in the temperature range between 0 ° C and 80 ° C.

The preferred polymers according to the invention for coating are vinyl ester homo- and copolymers, in particular vinyl acetate-homopolymer and vinyl acetate-copolymer with ethylene, acrylates, maleic esters, vinylamides and / or other vinylacyl derivatives.

Preferred are also homo- and copolymers of esters of acrylic and methacrylic acids, such as, for example, copolymers of methyl methacrylate and n-butyl ester of acrylic acid or 2-ethylhexyl ester of acrylic acid.

The above copolymers based on vinyl esters, esters of acrylic and methacrylic acids may contain further comonomers, for example styrene, butadiene, vinyl amides, olefinic unsaturated carboxylic acids and their derivatives, olefinic unsaturated sulfonic acids and their derivatives, vinylphosphonic acids and their derivatives or polyglycol unsaturated esters acids.

Examples of vinyl amides are, in particular, N-vinylformamide, N-vinyl-N-methylacetamide and N-vinylpyrrolidone.

Examples of olefinic unsaturated carboxylic acids are, first of all, acrylic acid, methacrylic acid, itaconic acid and maleic acid, as well as their alkaline, ammonium and amine salts. Examples of olefinic unsaturated carboxylic acid derivatives are primarily amides, such as (meth) acrylamide, N-tert-butyl (meth) acrylamide and N-isopropyl (meth) acrylamide, but also N-methylolamides or N-methylolamide ether, semi-amides and imides of aliphatic amines, such as acrylonitrile.

Examples of olefinic unsaturated sulfonic acids are salts of vinyl sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, styrene sulfonic acid, allyl and metal sulfonic acid, in particular their alkaline, ammonium and amine salts.

Examples of vinylphosphonic acid derivatives are predominantly mono- and diesters of alcohols having 1-18 carbon atoms, such as esters of methyl, propyl or stearyl alcohols. Vinyl phosphonic acid itself is provided, in particular, in the form of mono- and disols, with alkaline, ammonium and amine salts being preferred.

Polyglycol esters of unsaturated acids are, in particular, hydroxyethyl (meth) acrylate or an ester of acrylic and methacrylic acid with polyalkylene oxide compounds of the general formula

and mean

X is hydrogen or methyl,

n is from 0 to 50 and

R is an aliphatic, araliphatic or cycloaliphatic residue containing 1-24 carbon atoms, for example nonylphenyl.

In a preferred embodiment of the invention, film-forming polymers are used for coating. These are special polymers that have a tensile strength of the polymer film of 0.5-25 N / mm, preferably 1-20 N / mm, more preferably 2-15 and most preferably 5-10 N / mm and an elongation of 10-10000% preferably 20-5000%, more preferably 50-2000% and highly preferably 500-1000%. Tensile strength and tensile elongation are determined according to DIN EN ISO 527.

Preferred according to the invention used for coating polymers are the following film-forming polymers based on:

- polyacetals, that is, the products of the interaction of polyvinyl alcohols with aldehydes, such as butyraldehyde;

- polyurethanes, that is, polymers accessible through polyaddition of two or more valence alcohols, for example, obtained from polyester and / or polyether diols and, for example, 2,4- or 2,6-toluene diisocyanates, 4,4-methylenedi (phenylisocyanates) or hexamethylene diisocyanates (see Methods of Organic Chemistry (Methoden der Organischen Chemie), 4, Houben-Weyl, Volume E20 / 2, pp. 1561-1721);

- polyureas, i.e. polymers that are available by polyaddition of diamines and diisocyanates or by polycondensation of diamines with carbon dioxide, phosgene, esters of carboxylic acids (e.g. activated diphenyl carbonates) or ureas, respectively, by the interaction of diisocyanates with water (see Organic chemistry methods, 4, Houben-Weyl, Volume E20 / 2, pp. 1721-1752);

- polysiloxanes, moreover, linear dimethylpolysiloxane, the end groups of which can be variously modified, is used as the main polymer (see "Chemie und Technologie des kalthärtenden Siliconkautschuks", pages 49-64 in SILICONE - Chemistry and Technology [Symposium 28.04. 1989] VULKAN-VERLAG, Essen);

- polyamides, with copolyamides (see Plaste Kautsch. Jahrgang 25, pages 440-444 (1978)), as they are used, for example, for the manufacture of varnishes, an advantage is given;

- polyesters, that is, polymers that are polymerized by opening the cycle of lactones or by polycondensing hydroxycarboxylic acids or diols and derivatives of dicarboxylic acids, respectively (see Houben-Weyl, Methoden der Organischen Chemie, 4th edition, volume E20 / 2, pages 1404- 1429);

- epoxy resins which are prepared from polyepoxides by polyaddition reaction with suitable hardeners or by polymerization of epoxy groups (see Organic Chemistry Methods, 4, Houben-Weyl, Volume 14/2, pages 462-552, and Volume E20 / 2, pages .1891-1994 (e.g., reaction products of bisphenol A with epichlorohydrin) or based on;

- polycarbonates, which are easily made by reacting diglycols or bisphenols with phosgene, or carbonic acid diesters, respectively, in polycondensation or transesterification reactions (see Organic Chemistry Methods, 4, Houben-Weyl, Volume E20 / 2, pp. 1443-1457).

Particularly preferred polymers used for coating according to the invention are homo- and copolymers of esters of acrylic and methacrylic acid, as well as polymers based on polyacetals.

Mixtures of two or more of the above polymers may also be used. The ratio of the components of the mixture are completely uncritical and adapt to real conditions.

Preferred polymers used in the method according to the invention are, in addition, elastic polymers. These are polymers with rubber-elastic properties, which can repeatedly stretch at least two of their lengths at 23 ° C, and after removing the force necessary for stretching, they immediately assume their length close to the initial one.

Representative examples of suitable elastic polymers are natural and synthetic latexes, which are commonly used as a binder and elastomeric adhesive in the manufacture of air-drying absorbent products. In addition to the latexes described in the examples, it is appreciated that each natural and synthetic elastomer suitable for forming the latex dispersion is suitable for use in the present invention. Therefore, it can be used: natural rubber, polybutadiene rubber, styrene-butadiene rubber, aryl nitrile-butadiene rubber, poly-2-chlorobutadiene rubber, polyisoprene rubber, isoprene isobutylene copolymers, ethylene-propylene rubber, ethylene-polyethylene chloropolyethylene, polyethylene-ethylene-polyethylene-polyethylene rubber, ethylene-acrylate copolymers, epichlorohydrin rubber, polypropylene oxide rubber and polyurethanes.

In a further preferred embodiment of the invention, waxes are applied to the surface of the crosslinked water-absorbing polymer particles.

Under wax, first of all, in accordance with the wording of the German Society for Fat Sciences (Deutschen Gesellschaft für Fettwissenschaft (DGF)) of 1974 (see DGF-Einheitsmethoden: Untersuchung von Fetten, Fettprodukten und verwandten Stoffen, Abteilung Mündicht; Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1975) refers to a substance that is independent of its chemical composition and its natural and artificial origin, usually characterized by the following mechanical and physical properties:

- at 20 ° C plastic, from durable to brittle hard;

- from coarse to fine crystalline, from clear to cloudy, nonetheless not glassy;

- at more than 40 ° С it melts without decomposition;

- already at a temperature slightly above the melting point, it is relatively low-viscosity and not malleable;

- consistency and solubility are highly temperature dependent

- polished under light pressure.

Preferred waxes are especially those in which the melting point or, correspondingly, the dropping point lies in the temperature range between 30 and 180 ° C, particularly preferably between 40 and 180 ° C, highly preferably between 40 and 170 ° C. The dropping point is determined in accordance with the DGF-standard method DGF-M-III 3 (75) (Wissenschaftliche Verlagsgesellschaft, Stuttgart).

The waxes used according to the invention are natural waxes, modified natural waxes, partially synthetic and fully synthetic waxes. An example of natural waxes is modern waxes such as vegetable waxes or animal waxes. Examples of plant waxes are carnauba wax, candle wax, urikuri wax, sugarcane wax, and retam wax. An example of animal waxes is insect wax, such as beeswax, Austindian bee wax and shellac wax, as well as wool wax. Further examples for natural waxes include fossil waxes, petroleum wax or brown coal wax, and peat waxes. Examples of petroleum waxes are ozokerite and paraffin, and raw mountain wax is an example of brown coal and peat waxes. Examples of modified natural waxes are both waxes obtained by refining, and macro- and microcrystalline solid paraffin waxes or chemically modified waxes, such as, for example, double-bleached raw mountain wax, obtained from distillates of oil or, respectively, distillate residues. Examples of partially synthetic waxes are those made from mountain wax, acid waxes and ester waxes, wax acids made using paraffin oxidation, and alcohol and amide waxes. Examples of fully synthetic waxes are hydrocarbon waxes such as polyolefin waxes and Fischer-Tropsch waxes, as well as synthetic waxes with oxygen functional groups. Examples of synthetic waxes with oxygen functional groups are acidic waxes, which are formed by oxidizing synthetically produced hydrocarbon waxes or by copolymerizing or telomerizing olefins with unsaturated carboxylic acids, ester waxes, which are obtained by esterification of synthetic wax acids with synthetic alcohols and using copolymerization of olefins with unsaturated esters such as vinyl acetate, alcohol waxes, which are produced by oxosynthesis followed by hydrogenation, as well as by hydrogenation of synthetic fatty acids, as well as amide waxes, which are obtained by the interaction of synthetic acids with amines. Examples of waxes that are obtained by oxidizing synthetically produced hydrocarbon waxes are polyoxid wax oxidates.

The preferred waxes used according to the invention are ennobled (i.e., tarless and bleached) mountain waxes, such as polyolefin waxes.

Particularly preferred waxes used according to the invention are polyolefin waxes, polyethylene waxes (waxes from high pressure polyethylene, waxes from low pressure polyethylene, waxes from degraded polyethylene), oxidants of this polyethylene wax, waxes based on ethylene-α-olefin copolymers, waxes based on ethylene-vinyl acetate copolymers, waxes based on ethylene-styrene copolymers, waxes based on copolymers of ethylene and acrylic acids, as well as wax , Based on a mixture of polyethylene waxes with polytetrafluoroethylene waxes.

Mixtures of two or more of the above waxes may also be used. The ratio of the components of the mixture is completely uncritical and adapts to real conditions.

Monomeric solutions used in the method according to the invention for the manufacture of water-absorbing polymer particles contain at least one ethylene unsaturated monomer a), optionally crosslinking agent b), at least one initiator c) and water d).

Monomers a) are predominantly soluble in water, i.e. the solubility in water at 23 ° C. is usually at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, particularly preferably at least 25 g / 100 g of water and highly preferably at least 50 g / 100 g of water, and have at least one acid group.

Suitable monomers a) are, for example, ethylene unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid. More preferred monomers are acrylic acid and methacrylic acid. Of these, acrylic acid is particularly preferred.

Preferred monomers a) have at least one acid group, the acid groups being advantageously at least partially neutralized.

The proportion of acrylic acids and / or their salts in the total amount of monomer a) is preferably at least 50 mol%, particularly preferably at least 90 mol%. and highly preferably at least 95 mol%.

The acid groups of monomer a) are usually partially neutralized, preferably from 25 to 85 mol%, particularly preferably from 50 to 80 mol%. and highly preferably 60-75 mol%, a standard neutralizing agent, preferably alkali metal hydroxide, alkali metal carbonate or hydrogen carbonate, as well as mixtures thereof, can be used. Instead of alkali metal salts, an ammonium salt may be used. Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium hydrogen carbonate, as well as mixtures thereof, are still highly preferred. Typically, neutralization is achieved by adding a neutralizing agent in the form of an aqueous solution, in the form of a melt, or preferably also in the form of a solid. For example, sodium hydroxide with a fraction of water is clearly below 50% wt. can be represented as a waxy mass with a melting point above 23 ° C. In this case, the dosage is possible in the form of piece material or in the form of a melt at elevated temperature.

Monomers a), in particular acrylic acids, contain predominantly up to 0.025% wt. half ester of hydroquinone. Preferred half-esters of hydroquinone are hydroquinone monomethyl ether (MEHQ) and / or tocopherol.

By tocopherol are meant compounds of the following formula:

wherein R ¹ is hydrogen or methyl, R ² is hydrogen or methyl, R ³ is hydrogen or methyl, and R ⁴ is hydrogen or an acid residue with 1 to 20 carbon atoms.

Preferred radicals for R ⁴ are acetyl, ascorbyl, succinyl, nicotinyl and other physiologically compatible carboxylic acids. Carboxylic acids may be mono-, di- or tricarboxylic.

Alpha tocopherol with R ¹ = R ² = R ³ = methyl, especially racemic alpha tocopherol, is preferred. R ¹ particularly preferably means hydrogen or acetyl. Particularly preferred is RRR-alpha-tocopherol.

The monomer solution preferably contains at most 130 wt. Ppm, particularly preferably not more than 70 wt. Ppm, at least 10 wt. Ppm, particularly preferably at least 30 wt. Ppm, and most preferably about 50 wt. MD half-hydroquinone in each case relative to the content of acrylic acids, and salts of acrylic acids are taken into account as acrylic acids. For example, for the manufacture of a monomer solution, acrylic acid with an appropriate content of hydroquinone half ester can be used.

Polymerization inhibitors can also be removed from the monomer solution by absorption, for example, on activated carbon.

The crosslinking agent b) are compounds with at least two polymerizable groups that can be radically polymerized into a polymer network. Suitable crosslinking agents b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylol propane triacrylate, triallylamine, tetraallyloxyethane, as described in European application EP 530438 A1, di- and triacrylates, as described in 54 European Patent Application Advantages, European Patent Application No. 55, European Patent Application. international WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and German applications DE 10331450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylene unsaturated groups as described in German applications DE 10331456 A1 and DE 10355 401 A1, or mixtures of crosslinking agents, which are described, for example, in German DE 19543368 A1, DE 19646484 A1 and international applications WO 90/15830 A1 and WO 2002/32962 A2.

Suitable crosslinking agents b) are, in particular, N, N′-methylenebisacrylamide and N, N′-methylenebismethacrylamide, a polyol ester of unsaturated mono- or polycarboxylic acids, such as diacrylate or triacrylate, for example, butanediol or ethylene glycol diacrylate, or -methacrylate as well as trimethylol propane triacrylate and allyl compounds such as allyl (meth) acrylate, triallyl cyanurate, maleic acid diallyl ester, polyallyl ester, tetraallyloxyethane, triallylamide, tetraallyl ethylenediamide, allyl e firms of phosphoric acid, as well as derivatives of vinylphosphoric acid, as described, for example, in European application EP 343427 A2.

Further suitable crosslinking agents b) are pentaerythritol di-, pentaerythritol tri- and pentaerythritol tetraallyl ether, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol di and glyceryl triethyl ether, and also sorbitol ethyl allyl ether. In the method according to the invention, polyethylene glycol di (meth) acrylates are used, the polyethylene glycol used having a molecular weight of between 100 and 1000.

A particularly advantageous crosslinking agent b) is nevertheless di- and triacrylates of 3-20-fold ethoxylated glycerol, 3-20-fold ethoxylated trimethylolpropane, 3-20-fold ethoxylated trimethylol ethane, in particular 2-6- and tri- acrylates ethoxylated glycerol or trimethylolpropane; 3 times propoxylated glycerol or trimethylolpropane; and 3 times ethoxylated or propoxylated glycerol or trimethylolpropane; 15 times ethoxylated glycerol or trimethylolpropane; at least 40-fold ethoxylated glycerol, trimethylol ethane or trimethylol propane.

The highly preferred crosslinking agents b) are acrylic acids or methacrylic acids esterified to multiple ethoxylated or propoxylated glycerols esterified to di- and triacrylates, as, for example, described in international application WO 2003/104301 A1. Especially advantageous are the di- and / or triacrylates of 3-10-fold ethoxylated glycerols. Particularly preferred are di- or triacrylates of 1-5-fold ethoxylated and / or propoxylated glycerols. Triacrylates of 3-5-fold ethoxylated and / or propoxylated glycerols are highly preferred.

The monomer solution contains predominantly at least 0.1% by weight, preferably at least 0.2% by weight, more preferably at least 0.3% by weight, and particularly preferably at least 0.4% wt. and not more than 2.5% wt., preferred is not more than 2% wt., more preferably not more than 1.5% wt., and particularly preferred is not more than 1% wt. crosslinking agent b), in each case, relative to monomer a).

As initiator c), all, without exception, decomposing compounds under the polymerization conditions into radicals, for example, peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds, and so-called redox initiators, can be used. The use of water soluble initiators is preferred. In some cases, it is advantageous to use mixtures of various initiators, for example mixtures of hydrogen peroxide and sodium or potassium persulfate. Mixtures of hydrogen peroxide and sodium persulfate can be used in any ratio.

Particularly preferred initiators c) are azo initiators such as 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride and 2,2′-azobis [2- (5-methyl-2-imidazolin- 2-yl) propane] dihydrochloride, and photoinitiators such as 2-hydroxy-2-methylpropiophenone and 1- [4- (2-hydroxyethoxy) phenyl] -2-hydroxy-2-methyl-1-propan-1-one redox initiators such as sodium persulfate / hydroxymethyl sulfinic acid, ammonium peroxodisulfate / hydroxymethyl sulfinic acid, hydrogen peroxide / hydroxymethyl sulfinic acid, sodium persulfate ia / ascorbic acid, ammonium peroxodisulfate / ascorbic acid and hydrogen peroxide / ascorbic acid, as well as mixtures thereof.

The initiators are used in conventional amounts, for example, in an amount of 0.001-5% wt., Preferably 0.01-1% wt., Relative to monomer a).

Preferred polymerization inhibitors need optimal action in dissolved oxygen, and therefore, the monomer solution can be freed from dissolved oxygen prior to polymerization by creating an inert atmosphere, i.e. transmitting an inert gas, mainly nitrogen. Preferably, the oxygen content in the monomer solution is lowered to less than 1 w / w before polymerization, particularly preferably to less than 0.5 w / w.

For polymerization in the gas phase, the monomer solution is added dropwise.

The solids content of the monomer solution is advantageously at least 35% by weight, preferably at least 38% by weight, particularly preferably at least 40% by weight, highly preferably at least 42% wt. The solids content is the sum of all non-volatile substances after polymerization. These are monomer a), crosslinking agent b) and initiator c).

The oxygen content in the gas phase is preferably 0.001-0.15% vol., Particularly preferably 0.002-0.1% vol., Highly preferred 0.005-0.05% vol.

The gas phase contains, in addition to oxygen, mainly only inert gases, i.e. gases that do not enter into the polymerization reaction under the reaction conditions, for example nitrogen and / or water vapor.

When dropping, the monomer solution is metered into the gas phase by droplet formation. The instillation of the monomer solution can be carried out, for example, using a drip plate.

A drip plate is a plate with at least one hole, and the liquid flows from above through the hole. A drip plate or, accordingly, a liquid can go into oscillations, as a result of which, on the bottom side of the plate, each hole ideally produces a monodisperse chain of drops.

The number and size of holes are selected according to the desired volume and size of the droplets. The droplet diameter in this case is usually 1.9 hole diameters. It is important in this case that the liquid to be dripped does not pass too quickly through the holes or the pressure drop in the holes is not too large.

Otherwise, the liquid does not drip, and the liquid stream breaks (splashes) due to the high kinetic energy. A droplet former is used in the region of the laminar flow decay, i.e. the Reynolds number relative to the throughput of the hole and the diameter of the hole is preferably less than 2000, preferably less than 1000, particularly preferably less than 500, highly preferably less than 100. The pressure loss in the hole is mainly less than 2.5 bar, particularly preferably less than 1, 5 bar and most preferably less than 1 bar.

The drip plate usually has at least one, preferably at least 10, particularly preferably at least 50, and usually up to 10,000, preferably up to 5,000, particularly preferably up to 1,000 holes, the holes being distributed evenly on the drip plate, mainly in the so-called triangular distribution, i.e. three holes respectively form the corners of an equilateral triangle.

The diameter of the holes is adjusted to the desired droplet size. The droplets produced have an average size of preferably at least 100 microns, particularly preferably at least 150 microns, highly preferably at least 200 microns, the diameter of the droplet being determined by light scattering.

It may be advantageous to place the drip plate on the carrier plate, which plate also has openings. The holes of the carrier plate have a larger diameter than the holes on the drip plate, and are located so that under each hole of the drip plate there is a concentric hole with it on the carrier plate. This arrangement makes it possible to quickly replace the drip plate, for example, in order to produce a drop of a different size.

Dropping can also be carried out using pneumatic nozzles, rotation, cutting the flow or quickly adjustable microvent nozzles.

In a pneumatic nozzle, a stream of liquid along with a gas stream is accelerated through the diaphragm. Through the gas flow, it is possible to influence the diameter of the liquid jet and, together with it, the diameter of the drop.

When dropping by rotation, fluid escapes through the holes of the rotating disc. With the help of centrifugal force affecting the liquid, drops of a certain size come off. Preferred rotary dropping devices are described, for example, in German application DE 4308842 A1.

The escaping stream of liquid can be cut with the help of a rotating knife into certain segments. Each segment then forms a drop.

When using microvent nozzles directly drops with the set volume of liquid are made.

Mostly the gas phase in the form of a carrier gas flows through the reaction space. In this case, the carrier gas is passed through the reaction space in a direct flow or in countercurrent to freely supplying drops of a monomer solution, mainly in a direct flow. Advantageously, the carrier gas, after passing at least partially, preferably at least 50%, particularly preferably at least 75%, is returned as the circulation gas back to the reaction space. Typically, a portion of the carrier gas, preferably up to 10%, particularly preferably up to 3%, and most preferably up to 1% after passage, is discharged.

The gas velocity is mainly controlled so that the flow in the reactor is directed, for example, is the absence of convection vortices, opposite the general direction of flow, and is, for example, 0.01-5 m / s, preferably 0.02-4 m / s, especially preferably 0.05-3 m / s, highly preferably 0.1-2 m / s.

The carrier gas is heated to the reaction temperature, most preferably to the reactor.

The temperature of the reaction during thermally induced polymerization is preferably 70-250 ° C, particularly preferably 100-220 ° C and highly preferably 120-200 ° C.

The reaction can be carried out at an overpressure or a reduced pressure, a vacuum of up to 100 mbar with respect to the external pressure is preferred.

Exhaust reaction gas, i.e. The carrier gas that has left the reaction space can be cooled, for example, in a heat exchanger. In this case, water and the unreacted monomer a) condense. After that, the spent reaction gas can be at least partially reheated and introduced back into the reaction as a circulation gas. A portion of the exhaust gas may be removed from the reaction and replaced with fresh carrier gas, the water contained in the exhaust gas and the unconverted monomer a) being separated and may be returned to the reaction.

A thermal system is particularly preferred, which means that part of the waste heat when cooling the exhaust gas is used to heat the circulating gas.

Reactors can be additionally heated. The additional heating is set in such a way that the temperature of the walls is at least 5 ° C higher than the internal temperature of the reactor and condensation on the walls of the reactor is reliably prevented.

The reaction product can usually be removed from the reactor, preferably from the floor, by means of a screw conveyor and, if necessary, dried to the desired residual moisture and to the desired monomer content.

The polymer particles are subsequently crosslinked to further improve the properties. Suitable secondary crosslinking agents are compounds that contain at least 2 groups that can form covalent bonds with the carboxylate groups of hydrogels. Suitable compounds are, for example, polyacridine alkoxysilyl compounds, polyamines, polyamidoamines, di- or polyeloxides, as described, for example, in European patent applications EP 83022 A2, EP 543303 A1 and EP 937736 A2, di- and polyfunctional alcohols, as described in German DE 3314019 A1, DE 3523617 A1 and European EP 450 922 A2 patents, or β-hydroxyalkylamides, as described in German DE 10204938 A1 and US patent 6239230.

In addition, suitable secondary crosslinking agents are cyclic carbonates described in German patent DE 40 20-780 C1, in DE 19807502 A1 - 2-oxazolidone and its derivatives, such as 2-hydroxyethyl-2-oxazolidone, in DE 19807992 C1 - bis- and poly-2-oxazolidinones, in DE 19854573 A1 - 2-oxo-tetrahydro-1,3-oxazine and its derivatives, in DE 19854574 A1 - N-acryl-2-oxazolidones, in DE 10204937 A1 - cyclic urea, in DE 10334584 A1 - bicyclic amide acetates, in European patent EP 1199327 A2 - oxetanes and cyclic urea, and in international application WO 2003/31482 A - morpholine-2,3-dione and its derivative Wow.

Further, secondary crosslinking agents can be used that contain additional polymerizable ethylene unsaturated groups, as described in German patent DE 3713601 A1.

The amount of the secondary crosslinking agent is preferably 0.01 to 1% by weight, particularly preferably 0.05 to 0.5% by weight, highly preferably 0.1 to 0.2% by weight, in each case relative to the polymer.

In one preferred embodiment of the invention, in addition to the secondary crosslinking agents, polyvalent cations are applied to the surface of the particles.

The polyvalent cations used in the method according to the invention are, for example, divalent cations, such as, for example, zinc, magnesium, calcium and strontium cations, trivalent cations, such as aluminum, iron, chromium, rare earth metal and manganese oxides, tetravalent cations, such as cations of titanium and zirconium. As the counterion, chloride, bromide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate such as acetate and lactate can be used. Aluminum sulfate is preferred.

The amount used is, for example, 0.001 to 0.5% by weight, preferably 0.005 to 0.2% by weight, particularly preferably 0.02 to 0.1% by weight. relative to polymer particles.

Secondary crosslinking is usually carried out in such a way that the solution of the secondary crosslinking agent is sprayed onto a hydrogel or dry polymer particles. To attach, the sputtering is thermally dried, and the secondary crosslinking reaction can occur both before and during drying.

The spraying of the solution of the secondary crosslinking agent is carried out mainly in a mixer with moving mixing mechanisms, such as a screw mixer, paddle mixer, disk mixer, ploughshare mixer and blade mixer. Vertical mixers are particularly preferred; a ploughshare mixer and paddle mixer are highly preferred. Suitable mixers are Lödige, Bepex, Nauta, Processall and Schugi.

Thermal drying is carried out mainly in a contact drier, a blade dryer is particularly preferred, and a disk dryer is particularly preferred. Suitable are, for example, dryers from Bepex und Nara. Fluid bed dryers may also be used.

Drying can be carried out in the mixer itself, by heating the housing or blowing warm air. In the same way, a separately connected dryer, such as, for example, a grid dryer, a rotary tube furnace or a heated screw, is suitable. It is especially advantageous to dry and mix in a fluid bed dryer.

The preferred drying temperature lies in the range of 100-250 ° C, preferably 120-220 ° C and particularly preferably 140-200 ° C. The preferred processing time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, particularly preferably at least 20 minutes, highly preferably at least 30 minutes.

The method according to the invention makes it possible to manufacture water-absorbing polymer particles with a high centrifugal retention capacity (CRC), with high absorption under pressure of 4.83 kPa (AUP 0.7 psi) and high mechanical stability.

In accordance with the method according to the invention, the resulting water-absorbing polymer particles are usually in the form of hollow balls. A further subject of this invention is water-absorbing polymer particles containing at least one cavity within the particle and at least 1% wt. water and / or at least one alkanolamine, and the density of crosslinking on the surface of the particles is increased.

The water content of the polymer particles described in the invention is preferably 1-30% by weight, particularly preferably 2-20% by weight, highly preferably 3-15% by weight.

The polymer particles described in the invention preferably comprise 0.5-30% by weight, particularly preferably 1-20% by weight, highly preferably 2-15% by weight, of at least one alkanolamine.

The water-absorbing polymer particles described in the invention are almost round, i.e. the polymer particles have an average sphericity (mSPHT) of usually at least 0.84, preferably at least 0.86, particularly preferably at least 0.88, highly preferably at least 0, 9. Sphericity is defined as

,

,

moreover, A is the cross-sectional area and U is the circumference of the cross-section. Average sphericity is volume averaged sphericity.

Average sphericity (mSPHT) is determined, for example, by a Camsizer® image processing system (Retsch Technolgy GmbH; DE).

The relatively low average sphericity polymer particles (mSPHTs) are prepared by reverse suspension polymerization, where the polymer particles agglomerate during or after polymerization.

The water-absorbing polymer particles obtained through conventional solution polymerization (gel polymerization) are crushed and sorted after drying, and disordered polymer particles are obtained. The average sphericity of these polymer particles lies between about 0.72 and about 0.78.

A further subject of the invention are polymer particles containing at least one cavity within the particle, the crosslink density on the surface of the particle being increased, the particle exhibiting an average sphericity (mSPHT) of at least 0.84, preferably at least 0 86, particularly preferably at least 0.88, highly preferably at least 0.9, and particles having an absorption index (AUP0.7 psi-Index) of at least 60%, preferably at least 70%, particularly preferably at least 80% and in highly preferred, at least 85%.

A further subject of the invention are polymer particles containing at least one cavity within the particle, the crosslink density on the surface of the particle being increased and having an average sphericity (mSPHT) of at least 0.84, preferably at least 0 86, particularly preferably at least 0.88, highly preferably at least 0.9, and a stability index of less than 0.15, preferably less than 0.10, particularly preferably less than 0.075, highly preferred, less than 0.05.

The water content of the water-absorbing polymer particles is typically less than 10% by weight, preferably less than 8% by weight, particularly preferably less than 6% by weight, highly preferably less than 4% by weight, the water content being determined in accordance with the recommended EDANA (European Disposables and Nonwovens Association) research method Nr. WSP 230.2-05 "Moisture content".

The ratio of the maximum diameter of the cavity to the maximum diameter of the polymer particle is preferably at least 0.1, particularly preferably at least 0.3, highly preferably at least 0.4.

The polymer particles preferably contain at least 50 mol%, particularly preferably at least 90 mol%, highly preferably at least 95 mol%. polymerized acrylic acids. Polymerized acrylic acids, preferably up to 25-85 mol%, particularly preferably up to 50-80 mol%, highly preferably up to 60-75 mol%, are neutralized.

A further subject of the invention are polymer particles, which can be obtained by the method according to the invention.

In accordance with the method according to the invention, the resulting water-absorbing polymer particles have a retention capacity during centrifugation (CRC), usually at least 20 g / g, preferably at least 25 g / g, preferably at least 30 g / g , particularly preferably at least 35 g / g, highly preferably at least 40 g / g. The centrifugal retention capacity (CRC) of the water-absorbing polymer particles is usually less than 60 g / g.

In accordance with the method according to the invention, the resulting water-absorbing polymer particles have a pressure absorption of 4.83 kPa (AUP0.7 psi), usually at least 15 g / g, preferably at least 20 g / g, particularly preferably at least 25 g / g, highly preferably at least 30 g / g. Absorption under pressure of 4.83 kPa (AUP0.7 psi) of water-absorbing polymer particles is usually less than 50 g / g.

In accordance with the method according to the invention, the resulting water-absorbing polymer particles have a permeability (SFC) of usually at least 2 × 10 ⁻⁷ cm ³ s / g, preferably at least 10 × 10 ⁻⁷ cm ³ s / g, preferably at least 30 × 10 ⁻⁷ cm ³ s / g, particularly preferably at least 60 × 10 ⁻⁷ cm ³ s / g, highly preferably at least 200 × 10 ⁻⁷ cm ³ s / g The permeability (SFC) of the water-absorbing polymer particles is usually less than 500 × 10 ⁻⁷ cm ³ s / g.

The average diameter of the polymer particles is preferably at least 200 microns, particularly preferably from 250 to 600 microns, highly preferably from 300 to 500 microns, and the diameter of the polymer particles can be determined through light scattering and means a volume-averaged average diameter. 90% of the polymer particles have a diameter of preferably from 100 to 800 microns, particularly preferably from 150 to 700 microns, highly preferably from 200 to 600 microns.

A further subject of this invention are methods for the manufacture of hygiene products, in particular diapers, covering the use of water-absorbing polymer particles obtained according to the above methods.

A further object of this invention is the use of the water-absorbing polymer particles described in the invention in hygiene products, for thickening waste, especially medical waste, or as a water-holding agent in agriculture.

Water-absorbing polymer particles are tested using the following test methods.

Methods

Measurements should be taken, unless otherwise indicated, at an ambient temperature of 23 ± 2 ° C and a relative humidity of 50 ± 10%. Water-absorbing polymer particles are thoroughly mixed before measurement.

Average sphericity (mSPHT)

Average sphericity (mSPHT) is determined using the Camsizer® image processing system (Retsch Technolgy GmbH; DE).

For measurement, the product is poured through a funnel and delivered via a vibrating metering trough to the loading shaft. When particles fly past the luminous screen, they are selectively captured by the camera. The resulting images are processed by the software according to the selected parameters.

To characterize the roundness, the indicators indicated in the program along with sphericity are used. The average sphericities correlated with the volume are indicated, and the particle volume is determined using the equivalent diameter xc _min . To determine the equivalent diameter xc _min , the longest chordal diameter is measured in each case for 32 different directions of space. The equivalent diameter xc _min will be the shortest of the 32 chordal diameters. The equivalent diameter corresponds to the size of the filter opening, which the particle can still pass directly. To detect particles, the so-called CCD-Zoom camera (CAM-Z) is used. To control the vibratory metering chute, the fraction of the charged surface is set from 0.5%.

Water content

The water content in water-absorbing polymer particles is determined according to the recommended EDANA (European Disposables and Nonwovens Association) research methods Nr. WSP 230.2-05 "Moisture content".

Centrifuge Retention Capacity (CRC)

The retention capacity during centrifugation of water-absorbing polymer particles is determined according to the recommended EDANA (European Disposables and Nonwovens Association) research methods Nr. WSP 241.2-05 "Centrifuge retention capacity".

Absorption Under Pressure (AUP0.7 psi Absorption Under Pressure)

Pressure absorption is determined according to the recommended EDANA (European Disposables and Nonwovens Association) research methods Nr. WSP 242.2-05 "Absorption under pressure", using a weight of 49 g / cm ² (0.7 psi) instead of 21 g / cm ² (0.3 psi).

Liquid Permeability (SFC Saline Flow Conductivity)

The fluid permeability of the swollen gel layer under a compressive load of 0.3 psi (2070 Pa), as described in European patent application EP 640330 A1, is defined as the Gel-Layer-Permeability of the swollen gel layer of water-absorbing polymer particles, moreover, in the previously mentioned patent application on page 19 and figure 8, the described equipment is modified in that the glass filter (40) is no longer used, the plunger (39) consists of the same synthetic material as the cylinder (37), and now contains uniformly distributed on the entire supporting surface 21 avnovelikoe hole. The measurement principle and processing remain unchanged in accordance with European application EP 640330 A1. Leaks are recorded automatically.

Liquid Permeability (SFC) is calculated as follows:

SFC [cm ³ s / g] = (Fg (t = 0) × L0) / (d × A × WP),

moreover, Fg (t = 0) expresses the flow of a solution of sodium chloride NaCl in g / s, which is obtained by linear regression analysis of the data Fg (t) to determine the flow using extrapolation to t = 0, L0 is the gel layer thickness in cm, d is the density a solution of sodium chloride NaCl in g / cm ³ , A is the area of the gel layer in cm ² , and WP is the hydrostatic pressure on the gel layer in dyne / cm ² .

Absorption Index (AUP0.7 psi-Index)

The absorption index (AUP0.7 psi-Index) describes the mechanical stability of water-absorbing polymer particles.

For this, 20 g of water-absorbing polymer particles are weighed in a cylindrical porcelain mill with an internal volume of approximately 360 ml. The porcelain mill has an internal length of 8.8 cm and an inner diameter of 7.2 cm. Additionally, 24 cylindrical porcelain capsules are poured. Porcelain capsules have a height of 1.25 cm and a diameter of 1.25 cm. The weight of the porcelain capsule is 5.3 g. The cylindrical porcelain mill is closed and rotated for 15 minutes at a speed of 150 rpm using a roller drive.

Pressure absorption (AUP0.7 psi) of water-absorbing polymer particles is measured before and after mechanical stress.

The absorption index (AUP0.7 psi-Index) is calculated as follows:

AUP0.7 psi-Index = AUP0.7 psi _after /AUP0.7psi _to × 100%,

moreover, AUP0.7 psi _before , absorption under pressure (AUP0.7 psi) of polymer particles before mechanical loads, and AUP0.7 psi _after - absorption under pressure (AUP0.7 psi) of polymer particles after mechanical loads.

Stability index

The stability index describes the mechanical stability of water-absorbing polymer particles.

Mechanical loading is carried out, as already described in the absorption index.

Part of the water-absorbing polymer particles with a particle size of less than 100 μm is measured before and after mechanical loading. Water-absorbing polymer particles contain more than 1% wt. particles with a size of less than 100 microns, so they are separated in advance.

The stability index is calculated as follows:

Stability Index = (Particles <100 μm _after -Particles <100 μM _before ) / 100% wt.

moreover, Particles <100 μm _before - this is the mass fraction of polymer particles with a particle size of less than 100 microns before mechanical loads, and Particles <100 microns _after - this is the mass fraction of polymer particles with a particle size of less than 100 microns after mechanical loads.

Particles with particle sizes less than 100 are detected optically using the PartAn Particle Analyzer Tour 2001 F / L (Fa. AnaTec, Duisburg, DE). For measurements, 20 g of polymer particles are used.

Examples

Example 1

14.3 kg of sodium acrylate (37.5% by weight solution in water) and 1.6 kg of acrylic acid were mixed with 29 g of 15-fold ethoxylated trimethylol propane triacrylate. The solution was added dropwise in a heated drop tower filled with nitrogen atmosphere (180 ° C, 12 m high, 2 m wide, gas velocity in the forward flow 0.1 m / s).

The dosing rate was 16 kg / h. The drip plate had 30 holes with a diameter of 170 μm. The diameter of the drip plate was 65 mm. The initiator was mixed using a static mixer with a solution of monomer shortly before the dropping agent.

A 6.5% by weight solution of 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride in water was used as an initiator. The dosage rate of the initiator solution is 0.44 kg / h. The gas temperature at the outlet of the drip tower was 126 ° C.

In conclusion, the resulting water-absorbing polymer particles were crosslinked a second time. For this, 200 g of water-absorbing polymer particles were sprayed with 6.3 g of a solution of a secondary crosslinking agent using a two-component nozzle in a food processor with a moderate degree of mixing. The solution of the secondary crosslinking agent consisted of 0.3 g of 2-hydroxyethyl-2-oxazolidone, 4.2 g of water and 1.8 g of isopropanol. The wet polymer was homogenized again with a spatula and heated for 90 min at 170 ° C in an air circulation oven. Secondary crosslinked polymer particles were freed from lumps through 850 μm sieves.

Example 2

According to example 1, the obtained second-crosslinked water-absorbing polymer particles were coated with a layer. For this, 200 g of second-crosslinked water-absorbing polymer particles were heated in a heating cabinet for 30 minutes at a temperature of 80 ° C and sprayed with coating agents using a two-component nozzle in a food processor with medium mixing. As coating agents, 8.0 g of water was used. Then everything was mixed for 1 minute. After 16 hours of storage, the coated polymer particles were sieved through 850 μm sieves to free from lumps.

The results are combined in a table.

Example 3

The procedure was the same as in Example 2. As a coating agent, a solution of 8.0 g of water and 0.306 g of sorbitan monolaurate was used.

The results are combined in a table.

Example 4

The procedure was the same as in Example 2. A solution of 8.0 g of water and 0.306 g of sorbitan monolaurate and 3.0 g of triethanolamine was used as a coating agent.

The results are combined in a table.

Example 5

The procedure was the same as in Example 2. A solution of 8.0 g of water and 0.306 g of sorbitan monolaurate and 6.0 g of triethanolamine was used as a coating agent.

The results are combined in a table.

Research results. Example AUP0.7psi (up to) AUP0.7psi (after) AUP0.7psi index Particles <100 μm (up to) Particles <100 μm (after) Stability index one*) 24.7 g / g 12.8 g / g 51.8% 0.2% wt. 30.9% wt. 0.31 2 24.2 g / g 16.4 g / g 67.9% 0.1% wt. 20.3% wt. 0.20 3 24.1 g / g 19.0 g / g 78.8% 0.1% wt. 6.6% wt. 0,065 four 23.6 g / g 18.4 g / g 78.0% 0.1% wt. 6.6% wt. 0,065 5 22.8 g / g 19.4 g / g 85.1% 0.1% wt. 4.8% wt. 0,047 *) test case

The polymer particles obtained by conventional gel polymerization are milled after drying. Such particles, due to the edges of the debris, are less rounded and therefore have a low sphericity. This is confirmed by comparing (see FIGS. 1-7 below) a commercially available BASF product (Hysoer® B 7015) with polymer particles prepared according to the invention (polymer in the form of drops 1 and polymer in the form of drops 2).

Claims (12)

1. A method of manufacturing a water-absorbing polymer particles containing at least one cavity inside a particle through the polymerization of drops of a monomer solution containing
a) at least one ethylene unsaturated monomer having at least one acid group,
b) at least one crosslinking agent,
c) at least one initiator,
d) water
in the gas phase surrounding the droplet, wherein the obtained polymer particles are subsequently crosslinked, characterized in that the secondly crosslinked polymer particles are at least partially coated with water, an aqueous solution, alkanolamine, a polymer and / or wax. 2. The method according to claim 1, characterized in that the solids content in the monomer solution is at least 35 wt.%. 3. The method according to claim 1, characterized in that the monomer a) at least 50 mol.% Represents at least partially neutralized acrylic acid. 4. The method according to one of claims 1 to 3, characterized in that the droplets have an average diameter of at least 100 microns. 5. Water-absorbing polymer particles obtained by the method according to claim 1 and containing at least one cavity inside the particle and at least 1 wt.% Water and / or at least alkanolamine, wherein the crosslink density is on the outer surface particles increased. 6. Water-absorbing polymer particles according to claim 5, characterized in that the polymer particles have an average sphericity of at least 0.84. 7. Water-absorbing polymer particles obtained by the method according to claim 1 and containing at least one cavity inside the particle, and the crosslink density on the outer surface of the particles is increased, the particles have an average sphericity of at least 0.84 and an absorption index of at least 60%. 8. Water-absorbing polymer particles obtained by the method according to claim 1 and containing at least one cavity inside the particle, and the crosslink density on the outer surface of the particles is increased, the particles have an average sphericity of at least 0.84 and a stability index of less than 0.15. 9. Water-absorbing polymer particles according to one of claims 5 to 8, characterized in that the polymer particles have at least one cavity and the ratio of the maximum diameter of the cavity to the maximum diameter of the polymer particle is at least 0.1. 10. Water-absorbing polymer particles according to one of claims 5 to 8, characterized in that the polymer particles contain at least 50 mol%, at least partially neutralized polymerized acrylic acid. 11. Water-absorbing polymer particles according to one of claims 5 to 8, characterized in that the polymer particles have an absorption under pressure of 4.83 kPa of at least 15 g / g. 12. Hygiene products containing polymer particles according to one of paragraphs.5-11.

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