# Patent

## Title
Process for producing water-absorbing polymer particles in a polymerization reactor with at least two axially parallel rotating shafts

## Abstract

A process for producing water-absorbing polymer particles by polymerizing an aqueous monomer solution in a polymerization reactor with at least two axially parallel rotating shafts, wherein the polymerization is performed in the presence of an inert gas and under positive pressure.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. provisional patent application No. 61/658,928, filed Jun. 13, 2012, incorporated herein by reference in its entirety.


The present invention relates to a process for producing water-absorbing polymer particles by polymerizing an aqueous monomer solution in a polymerization reactor with at least two axially parallel rotating shafts, wherein the polymerization is performed in the presence of an inert gas and under positive pressure.
Water-absorbing polymer particles are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening. The water-absorbing polymer particles are also referred to as superabsorbents.
The production of water-absorbing polymer particles is described in the monograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, pages 71 to 103.
The properties of the water-absorbing polymer particles can be adjusted, for example, via the amount of crosslinker used. With an increasing amount of crosslinker, the centrifuge retention capacity (CRC) falls and the absorption under a pressure of 21.0 g/cm2 (AUL0.3 psi) passes through a maximum.
To improve the use properties, for example, permeability of the swollen gel bed (SFC) in the diaper and absorption under a pressure of 49.2 g/cm2 (AUL0.7 psi), water-absorbing polymer particles are generally surface postcrosslinked. This increases the level of crosslinking of the particle surface, which can at least partly decouple the absorption under a pressure of 49.2 g/cm2 (AUL0.7 psi) and the centrifuge retention capacity (CRC). This surface postcrosslinking can be performed in the aqueous gel phase. Preferably, however, dried, ground and sieved polymer particles (base polymer) are surface coated with a surface postcrosslinker and thermally surface postcrosslinked. Crosslinkers suitable for that purpose are compounds which can form covalent bonds to at least two carboxylate groups of the water-absorbing polymer particles.
The production of water-absorbing polymer particles in a polymerization reactor with at least two axially parallel rotating shafts (kneaders) is described, for example, in WO 01/038402 A1, WO 03/022896 A1, WO 03/051415 A1, WO 2006/034806 A1, WO 2006/034853 A1 and WO 2009/115472 A1.
WO 01/038402 A1 discloses the evaporation of water during the polymerization for removal of the heat of reaction.
WO 03/022896 A1 teaches the supply of steam into the polymerization reactor.
WO 2003/051415 A1 describes the advantages of supplying a hot monomer solution to the polymerization reactor.
WO 2006/034806 A1 relates to further aspects of the polymerization, such as the fill level of the polymerization reactor, the inhibitor content of the monomer solution, the temperature in the reaction zone and backmixing during the polymerization.
WO 2006/034853 A1 relates to the configuration of the polymerization reactor itself.
WO 2009/115472 A1 describes the use of a small amount of inert gas for production of water-absorbing polymer particles with high centrifuge retention capacity (CRC).
It was an object of the present invention to provide an improved process for producing water-absorbing polymer particles, especially water-absorbing polymer particles with a high absorption under a pressure of 21.0 g/cm2 (AUL0.3 psi), a low level of extractables and a low level of residual monomer.
The object was achieved by a process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising
a) an ethylenically unsaturated monomer which bears acid groups and may be at least partly neutralized,
b) at least one crosslinker,
c) at least one initiator,
d) at least one ethylenically unsaturated monomer copolymerizable with the monomers mentioned under a) and
e) optionally one or more water-soluble polymers,
in a polymerization reactor with at least two axially parallel rotating shafts, which comprises performing the polymerization in the presence of an inert gas and under positive pressure.
Suitable inert gases are nitrogen, carbon dioxide, steam and argon. The polymerization reaction is inhibited by oxygen. Therefore, the inert gas should comprise preferably less than 0.001% by volume, more preferably less than 0.0005% by volume and most preferably less than 0.0002% by volume of oxygen. Advantageously, the inert gas flows continuously through the polymerization reactor. The inert gas volume flow rate is preferably from 0.001 to 5 m3/h per m3 of reactor volume, more preferably from 0.01 to 2 m3/h per m3 of reactor volume and most preferably from 0.2 to 1 m3/h per m3 of reactor volume.
The inert gas used is preferably nitrogen, more preferably in technical grade quality. Technical grade nitrogen comprises typically at least 99.8% by volume of nitrogen and less than 0.0005% by volume of oxygen.
The positive pressure in the polymerization reactor is preferably from 1 to 500 mbar, more preferably from 10 to 100 mbar and most preferably from 15 to 50 mbar, positive pressure meaning a higher pressure than ambient pressure.
The kneaders usable in the process according to the invention have at least two axially parallel rotating shafts, typically with several kneading and transport elements present on the shafts.
Kneaders usable in the process according to the invention are available, for example, from List AG (Arisdorf; Switzerland) and are described in CH 664 704 A5, EP 0 517 068 A1, WO 97/12666 A1, DE 21 23 956 A1, EP 0 603 525 A1, DE 195 36 944 A1 and DE 41 18 884 A1.
Such kneaders with at least two shafts achieve, by virtue of the arrangement of the kneading and transport elements, a high level of self-cleaning, which is an important requirement for a continuous polymerization. The two shafts preferably rotate counter to one another.
On the stirrer shaft, the disk segments are arranged in the manner of a propeller. Suitable kneading and transport elements are, for example, close-clearance mixing bars and L- or U-shaped attachments.
The present invention is based on the finding that the properties of water-absorbing polymer particles can be improved when a switch is made from customary polymerization under atmospheric pressure to a polymerization under positive pressure.
The production of the water-absorbing polymer particles is described in detail hereinafter:
The water-absorbing polymer particles are produced by polymerizing a monomer solution or suspension, and are typically water-insoluble.
The monomers a) are preferably water-soluble, i.e. the solubility in water at 23° C. is typically at least 1 g/100 g of water, preferably at least 5 g/100 g of water, more preferably at least 25 g/100 g of water and most preferably at least 35 g/100 g of water.
Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particular preference is given to acrylic acid.
Further suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
Impurities can have a considerable influence on the polymerization. The raw materials used should therefore have a maximum purity. It is therefore often advantageous to specially purify the monomers a). Suitable purification processes are described, for example, in WO 02/055469 A1, WO 03/078378 A1 and WO 2004/035514 A1. A suitable monomer a) is, for example, acrylic acid purified according to WO 2004/035514 A1 and comprising 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid, 0.0332% by weight of water, 0.0203% by weight of propionic acid, 0.0001% by weight of furfurals, 0.0001% by weight of maleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether.
The proportion of acrylic acid and/or salts thereof in the total amount of monomers a) is preferably at least 50 mol %, more preferably at least 90 mol %, most preferably at least 95 mol %.
The monomers a) typically comprise polymerization inhibitors, preferably hydroquinone monoethers, as storage stabilizers.
The monomer solution comprises preferably up to 250 ppm by weight, preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight, and preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight and especially around 50 ppm by weight, of hydroquinone monoether, based in each case on the unneutralized monomer a). For example, the monomer solution can be prepared by using an ethylenically unsaturated monomer bearing acid groups with an appropriate content of hydroquinone monoether.
Preferred hydroquinone monoethers are hydroquinone monomethyl ether (MEHQ) and/or alpha-tocopherol (vitamin E).
Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be polymerized free-radically into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomer a). In addition, polyvalent metal salts which can form coordinate bonds with at least two acid groups of the monomer a) are also suitable as crosslinkers b).
Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be polymerized free-radically into the polymer network. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO 03/104299 A1, WO 03/104300 A1, WO 03/104301 A1 and DE 103 31 450 A1, mixed acrylates which, as well as acrylate groups, comprise further ethylenically unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, as described, for example, in DE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 02/032962 A2.
Preferred crosslinkers b) are pentaerythrityl triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, 15-tuply ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine.
Very particularly preferred crosslinkers b) are the polyethoxylated and/or -propoxylated glycerols which have been esterified with acrylic acid or methacrylic acid to give di- or triacrylates, as described, for example, in WO 03/104301 A1. Di- and/or triacrylates of 3- to 10-tuply ethoxylated glycerol are particularly advantageous. Very particular preference is given to di- or triacrylates of 1- to 5-tuply ethoxylated and/or propoxylated glycerol. Most preferred are the triacrylates of 3- to 5-tuply ethoxylated and/or propoxylated glycerol, especially the triacrylate of 3-tuply ethoxylated glycerol.
The amount of crosslinker b) is preferably 0.05 to 1.5% by weight, more preferably 0.1 to 1% by weight and most preferably 0.2 to 0.6% by weight, based in each case on unneutralized monomer a). With rising crosslinker content, the centrifuge retention capacity (CRC) falls and the absorption under a pressure of 21.0 g/cm2 passes through a maximum.
The initiators c) used may be all compounds which generate free radicals under the polymerization conditions, for example thermal initiators, redox initiators or photoinitiators. Suitable redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodium peroxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite. Preference is given to using mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate/hydrogen peroxide/ascorbic acid. The reducing component used is, however, preferably a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures are obtainable as Brüggolite® FF6 and Brüggolite® FF7 (Bruggemann Chemicals; Heilbronn; Germany).
Ethylenically unsaturated monomers d) copolymerizable with the ethylenically unsaturated monomers a) bearing acid groups are, for example, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.
The water-soluble polymers e) used may be polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.
Typically, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75% by weight, more preferably from 45 to 70% by weight and most preferably from 50 to 65% by weight. It is also possible to use monomer suspensions, i.e. monomer solutions with excess monomer a), for example sodium acrylate. With rising water content, the energy requirement in the subsequent drying rises, and, with falling water content, the heat of polymerization can only be removed inadequately.
For optimal action, the preferred polymerization inhibitors require dissolved oxygen. The monomer solution can therefore be freed of dissolved oxygen before the polymerization by inertization, i.e. flowing an inert gas through, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution is preferably lowered before the polymerization to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.
To improve the drying properties, the comminuted polymer gel obtained can additionally be extruded.
The acid groups of the resulting polymer gels have typically been partially neutralized. Neutralization is preferably carried out at the monomer stage. This is typically accomplished by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid. The degree of neutralization is preferably from 25 to 85 mol %, more preferably from 30 to 80 mol % and most preferably from 40 to 75 mol %, for which the customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogencarbonates and also mixtures thereof. Instead of alkali metal salts, it is also possible to use ammonium salts. Particularly preferred alkali metals are sodium and potassium, but very particular preference is given to sodium hydroxide, sodium carbonate or sodium hydrogencarbonate and also mixtures thereof.
However, it is also possible to carry out neutralization after the polymerization, at the stage of the polymer gel formed in the polymerization. It is also possible to neutralize up to 40 mol %, preferably 10 to 30 mol % and more preferably 15 to 25 mol % of the acid groups before the polymerization by adding a portion of the neutralizing agent directly to the monomer solution and setting the desired final degree of neutralization only after the polymerization, at the polymer gel stage. When the polymer gel is neutralized at least partly after the polymerization, the polymer gel is preferably comminuted mechanically, for example by means of an extruder, in which case the neutralizing agent can be sprayed, sprinkled or poured on and then carefully mixed in. To this end, the gel mass obtained can be repeatedly extruded for homogenization.
The polymer gel is then preferably dried with a belt drier until the residual moisture content is preferably 0.5 to 15% by weight, more preferably 1 to 10% by weight and most preferably 2 to 8% by weight, the residual moisture content being determined by EDANA recommended test method No. WSP 230.2-05 “Mass Loss Upon Heating”. In the case of too high a residual moisture content, the dried polymer gel has too low a glass transition temperature Tg and can be processed further only with difficulty. In the case of too low a residual moisture content, the dried polymer gel is too brittle and, in the subsequent comminution steps, undesirably large amounts of polymer particles with an excessively low particle size are obtained (“fines”). The solids content of the gel before the drying is preferably from 25 to 90% by weight, more preferably from 35 to 70% by weight and most preferably from 40 to 60% by weight. However, a fluidized bed drier or a paddle drier may optionally also be used for drying purposes.
Thereafter, the dried polymer gel is ground and classified, and the apparatus used for grinding may typically be single or multistage roll mills, preferably two- or three-stage roll mills, pin mills, hammer mills or vibratory mills.
The mean particle size of the polymer particles removed as the product fraction is preferably at least 200 μm, more preferably from 250 to 600 μm and very particularly from 300 to 500 μm. The mean particle size of the product fraction may be determined by means of EDANA recommended test method No. WSP 220.2-05 “Particle Size Distribution”, where the proportions by mass of the screen fractions are plotted in cumulated form and the mean particle size is determined graphically. The mean particle size here is the value of the mesh size which gives rise to a cumulative 50% by weight.
The proportion of particles having a particle size of greater than 150 μm is preferably at least 90% by weight, more preferably at least 95% by weight and most preferably at least 98% by weight.
Polymer particles with too small a particle size lower the permeability (SFC). The proportion of excessively small polymer particles (“fines”) should therefore be low.
Excessively small polymer particles are therefore typically removed and recycled into the process. This is preferably done before, during or immediately after the polymerization, i.e. before the drying of the polymer gel. The excessively small polymer particles can be moistened with water and/or aqueous surfactant before or during the recycling.
It is also possible to remove excessively small polymer particles in later process steps, for example after the surface postcrosslinking or another coating step. In this case, the excessively small polymer particles recycled are surface postcrosslinked or coated in another way, for example with fumed silica.
When a kneading reactor is used for polymerization, the excessively small polymer particles are preferably added during the last third of the polymerization.
When the excessively small polymer particles are added at a very early stage, for example actually to the monomer solution, this lowers the centrifuge retention capacity (CRC) of the resulting water-absorbing polymer particles. However, this can be compensated for, for example, by adjusting the amount of crosslinker b) used.
When the excessively small polymer particles are added at a very late stage, for example not until an apparatus connected downstream of the polymerization reactor, for example an extruder, the excessively small polymer particles can be incorporated into the resulting polymer gel only with difficulty. Insufficiently incorporated, excessively small polymer particles are, however, detached again from the dried polymer gel during the grinding, are therefore removed again in the course of classification and increase the amount of excessively small polymer particles to be recycled.
The proportion of particles having a particle size of at most 850 μm is preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight.
The proportion of particles having a particle size of at most 600 μm is preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight.
Polymer particles of excessively large particle size lower the free swell rate. The proportion of excessively large polymer particles should therefore likewise be small.
Excessively large polymer particles are therefore typically removed and recycled into the grinding of the dried polymer gel.
To further improve the properties, the polymer particles can be surface postcrosslinked. Suitable surface postcrosslinkers are compounds which comprise groups which can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional amido amines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and U.S. Pat. No. 6,239,230.
Additionally described as suitable surface postcrosslinkers are cyclic carbonates in DE 40 20 780 C1, 2-oxazolidinone and derivatives thereof, such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 502 A1, bis- and poly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazine and derivatives thereof in DE 198 54 573 A1, N-acyl-2-oxazolidinones in DE 198 54 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals in DE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 and morpholine-2,3-dione and derivatives thereof in WO 03/031482 A1.
Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.
Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol.
In addition, it is also possible to use surface postcrosslinkers which comprise additional polymerizable ethylenically unsaturated groups, as described in DE 37 13 601 A1.
The amount of surface postcrosslinker is preferably 0.001 to 2% by weight, more preferably 0.02 to 1% by weight and most preferably 0.05 to 0.2% by weight, based in each case on the polymer particles.
In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers before, during or after the surface postcrosslinking.
The polyvalent cations usable in the process according to the invention are, for example, divalent cations such as the cations of zinc, magnesium, calcium, iron and strontium, trivalent cations such as the cations of aluminum, iron, chromium, rare earths and manganese, tetravalent cations such as the cations of titanium and zirconium. Possible counterions are hydroxide, chloride, bromide, sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate and carboxylate, such as acetate, citrate and lactate. Salts with different counterions are also possible, for example basic aluminum salts such as aluminum monoacetate or aluminum monolactate. Aluminum sulfate, aluminum monoacetate and aluminum lactate are preferred. Apart from metal salts, it is also possible to use polyamines as polyvalent cations.
The amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight and more preferably 0.02 to 0.8% by weight, based in each case on the polymer particles.
The surface postcrosslinking is typically performed in such a way that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. After the spray application, the polymer particles coated with surface postcrosslinker are surface postcrosslinked and dried, and the surface postcrosslinking reaction can take place both before and during the drying.
The spray application of a solution of the surface postcrosslinker is preferably performed in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Particular preference is given to horizontal mixers such as paddle mixers, very particular preference to vertical mixers. The distinction between horizontal mixers and vertical mixers is made by the position of the mixing shaft, i.e. horizontal mixers have a horizontally mounted mixing shaft and vertical mixers a vertically mounted mixing shaft. Suitable mixers are, for example, horizontal Pflugschar® plowshare mixers (Gebr. Lodige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV; Doetinchem; the Netherlands), Processall Mixmill mixers (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; the Netherlands). However, it is also possible to spray on the surface postcrosslinker solution in a fluidized bed.
The surface postcrosslinkers are typically used in the form of an aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted via the content of nonaqueous solvent and total amount of solvent.
When exclusively water is used as the solvent, a surfactant is advantageously added. This improves the wetting characteristics and reduces the tendency to form lumps. However, preference is given to using solvent mixtures, for example isopropanol/water, 1,3-propanediol/water and propylene glycol/water, where the mixing ratio in terms of mass is preferably from 20:80 to 40:60.
The surface postcrosslinking is preferably performed in contact driers, more preferably shovel driers, most preferably disk driers. Suitable driers are, for example, Hosokawa Bepex® Horizontal Paddle Dryers (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Dryers (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® driers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryers (NARA Machinery Europe; Frechen; Germany). Moreover, fluidized bed driers may also be used.
The surface postcrosslinking can be effected in the mixer itself, by heating the jacket or blowing in warm air. Equally suitable is a downstream drier, for example a shelf drier, a rotary tube oven or a heatable screw. It is particularly advantageous to effect mixing and drying in a fluidized bed drier.
Preferred reaction temperatures are in the range of 100 to 250° C., preferably 120 to 220° C., more preferably 130 to 210° C. and most preferably 150 to 200° C. The preferred residence time at this temperature is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typically at most 60 minutes.
In a preferred embodiment of the present invention, the water-absorbing polymer particles are cooled after the surface postcrosslinking. The cooling is preferably performed in contact coolers, more preferably paddle coolers and most preferably disk coolers. Suitable coolers are, for example, Hosokawa Bepex® Horizontal Paddle Coolers (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Coolers (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® coolers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Coolers (NARA Machinery Europe; Frechen; Germany). Moreover, fluidized bed coolers may also be used.
In the cooler, the water-absorbing polymer particles are cooled to 20 to 150° C., preferably 30 to 120° C., more preferably 40 to 100° C. and most preferably 50 to 80° C.
Subsequently, the surface postcrosslinked polymer particles can be classified again, excessively small and/or excessively large polymer particles being removed and recycled into the process.
To further improve the properties, the surface postcrosslinked polymer particles can be coated or remoisturized.
The remoisturizing is preferably performed at 30 to 80° C., more preferably at 35 to 70° C., most preferably at 40 to 60° C. At excessively low temperatures, the water-absorbing polymer particles tend to form lumps, and, at higher temperatures, water already evaporates to a noticeable degree. The amount of water used for remoisturizing is preferably from 1 to 10% by weight, more preferably from 2 to 8% by weight and most preferably from 3 to 5% by weight. The remoisturizing increases the mechanical stability of the polymer particles and reduces their tendency to static charging. The remoisturizing is advantageously performed in the cooler after the thermal drying.
Suitable coatings for improving the free swell rate and the permeability (SFC) are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers and di- or polyvalent metal cations. Suitable coatings for dust binding are, for example, polyols. Suitable coatings for counteracting the undesired caking tendency of the polymer particles are, for example, fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20.
The water-absorbing polymer particles produced by the process according to the invention have a moisture content of preferably 0 to 15% by weight, more preferably 0.2 to 10% by weight and most preferably 0.5 to 8% by weight, the moisture content being determined by EDANA recommended test method No. WSP 230.2-05 “Mass Loss Upon Heating”.
The water-absorbing polymer particles produced by the process according to the invention have a proportion of particles having a particle size of 300 to 600 μm of preferably at least 30% by weight, more preferably at least 50% by weight and most preferably at least 70% by weight.
The water-absorbing polymer particles produced by the process according to the invention have a centrifuge retention capacity (CRC) of typically at least 15 g/g, preferably at least 20 g/g, more preferably at least 22 g/g, especially preferably at least 24 g/g and most preferably at least 26 g/g. The centrifuge retention capacity (CRC) of the water-absorbing polymer particles is typically less than 60 g/g. The centrifuge retention capacity (CRC) is determined by EDANA recommended test method No. WSP 241.2-05 “Fluid Retention Capacity in Saline, After Centrifugation”.
The water-absorbing polymer particles produced by the process according to the invention have an absorption under a pressure of 49.2 g/cm2 of typically at least 15 g/g, preferably at least 20 g/g, more preferably at least 22 g/g, especially preferably at least 24 g/g and most preferably at least 26 g/g. The absorption under a pressure of 49.2 g/cm2 of the water-absorbing polymer particles is typically less than 35 g/g. The absorption under a pressure of 49.2 g/cm2 is determined analogously to EDANA recommended test method No. WSP 242.2-05 “Absorption Under Pressure, Gravimetric Determination”, except that a pressure of 49.2 g/cm2 is established instead of a pressure of 21.0 g/cm2.
Methods:
The standard test methods described hereinafter and designated “WSP” are described in: “Standard Test Methods for the Nonwovens Industry”, 2005 edition, published jointly by the Worldwide Strategic Partners EDANA (Avenue Eugene Plasky, 157, 1030 Brussels, Belgium, www.edana.org) and INDA (1100 Crescent Green, Suite 115, Cary, N.C. 27518, U.S.A., www.inda.org). This publication is available both from EDANA and from INDA.
The measurements should, unless stated otherwise, be conducted at an ambient temperature of 23±2° C. and a relative air humidity of 50±10%. The water-absorbing polymer particles are mixed thoroughly before the measurement.
Residual Monomers
The residual monomer content of the water-absorbing polymer particles is determined by EDANA recommended test method WSP No. 210.2-05 “Residual Monomers”.
Centrifuge Retention Capacity
The centrifuge retention capacity (CRC) is determined by EDANA recommended test method No. WSP 241.2-05 “Fluid Retention Capacity in Saline, After Centrifugation”.
Absorption under a pressure of 21.0 g/cm2 (absorption under load)
The absorption under a pressure of 21.0 g/cm2 (AUL0.3 psi) is determined by EDANA recommended test method No. WSP 242.2-05 “Absorption Under Pressure, Gravimetric Determination”.
Extractables
The content of extractables of the water-absorbing polymer particles is determined by EDANA recommended test method No. WSP 270.2-05 “Extractable”.


EXAMPLES
Example 1
A monomer solution composed of 492.9 g of acrylic acid, 4283.9 g of aqueous sodium acrylate solution (37.3% strength by weight), 681.1 g of water and 6.05 g of polyethylene glycol diacrylate (diacrylate of a polyethylene glycol having a mean molar mass of 400 g/mol), having been freed of atmospheric oxygen with nitrogen gas for 30 minutes, was polymerized in an LUK 8.0 K2 polymerization reactor with two axially parallel rotating shafts (Coperion Werner & Pfleiderer GmbH & Co. KG, Stuttgart, Germany), which was purged with an N2 stream before, during and after the filling with the monomer solution. The pressure within the reaction chamber corresponded to a positive pressure of 18 mbar relative to the area outside the polymerization reactor (atmospheric pressure). The pressures were measured with CVC 24 gauges from Vacuubrand (Vacuubrand GmbH & Co. KG, Wertheim, Germany), with connection of the interior of the polymerization reactor to the gauge via a vacuum hose. The polymerization was initiated, under further gassing with N2, at 25° C. by adding 29 g of aqueous ammonium persulfate solution (10% strength by weight) and 16 g of aqueous ascorbic acid solution (0.5% strength by weight), with the thermostat for the trace heating of the polymerization reactor set to 75° C. and switched off again 10 minutes after the initiation. 11 minutes after initiator addition, an internal temperature of 74° C. was attained. 30 minutes after the initiator addition, the polymer gel was removed and was dried at 180° C. in a forced air drying cabinet on a wire mesh sheet with a loading of 0.519 g/cm2 for 50 minutes, ground in three stages with a roll mill (1000 μm, 600 μm and 400 μm) and screened to a particle size range from 150 to 850 μm.
CRC=38.0 g/g
AUL0.3 psi=11.5 g/g
Extractables=12.3% by weight
Residual monomer=1103 ppm
Example 2
Comparative Experiment
Example 1 was repeated, except that the pressure within the reaction chamber corresponded to the pressure outside the polymerization reactor, i.e. there was no positive pressure in the polymerization reactor. The polymerization proceeded similarly to that in example 1; the internal temperature reached 71° C.
CRC=38.6 g/g
AUL0.3 psi=9.9 g/g
Extractables=15.4% by weight
Residual monomer=1962 ppm
The experiments show that, under positive pressure, a better-crosslinked polymer (lower CRC, higher AUL0.3 psi and lower extractables) with less residual monomer is obtained.

## Claims

1. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising
a) an ethylenically unsaturated monomer which bears an acid group and may be at least partly neutralized,
b) at least one crosslinker,
c) at least one initiator,
d) at least one ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), and
e) optionally one or more water-soluble polymer,
in a polymerization reactor with at least two axially parallel rotating shafts, which comprises performing the polymerization in the presence of an inert gas and under a positive pressure from 15 to 500 mbar higher than ambient pressure.

2. The process according to claim 1, wherein the inert gas is nitrogen.

3. The process according to claim 1, wherein the inert gas comprises less than 0.0005% by volume of oxygen.

4. The process according to claim 1, wherein the positive pressure in the polymerization reactor is from 15 to 50 mbar.

5. The process according to claim 1, wherein the inert gas flows continuously through the polymerization reactor.

6. The process according to claim 1, wherein monomer a) is acrylic acid partly neutralized to an extent of at least 50 mol %.

7. The process according to claim 1, wherein monomer a) has been neutralized to an extent of 25 to 85 mol %.

8. The process according to claim 1, wherein the monomer solution comprises from 0.05 to 1.5% by weight of crosslinker b), based on the unneutralized monomer a).

9. The process according to claim 1, wherein the water-absorbing polymer particles are additionally surface postcrosslinked.