[Invention Title]

SURFACE-CROSSLINKING AND COOLING OF SURFACE-CROSSLINKED WATER- ABSORBENT POLYMER PARTICLES IN THE PRODUCTION THEREOF

[Technical Field]

The invention relates to a process for the preparation of surface-crosslinked water- absorbent polymer particles; to a surface-crosslinked water-absorbent polymer particle obtainable by such a process; to a composite material comprising such a surface-crosslinked water-absorbent polymer particle; to a process for the production of a composite material, to a composite material obtainable by such a process; to a use of the surface-crosslinked water- absorbent polymer particle; to a device for the preparation of surface-crosslinked water- absorbent polymer particles; and to a process for the preparation of surface-crosslinked water- absorbent polymer particles in such a device.

[Background Art]

Superabsorbers are water-insoluble, crosslinked polymers which are able to absorb large amounts of aqueous fluids, especially body fluids, more especially urine or blood, with swelling and the formation of hydrogels, and to retain such fluids under a certain pressure. By virtue of those characteristic properties, such polymers are chiefly used for incorporation into sanitary articles, such as, for example, baby's nappies/diapers, incontinence products or sanitary towels.

The preparation of superabsorbers is generally carried out by free-radical polymerization of acid-group-carrying monomers in the presence of crosslinkers, it being possible for polymers having different absorber properties to be prepared by the choice of the monomer composition, the crosslinkers and the polymerization conditions and of the processing conditions for the hydrogel obtained after the polymerization (for details see, for example, Modern Superabsorbent Polymer Technology, FL Buchholz, GT Graham, Wiley-VCH, 1998).

The acid-group-carrying monomers can be polymerized in the presence of the cross- linkers in a batch process or in a continuous process. Both in continuous and in batchwise polymerization, partially neutralized acrylic acid is typically used as the monomer. Suitable neutralization processes are described, for example, in EP 0 372 706 A2, EP 0 574 260 Al , WO 2003/051415 Al, EP 1 470 905 Al, WO 2007/028751 Al, WO 2007/028746 Al and WO 2007/028747 Al .

[Disclosure] [Technical Problem]

The polymer gel, also called hydrogel, obtained after the polymerization is usually comminuted, dried and classified in order to obtain a particulate superabsorber with a well defined particles size distribution. In a further process step these superabsorbent particles are often surface crosslinked in order to improve the absorption behavior. For this purpose the particles are mixed with an aqueous solution containing a surface crosslinking agent and optionally further additives and the thus obtained mixture is heat treated in order to promote the crosslinking reaction. In the prior art adding an aqueous solution to water-absorbent polymer particles results in caking, agglomeration or a formation of lumps of the particles. Hence, a mixing of the particles and the surface-crosslinking solution is at least partly inhibited. This reduces the desired improvement of the absorption behavior of the superabsorber product, i.e. the retention capacity is restrained and gel blocking is promoted.

[Technical Solution]

Generally, it is an object of the present invention to at least partly overcome a disad- vantage arising from the prior art in the context of the production of superabsorbers.

It is a further object of the invention to provide a process for the production of surface- crosslinked water-absorbent polymer particles, wherein one selected from the group of caking, agglomeration and lump formation of polymer particles or a combination of at least two thereof is reduced. It is a further object of the invention to provide a process for the produc- tion of surface-crosslinked water-absorbent polymer particles, wherein a dust content of the surface-crosslinked water- absorbent polymer particles is reduced. It is a further object of the invention to provide a process for the production of surface-crosslinked water-absorbent polymer particles, wherein a water-soluble polymer content of the surface-crosslinked water- absorbent polymer particles is reduced. It is a further object of the invention to provide a pro- cess for the production of surface-crosslinked water-absorbent polymer particles, wherein a storability of the surface-crosslinked water- absorbent polymer particles is improved. It is a further object of the invention to provide a process for the production of surface-crosslinked water-absorbent polymer particles, wherein the surface-crosslinked water- absorbent polymer particles can be stored, preferably in a humid environment or at an elevated temperature or both, without deteriorating a quality, preferably an absorption under load, of the surface- crosslinked water-absorbent polymer particles. It is a further object of the invention to provide a process for the production of surface-crosslinked water-absorbent polymer particles, wherein a mixing of polymer particles with a further component, such as Si-oxide or a surface- crosslinking composition, is improved or accelerated or both. It is a further object of the invention to provide a process for the production of surface-crosslinked water-absorbent polymer particles, wherein an output efficiency of the process is increased. It is a further object of the invention to provide a process for the production of superabsorbent polymer particles, wherein the process shows a balanced combination of at least two, preferably at least three, of the above advantages. A further object is to provide water-absorbent polymer particles which have been produced by a less expensive process. It is a further object of the present invention to provide a water-absorbent polymer particle or a plurality of water-absorbent polymer particles produced by a process having at least one, preferably a balanced combination of at least two, of the above advantages, wherein the water-absorbent polymer particle shows no reduction of quality. It is a further object of the present invention to provide a composite material comprising a water-absorbent polymer particle produced by a process having at least one of the above advantages, wherein the composite material shows no reduction of quality. It is a further object of the present invention to provide a device for producing water-absorbent polymer particles by a process having at least one of the above advantages.

A contribution to the solution of at least one of the above objects is given by the independent claims. The dependent claims provide preferred embodiments of the present invention which also serve solving at least one of the above mentioned objects.

[Advantageous Effects] One selected from the group of caking, agglomeration and lump formation of polymer particles or a combination of at least two thereof may be reduced. Dust content of the surface-crosslinked water-absorbent polymer particles may be reduced. A water-soluble polymer content of the surface-crosslinked water-absorbent polymer particles may be reduced. A stor- ability of the surface-crosslinked water-absorbent polymer particles may be improved. The surface-crosslinked water-absorbent polymer particles may be stored, preferably in a humid environment or at an elevated temperature or both, without deteriorating a quality, preferably an absorption under load, of the surface-crosslinked water-absorbent polymer particles. A mixing of polymer particles with a further component, such as Si-oxide or a surface- crosslinking composition, may be improved or accelerated or both. An output efficiency of the process may be increased.

[Description of Drawings]

Figure 1 is a flow chart diagram depicting the steps of a process accord- ing to the invention;

Figure 2 is a flow chart diagram depicting the steps of another process according to the invention;

Figure 3 is a flow chart diagram depicting the steps of another process according to the invention;

Figure 4 is a flow chart diagram depicting the steps of another process according to the invention;

Figure 5 is a flow chart diagram of process steps (xi) to (xiii) according to the invention;

Figure 6 is a flow chart diagram of process steps (xi) to (xiv) according to the invention;

Figure 7 is a block diagram of a device for the preparation of surface- crosslinked water-absorbent polymer particles according to the invention;

Figure 8 is a block diagram of another device for the preparation of sur- face-crosslinked water-absorbent polymer particles according to the invention;

Figure 9a is a scheme of a longitudinal cross section of a second mixing device according to the invention; and

Figure 9b is a scheme of a transversal cross section of the second mixing device in figure 9a).

100 process according to the invention

101 step (i)

102 step (ii)

103 step (iii)

104 step (iv)

105 step (v)

106 step (vi)

107 step (vii)

108 step (viii)

109 step (ix)

1 10 step (x)

1 11 step (xi)

1 12 step (xii)

1 13 step (xiii) 1 14 step (xiv)

501 water-absorbent polymer particles

502 first mixing device

503 treated water-absorbent polymer particles

504 second mixing device

505 surface-crosslinked water-absorbent polymer particles

506 cooling device

507 Si-oxide

508 further crosslinker

509 water

601 third mixing device

602 drying device

603 further sizing device

604 reducing agent

605 fine surface-crosslinked water-absorbent polymer particles

700 device for the preparation of surface-crosslinked water-absorbent polymer particles

701 first container

702 further container

703 fourth mixing device

704 polymerization reactor

705 comminuting device

706 belt dryer

707 grinding device

708 first sizing device

709 process stream

901 inlet

902 mixing chamber

903 mixing chamber wall

904 outlet

905 annular layer of ground and sized water-absorbent polymer particles

906 axial position

907 rotating shaft

908 mixing tool [Best Mode]

A contribution to the solution of at least one of these objects is made by a process for the preparation of surface-crosslinked water-absorbent polymer particles, comprising the process steps of

(i) preparing an aqueous monomer solution comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al) and at least one crosslinker (a3);

(ii) optionally adding fine water-absorbent polymer particles or fine surface- crosslinked water-absorbent polymer particles or both to the aqueous monomer solution; (iii) adding a polymerization initiator or a at least one component of a polymerization initiator system that comprises two or more components to the aqueous monomer solution;

(iv) optionally decreasing the oxygen content of the aqueous monomer solution;

(v) charging the aqueous monomer solution into a polymerization reactor;

(vi) polymerizing the monomers in the aqueous monomer solution in the polymerization reactor, thereby obtaining a polymer gel;

(vii) discharging the polymer gel out of the polymerization reactor and optionally comminuting the polymer gel;

(viii) drying the polymer gel;

(ix) grinding the dried polymer gel, thereby obtaining water-absorbent polymer particles;

(x) subjecting the water-absorbent polymer particles to a first sizing step;

(xi) adding a Si-oxide to the sized water-absorbent polymer particles, thereby obtaining treated water-absorbent polymer particles;

(xii) adding a crosslinking composition, comprising a further crosslinker, to the treated water-absorbent polymer particles and heating the treated water-absorbent polymer particles, thereby obtaining surface-crosslinked water-absorbent polymer particles;

(xiii) forming a fluidized bed of the surface-crosslinked water-absorbent polymer particles and cooling the surface-crosslinked water-absorbent polymer particles in the fluid- ized bed in a cooling device.

Therein, subsequent steps of the process according to the invention may be performed simultaneously or may overlap in time or both. This holds particularly for the steps (i) to (iv), especially particularly for the steps (iii) and (iv). The process according to the present invention is preferably a continuous process in which the aqueous monomer solution is continuously provided and is continuously fed into the polymerization reactor. The polymer gel obtained is continuously discharged out of the polymerization reactor and is continuously comminuted, dried and ground in the subsequent process steps. This continuous process may, however, be interrupted in order to, for example, substitute certain parts of the process equipment, like the belt material of the conveyor belt if a conveyor belt is used as the polymerization reactor,

clean certain parts of the process equipment, especially for the purpose of removing polymer deposits in tanks or pipes, or

- start a new process when surface-crosslinked water-absorbent polymer particles with other absorption characteristics have to be prepared.

Surface-crosslinked water- absorbent polymer particles which are preferred according to the invention are particles that have an average particle size in accordance with WSP 220.2 (test method of„Word Strategic Partners" ED ANA and INDA) in the range of from 10 to 3,000 μιη, preferably 20 to 2,000 μηι and particularly preferably 150 to 850 μιη. In this context, it is particularly preferable for the content of surface-crosslinked water- absorbent polymer particles having a particle size in a range of from 300 to 600 μηι to be at least 30 wt.-%, particularly preferably at least 40 wt.-% and most preferably at least 50 wt.-%, based on the total weight of the surface-crosslinked water-absorbent polymer particles.

In process step (i) of the process according to the present invention an aqueous monomer solution containing at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al) and at least one crosslinker (a3) is prepared.

Preferred monoethylenically unsaturated monomers bearing carboxylic acid groups (al) are those cited in DE 102 23 060 Al as preferred monomers (al), whereby acrylic acid is particularly preferred.

It is preferred according to the present invention that the water-absorbent polymer produced by the process according to the invention comprises monomers bearing carboxylic acid groups to at least 50 wt.-%, preferably to at least 70 wt.-% and further preferably to at least 90 wt.-%, based on the dry weight. It is particularly preferred according to the invention, that the water- absorbent polymer produced by the process according to the invention is formed from at least 50 wt.-%, preferably at least 70 wt.-% of acrylic acid, which is preferably neutralized to at least 20 mol-%, particularly preferably to at least 50 mol-%. The concentration of the partially neutralized, monoethylenically unsaturated monomers bearing carboxylic acid groups (al) in the aqueous monomer solution that is provided in process step (i) is preferably in the range of from 10 to 60 wt.-%, preferably from 30 to 55 wt.-% and most preferably from 40 to 50 wt.-%, based on the total weight of the aqueous monomer solution.

The aqueous monomer solution may also comprise monoethylenically unsaturated monomers (a2) which are copolymerizable with (al). Preferred monomers (a2) are those monomers which are cited in DE 102 23 060 Al as preferred monomers (a2), whereby acrylamide is particularly preferred.

Preferred crosslinkers (a3) according to the present invention are compounds which have at least two ethylenically unsaturated groups in one molecule (crosslinker class I), compounds which have at least two functional groups which can react with functional groups of the monomers (al) or (a2) in a condensation reaction (= condensation crosslinkers), in an addition reaction or a ring-opening reaction (cross-linker class II), compounds which have at least one ethylenically unsaturated group and at least one functional group which can react with functional groups of the monomers (al) or (a2) in a condensation reaction, an addition reaction or a ring-opening reaction (crosslinker class III), or polyvalent metal cations (cross- linker class IV). Thus with the compounds of crosslinker class I a crosslinking of the polymer is achieved by radical polymerization of the ethylenically unsaturated groups of the crosslinker molecules with the monoethylenically unsaturated monomers (al) or (a2), while with the compounds of crosslinker class II and the polyvalent metal cations of crosslinker class IV a crosslinking of the polymer is achieved respectively via condensation reaction of the func- tional groups (crosslinker class II) or via electrostatic interaction of the polyvalent metal cation (crosslinker class IV) with the functional groups of the monomer (al) or (a2). With compounds of cross-linker class III a cross-linking of the polymers is achieved correspondingly by radical polymerization of the ethylenically unsaturated groups as well as by condensation reaction between the functional groups of the cross-linkers and the functional groups of the monomers (al ) or (a2).

Preferred crosslinkers (a3) are all those compounds which are cited in DE 102 23 060 Al as crosslinkers (a3) of the crosslinker classes I, II, III and IV, whereby

as compounds of crosslinker class I, N, ^" NT -methylene bisacrylamide, polyethylenegly- col di(meth)acrylates, triallylmethylammonium chloride, tetraallylammonium chloride and allylnonaethyleneglycol acrylate produced with 9 mol ethylene oxide per mol acrylic acid are particularly preferred, wherein N, 1ST -methylene bisacrylamide is even more preferred, and as compounds of crosslinker class IV, Al ₂ (S0 ₄ ) ₃ and its hydrates are particularly preferred. Preferred water-absorbent polymers produced by the process according to the invention are polymers which are crosslinked by crosslinkers of the following crosslinker classes or by crosslinkers of the following combinations of crosslinker classes respectively: I, II, III, IV, I II, I III, I IV, I II III, I II IV, I III IV, II III IV, II IV or III IV.

Further preferred water-absorbent polymers produced by the process according to the invention are polymers which are crosslinked by any of the crosslinkers disclosed in DE 102 23 060 Al as crosslinkers of crosslinker classes I, whereby Ν,Ν' -methylene bisacrylamide, polyethyleneglycol di(meth)acrylates, triallyl-methylammonium chloride, tetraallylammonium chloride and allylnonaethylene-glycol acrylate produced from 9 mol eth- ylene oxide per mol acrylic acid are particularly preferred as crosslinkers of crosslinker class I, wherein N, N' -methylene bisacrylamide is even more preferred.

The aqueous monomer solution may further comprise water-soluble polymers (a4). Preferred water-soluble polymers (a4) include partly or completely saponified polyvinyl alcohol, polyvinylpyrrolidone, starch or starch derivatives, polyglycols or polyacrylic acid. The molecular weight of these polymers is not critical, as long as they are water-soluble. Preferred water-soluble polymers (a4) are starch or starch derivatives or polyvinyl alcohol. The water- soluble polymers (a4), preferably synthetic, such as polyvinyl alcohol, can not only serve as a graft base for the monomers to be polymerized. It is also conceivable for these water-soluble polymers to be mixed with the polymer gel or the already dried, water-absorbent polymer.

The aqueous monomer solution can furthermore also comprise auxiliary substances

(a5), these auxiliary substances including, in particular, complexing agents, such as, for example, EDTA.

The relative amount of monomers (al) and (a2) and of crosslinking agents (a3) and water-soluble polymers (a4) and auxiliary substances (a5) in the aqueous monomer solution is preferably chosen such that the water-absorbent polymer structure obtained after drying the optionally comminuted polymer gel is based

to the extent of 20 to 99.999 wt.-%, preferably to the extent of 55 to 98.99 wt.-% and particularly preferably to the extent of 70 to 98.79 wt.-% on monomers (al),

to the extent of 0 to 80 wt.-%, preferably to the extent of 0 to 44.99 wt.-% and particu- larly preferably to the extent of 0.1 to 44.89 wt.-% on the monomers (a2),

to the extent of 0 to 5 wt.-%, preferably to the extent of 0.001 to 3 wt.-% and particularly preferably to the extent of 0.01 to 2.5 wt.-% on the crosslinking agents (a3),

to the extent of 0 to 30 wt.-%, preferably to the extent of 0 to 5 wt.-% and particularly preferably to the extent of 0.1 to 5 wt.-% on the water-soluble polymers (a4), to the extent of 0 to 20 wt.-%, preferably to the extent of 0 to 10 wt.-% and particularly preferably to the extent of 0.1 to 8 wt.-% on the auxiliary substances (a5), and

to the extent of 0.5 to 25 wt.-%, preferably to the extent of 1 to 10 wt.-% and particularly preferably to the extent of 3 to 7 wt.-% on water (a6)

the sum of the amounts by weight (al) to (a6) being 100 wt.-%.

Optimum values for the concentration in particular of the monomers, crosslinking agents and water-soluble polymers in the monomer solution can be determined by simple preliminary experiments or from the prior art, in particular from the publications US 4,286,082, DE 27 06 135 Al , US 4,076,663, DE 35 03 458 Al, DE 40 20 780 CI , DE 42 44 548 Al, DE 43 33 056 Al and DE 44 18 818 Al .

In process step (ii) fine particles of a water-absorbent polymer may optionally be added to the aqueous monomer solution. Independent of optional step (ii) fine water-absorbent polymer particles may be added to the aqueous monomer solution at one selected from the group consisting of after step (iii), after step (iv), and before step (v), or a combination of at least two thereof. Therein, surface-crosslinked fine particles or non-surface-crosslinked fine particles or both may optionally be added to the aqueous monomer solution. The term fine particles refers to surface-crosslinked fine particles and non-surface-crosslinked fine particles.

Fine particles are preferably water-absorbent polymer particles the composition of which corresponds to the composition of the above described surface-crosslinked water- absorbent polymer particles, wherein it is preferred that at least 90 wt.-% of the fine particles, preferably at least 95 wt.-% of the fine particles and most preferred at least 99 wt.-% of the fine particles have a particle size of less than 200 μηι, preferably less than 150 μηι and particular preferably less than 100 μηι.

In a preferred embodiment of the process according to the present invention the water- absorbent fine particles which may optionally be added to the aqueous monomer solution in process step (ii) are fine particles which are obtained in process step (x) of the process according to the present invention and which are thus recycled.

In another preferred embodiment of the process according to the present invention the surface-crosslinked water- absorbent fine particles which may optionally be added to the aqueous monomer solution in process step (ii) are surface-crosslinked fine particles which are obtained after process step (xiii) of the process according to the present invention and which are thus recycled.

The fine particles can be added to the aqueous monomer solution by means of any mixing device the person skilled of the art would consider as appropriate for this purpose. In a preferred embodiment of the present invention, which is especially useful if the process is performed continuously as described above, the fine particles are added to the aqueous monomer solution in a mixing device in which a first stream of the fine particles and a second stream of the aqueous monomer solution are directed continuously, but from different direc- tions, onto a rotating mixing device. Such a kind of mixing setup can be realized in a so called "Rotor Stator Mixer ^' " which comprises in its mixing area a preferably cylindrically shaped, non-rotating stator, in the centre of which a likewise preferably cylindrically shaped rotor is rotating. The walls of the rotor as well as the walls of the stator are usually provided with notches, for example notches in the form of slots, through which the mixture of fine particles and aqueous monomer solution can be sucked through and thus can be subjected to high shear forces.

In this context it is particularly preferred that the first stream of the fine particles and the second stream of the aqueous monomer solution form an angle δ in the range from 60 to 120°, more preferred in the range from 75 to 105°, even more preferably in the range from 85 to 95° and most preferred form an angle of about 90°. It is also preferred that the stream of the mixture of fine particles and aqueous monomer solution that leaves the mixer and the first stream of fine particles that enters the mixer form an angle ε in the range from 60 to 120°, preferably in the range from 75 to 105°, even more preferred in the range from 85 to 95° and most preferred form an angle of about 90°.

Such a kind of mixing set up can, for example, be realized by means of mixing devices which are disclosed in DE-A-25 20 788 and DE-A-26 17 612, the content of which is incorporated herein by reference. Concrete examples of mixing devices which can be used to add the fine particles to the aqueous monomer solution in process step (ii) of the present invention are the mixing devices which can be obtained by the IKA ^® Werke GmbH & Co. KG, Staufen, Germany, under designations MHD 2000/4, MHD 2000/05, MHD 2000/10, MDH 2000/20, MHD 2000/30 und MHD 2000/50, wherein the mixing device MHD 2000/20 is particularly preferred. Further mixing devices which can be used are those offered by ystral GmbH, Ballrechten-Dottingen, Germany, for example under designation„Conti TDS", or by Kine- matika AG, Luttau, Switzerland, for example under the trademark Megatron ^® .

The amount of fine particles that may be added to the aqueous monomer solution in process step (ii) is preferably in the range from 0.1 to 15 wt.-%, even more preferred in the range from 0.5 to 10 wt.-% and most preferred in the range from 3 to 8 wt.-%, based on the weight of the aqueous monomer solution. In process step (iii) of the process according to the present invention a polymerization initiator or at least one component of a polymerization initiator system that comprises two or more components is added to the aqueous monomer solution.

As polymerization initiators for initiation of the polymerization all initiators forming radicals under the polymerization conditions can be used, which are commonly used in the production of superabsorbers. Among these belong thermal catalysts, redox catalysts and photo-initiators, whose activation occurs by energetic irradiation. The polymerization initiators may be dissolved or dispersed in the aqueous monomer solution. The use of water- soluble catalysts is preferred.

As thermal initiators may be used all compounds known to the person skilled in the art that decompose under the effect of an increased temperature to form radicals. Particularly preferred are thermal polymerisation initiators with a half life of less than 10 seconds, more preferably less than 5 seconds at less than 180°C, more preferably at less than 140°C. Peroxides, hydroperoxides, hydrogen peroxide, persulfates and azo compounds are particularly preferred thermal polymerization initiators. In some cases it is advantageous to use mixtures of various thermal polymerization initiators. Among such mixtures, those consisting of hydrogen peroxide and sodium or potassium peroxodisulfate are preferred, which may be used in any desired quantitative ratio. Suitable organic peroxides are preferably acetylacetone peroxide, methyl ethyl ketone peroxide, benzoyl peroxide, lauroyl peroxide, acetyl peroxide, capryl peroxide, isopropyl peroxidicarbonate,2-ethylhexyle peroxidicarbonate, tert.-butyl hydroperoxide, cumene hydroperoxide, and peroxides of tert- amyl perpivalate, tert.-butyl perpivalate, tert.-butyl perneohexonate, tert.-butyl isobutyrate, tert.-butyl per-2-ethylhexenoate, tert.-butyl perisononanoate, tert.-butyl permaleate, tert.-butyl perbenzoate, tert.-butyl-3,5,5-trimethylhexanoate and amyl perneodecanoate. Furthermore, the following thermal polymerisation initiators are preferred: azo compounds such as azo-bis- isobutyronitril, azo-bis-dimethylvaleronitril, azo-bis-ami-dinopropane dihydrochloride, 2,2'- azobis-(N,N-dimethylene)isobutyramidine di-hydrochloride, 2-(carbamoylazo)isobutyronitrile and 4,4'-azobis-(4-cyano-valeric acid). The aforementioned compounds are used in conventional amounts, preferably in a range from 0.01 to 5 mol-%, more preferably 0.1 to 2 mol-%, respectively based on the amount of the monomers to be polymerized.

Redox catalysts comprise two or more components, usually one or more of the peroxo compounds listed above, and at least one reducing component, preferably ascorbic acid, glucose, sorbose, mannose, ammonium or alkali metal hydrogen sulfite, sulfate, thiosulfate, hyposulfite or sulfide, metal salts such as iron II ions or silver ions or sodium hydroxymethyl sulfoxylate. Preferably ascorbic acid or sodium pyrosulfite is used as reducing component of the redox catalyst. 1 ^χ 10° to 1 mol-% of the reducing component of the redox catalyst and 1 x 10° to 5 mol-% of the oxidizing component of the redox catalyst are used, in each case referred to the amount of monomers used in the polymerization. Instead of the oxidising component of the redox catalyst, or as a complement thereto, one or more, preferably water- soluble azo compounds may be used.

The polymerization is preferably initiated by action of energetic radiation, so-called photo -initiators are generally used as initiator. These can comprise for example so-called oc- splitters, H-abstracting systems or also azides. Examples of such initiators are benzophenone derivatives such as Michlers ketone, phenanthrene derivatives, fluorine derivatives, anthra- quinone derivatives, thioxanthone derivatives, cumarin derivatives, benzoinether and derivatives thereof, azo compounds such as the above-mentioned radical formers, substituted hex- aarylbisimidazoles or acylphosphine oxides. Examples of azides are: 2-(N,N- dimethylamino)ethyl-4-azidocinnamate, 2-(N,N-dimethylamino)ethyl-4-azidonaphthylketone, 2-(N,N-di-methylamino)ethyl-4-azidobenzoate, 5-azido-l-naphthyl-2'-(N,N-dimethylami- no)ethylsulfone, N-(4-sulfonylazidophenyl)maleinimide, N-acetyl-4-sulfonyl-azidoaniline, 4- sulfonylazidoaniline, 4-azidoaniline, 4-azidophenacyl bromide, p-azidobenzoic acid, 2,6- bis(p-azidobenzylidene)cyclohexanone and 2,6-bis(p-azidobenzylidene)-4- methylcyclohexanone. A further group of photo-initiators are di-alkoxy ketales such as 2,2- dimethoxy-l ,2-diphenylethan-l-one. The photo-initiators, when used, are generally employed in quantities from 0.0001 to 5 wt.-% based on the monomers to be polymerized.

According to a further embodiment of the process according to the invention it is preferred that in process step (iii) the initiator comprises the following components

iiia. a peroxodisulfate; and

iiib. an organic initiator molecule comprising at least three oxygen atoms or at least three nitrogen atoms;

wherein the initiator comprises the peroxodisulfate and the organic initiator molecule in a molar ratio in the range of from 20: 1 to 50: 1. In one aspect of this embodiment it is preferred that the concentration of the initiator component iiia. is in the range from 0.05 to 2 wt.- %, based on the amount of monomers to be polymerized. In another aspect of this embodiment it is preferred that the organic initiator molecule is selected from the group consisting of 2,2-dimethoxy-l ,2-diphenylethan-l-one, 2,2-azobis-(2-amidinopropane)dihydrochloride, 2,2- azobis-(cyano valeric acid) or a combination of at least two thereof. In a further aspect of this embodiment it is preferred that the peroxodisulfate is of the general formula M ₂ S ₂ O ₈ , with M being selected from the group consisting of NH ₄ , Li, Na, Ka or at least two thereof. The above described components are in particular suitable for UV initiation of the polymerization in step (vi) of the process of the present invention. Employing this composition further yields low residual monomer and reduced yellowing in the water-absorbent polymer particle, obtainable by the process according to the present invention.

In this context it should also be noted that step (iii), adding the polymerization initiator, may be realized before step (iv), simultaneously to step (iv), or overlapping in time with step (iv), i.e. when the oxygen content of the aqueous monomer solution is decreased. If a polymerization initiator system is used that comprises two or more components, one or more of the components of such a polymerization initiator system may, for example, be added before process step (iv), whereas the remaining component or the remaining components which are necessary to complete the activity of the polymerisation initiator system, are added after process step (iv), perhaps even after process step (v). Independent of optional step (iv), decreasing the oxygen content of the aqueous monomer solution may also be performed before process step (iii) according to the invention.

In process step (iv) of the process according to the present invention the oxygen content of the aqueous monomer solution is optionally decreased. Independent of optional step (iv), decreasing the oxygen content of the aqueous monomer solution may also be performed before, during or after process step (ii) according to the invention. Preferably, the oxygen content of the aqueous monomer solution is decreased after the fine particles have been added in process step (ii).

Whenever the oxygen content of the aqueous monomer solution is decreased, this may be realized by bringing the aqueous monomer solution into contact with an inert gas, such as nitrogen. The phase of the inert gas being in contact with the aqueous monomer solution is free of oxygen and is thus characterized by a very low oxygen partial pressure. As a consequence oxygen converts from the aqueous monomer solution into the phase of the inert gas until the oxygen partial pressures in the phase of the inert gas and the aqueous monomer solution are equal. Bringing the aqueous monomer phase into contact with a phase of an inert gas can be accomplished, for example, by introducing bubbles of the inert gas into the monomer solution in co-current, countercurrent or intermediate angles of entry. Good mixing can be achieved, for example, with nozzles, static or dynamic mixers or bubble columns. The oxygen content of the monomer solution before the polymerization is preferably lowered to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, based on the monomer solution. In process step (v) of the process according to the present invention the aqueous monomer solution is charged into a polymerization reactor, preferably onto a conveyor belt, especially preferred at an upstream position of the conveyor belt and in process step (vi) the monomers in the aqueous monomer solution are polymerized in the polymerization reactor, thereby obtaining a polymer gel. If polymerization is performed on a conveyor belt as the polymerization reactor, a polymer gel sheet is obtained in a downstream portion of the conveyor belt, which, before drying, is preferably comminuted in order to obtain polymer gel particles.

As the polymerization reactor every reactor can be used which the person skilled in the art would regard as appropriate for the continuous or batchwise polymerization of monomers like acrylic acid in aqueous solutions. An example of a suitable polymerization reactor is a kneading reactor. In a kneader the polymer gel formed in the polymerization of the aqueous monomer solution is comminuted continuously by, for example, contrarotatory stirrer shafts, as described in WO 2001/38402. In this example the polymerization reactor is identical to a first comminuting device according to the invention. Further, the rotating component according to the invention may be a rotating stirrer shaft.

Another example of a preferred polymerization reactor is a conveyor belt. As a conveyor belt that is useful for the process according to the present invention any conveyor belt can be used which the person skilled in the art considers to be useful as a support material onto which the above described aqueous monomer solution can be charged and subsequently polymerized to form a hydrogel.

The conveyor belt usually comprises an endless moving conveyor belt passing over supporting elements and at least two guide rollers, of which at least one is driven and one is configured so as to be adjustable. Optionally, a winding and feed system for a release sheet that may be used in sections on the upper surface of the conveyor belt is provided. The system includes a supply and metering system for the reaction components, and optional irradiating means arranged in the direction of movement of the conveyor belt after the supply and metering system, together with cooling and heating devices, and a removal system for the polymer gel strand that is arranged in the vicinity of the guide roller for the return run of the conveyor belt. In order to provide for the completion of polymerization with the highest possible space-time yield, according to the present invention, in the vicinity of the upper run of the conveyor belt on both sides of the horizontal supporting elements, starting in the area of the supply and metering systems, there are upwardly extending supporting elements, the longitudinal axes of which intersect at a point that is beneath the upper run, and which shape the conveyor belt that is supported by them so that it become suitably trough-shaped. Thus, according to the present invention, the conveyor belt is supported in the vicinity of the supply system for the reaction components by a plurality of trough-shaped supporting and bearing elements that form a deep trough-like or dish-like configuration for the reaction components that are introduced. The desired trough-like shape is determined by the shape and arrangement of the supporting elements along the length of the path of the upper run. In the area where the reaction components are introduced, the supporting elements should be relatively close to each other, whereas in the subsequent area, after the polymerization has been initiated, the supporting elements can be arranged somewhat further apart. Both the angle of inclination of the supporting elements and the cross-section of the supporting elements can be varied in order to flatten out the initially deep trough towards the end of the polymerization section and once again bring it to an extended state. In a further embodiment of the invention, each supporting element is preferably formed by a cylindrical or spherical roller that is rotatable about its longitudinal axis. By varying both the cross-section of the roller as well as the configuration of the roller it is easy to achieve the desired cross-sectional shape of the trough. In order to ensure proper formation of the trough by the conveyor belt, both when it makes the transition from a flat to a trough-like shape and when it is once again returned to the flat shape, a conveyor belt that is flexible in both the longitudinal and the transverse directions is preferred.

The belt can be made of various materials, although these preferably have to meet the requirements of good tensile strength and flexibility, good fatigue strength under repeating bending stresses, good deformability and chemical resistance to the individual reaction components under the conditions of the polymerization. These demands are usually not met by a single material. Therefore, a multi-layer material is commonly used as belt of the present invention. The mechanical requirements can be satisfied by a carcass of, for example, fabric inserts of natural and/or synthetic fibers or glass fibers or steel cords. The chemical resistance can be achieved by a cover of, for example, polyethylene, polypropylene, polyisobutylene, halogenated polyolefines such as polyvinyl chloride or polytetrafluorethylene, polyamides, natural or synthetic rubbers, polyester resins or epoxy resins. The preferred cover material is silicone rubber.

In process step (vii) of the process according to the present invention the polymer gel obtained in the polymerization reactor is optionally comminuted, thereby obtaining polymer gel particles. Preferred polymer gel particles are one selected from the group consisting of polymer gel strands, polymer gel flakes, and polymer gel nuggets, or a combination of at least two thereof. The optional comminuting according to the invention comprises preferably at least two comminuting steps. Therein, a first comminuting step is performed in a first comminuting device and a further comminuting step in a further comminuting device. The first comminuting device may be the polymerization reactor or a part of the polymerization reactor, or a separate device, or both. Hence, the first comminuting step may be performed before, during, or after discharging the polymer gel out of the polymerization reactor. A preferred polymerization reactor which is the first comminuting device is a kneading reactor. If the first comminuting step is performed in the polymerization reactor, the polymer gel particles obtained are preferably further comminuted in the further comminuting step after discharging out of the polymerization reactor. If the polymerization reactor is a conveyor belt, the first comminuting step is preferably performed after discharging the polymer gel as a polymer gel sheet from the conveyor belt in a first comminuting device, wherein the first comminuting device is a separate device. Preferably, the polymer gel sheet is discharged from the conveyor belt as a continuous sheet that is of a soft semi-solid consistency and is then passed on for further processing such as comminuting.

Comminution of the polymer gel is preferably performed in at least two, more preferably at least three steps:

in the first comminuting step, a cutting unit, preferably a knife, for example a knife as disclosed in WO-A-96/36464, is used for cutting the polymer gel into flat gel strips, prefera- bly with a length within the range of from 5 to 500 mm, preferably from 10 to 300 mm and particularly preferably from 100 to 200 mm, a height within the range of from 1 to 30 mm, preferably from 5 to 25 mm and particularly preferably from 10 to 20 mm as well as a width within the range of from 1 to 500 mm, preferably from 5 to 250 mm and particularly preferably from 10 to 200 mm;

- in a second step, a shredding unit, preferably a breaker, is used for shredding the gel strips into gel pieces, preferably with a length within the range of 3 to 100 mm, preferably from 5 to 50 mm, a height within the range from 1 to 25 mm, preferably from 3 to 20 mm as well as a width within the range from 1 to 100 mm, preferably from 3 to 20 mm and

in a third step, which is preferably the further comminuting step a "wolf (grinding) unit, preferably a mincer, preferably having a screw and a hole plate, whereby the screw conveys against the hole plate is used in order to grind and crush gel pieces into polymer gel particles which are preferably smaller than the gel pieces.

An optimal surface- volume ratio is achieved hereby, which has an advantageous effect on the drying behaviour in process step (viii). A polymer gel which has been comminuted in this way is particularly suited to belt drying. The three-step comminution offers a better "air- ability" because of the air channels located between the granulate kernels.

Another preferred comminution of the polymer gel is performed in at least two steps: in a first comminuting step the polymer gel is crushed by a plurality of rotating discs of the first comminuting device, preferably rotating toothed wheels. Thereby a plurality of polymer gel strands is produced.

in a second step, preferably the further comminuting step, a "wolf (grinding) unit, preferably a mincer, preferably having a screw and a hole plate, whereby the screw conveys against the hole plate is used in order to grind and crush the polymer gel strands into polymer gel particles which are preferably smaller than the polymer gel strands.

In process step (viii) of the process according to the present invention the polymer gel is dried.

The drying of the polymer gel can be effected in any dryer or oven the person skilled in the art considers as appropriate for drying the above described polymer gel and/or polymer gel particles. Rotary tube furnaces, fluidized bed dryers, plate dryers, paddle dryers and infrared dryers may be mentioned by way of example.

Especially preferred are belt dryers. A belt dryer is a convective system of drying, for the particularly gentle treatment of through-airable products. The product to be dried is placed onto an endless conveyor belt which lets gas through, and is subjected to the flow of a heated gas stream, preferably air. The drying gas is recirculated in order that it may become very highly saturated in the course of repeated passage through the product layer. A certain fraction of the drying gas, preferably not less than 10 %, more preferably not less than 15 % and most preferably not less than 20 % and preferably up to 50 %, more preferably up to 40 % and most preferably up to 30 % of the gas quantity per pass, leaves the dryer as a highly saturated vapor and carries off the water quantity evaporated from the product. The temperature of the heated gas stream is preferably not less than 50°C, more preferably not less than 100°C and most preferably not less than 150°C and preferably up to 250°C, more preferably up to 220°C and most preferably up to 200°C.

The size and design of the dryer depends on the product to be processed, the manufac- turing capacity and the drying duty. A belt dryer can be embodied as a single-belt, multi-belt, multi-stage or multistory system. The present invention is preferably practiced using a belt dryer having at least one belt. One-belt dryers are very particularly preferred. To ensure optimum performance of the belt-drying operation, the drying properties of the water-absorbent polymers are individually determined as a function of the processing parameters chosen. The hole size and mesh size of the belt is conformed to the product. Similarly, certain surface enhancements, such as electropolishing or Teflonizing, are possible.

The polymer gel and/or polymer gel particles to be dried are preferably applied to the belt of the belt dryer by means of a swivel belt. The feed height, i.e., the vertical distance be- tween the swivel belt and the belt of the belt dryer, is preferably not less than 10 cm, more preferably not less than 20 cm and most preferably not less than 30 cm and preferably up to 200 cm, more preferably up to 120 cm and most preferably up to 40 cm. The thickness on the belt dryer of the polymer gel and/or polymer gel particles to be dried is preferably not less than 2 cm, more preferably not less than 5 cm and most preferably not less than 8 cm and preferably not more than 20 cm, more preferably not more than 15 cm and most preferably not more than 12 cm. The belt speed of the belt dryer is preferably not less than 0.005 m/s, more preferably not less than 0.01 m/s and most preferably not less than 0.015 m/s and preferably up to 0.05 m/s, more preferably up to 0.03 m/s and most preferably up to 0.025 m/s.

Furthermore, it is preferable according to the invention that the polymer gel is dried to a water content in the range of from 0.5 to 25 wt.-%, preferably from 1 to 10 wt.-% and particularly preferably from 3 to 7 wt.-%, based on the dried polymer gel particles.

In process step (ix) of the process according to the present invention the dried polymer gel is ground thereby obtaining particulate water-absorbent polymer particles.

For grinding of the dried polymer gel any device can be used the person skilled in the art considers as appropriate for grinding the above described dried polymer gel and/or polymer gel particles. As an example for a suitable grinding device a single- or multistage roll mill, preferably a two- or three-stage roll mill, a pin mill, a hammer mill or a vibratory mill may be mentioned.

In process step (x) of the process according to the present invention the ground water- absorbent polymer particles are sized in a first sizing step, preferably using appropriate sieves. In this context it is particularly preferred that after sizing the water-absorbent polymer particles the content of polymer particles having a particle size of less than 150 μιη is less than 10 wt.-%, preferably less than 8 wt.-% and particularly less than 6 wt.-% and that the content of polymer particles having a particle size of more than 850 μηα is also less than 10 wt.-%, preferably less than 8 wt.-% and particularly preferably less than 6 wt.-%. It is also preferred that after sizing the water-absorbent polymer particles at least 30 wt.-%, more preferred at least 40 wt.-% and most preferred at least 50 wt.-% of the particles have a particle size in a range of from 300 to 600 μιη. In process step (xi) of the process according to the present invention a Si-oxide is added to the sized water-absorbent polymer particles, thereby obtaining treated water-absorbent polymer particles. A preferred Si-oxide is Si0 ₂ . Preferably, the Si-oxide is added to reduce a caking of the water-absorbent polymer particles.

In process step (xii) of the process according to the present invention the surfaces of the treated water-absorbent polymer particles are treated by contacting the treated water- absorbent polymer particles with a crosslinking composition, comprising a further crosslinker and heating the treated water-absorbent polymer particles, thereby obtaining surface- crosslinked water-absorbent polymer particles.

A preferred further crosslinker is a surface crosslinker. Preferred further crosslinkers are disclosed in US 201 1/0204288 Al and incorporated herein by reference and in the following. Preferred further crosslinkers 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 Al and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 Al, DE 35 23 617 Al and EP 0 450 922A2, or β-hydroxyalkylamides, as described in DE 102 04 938 Al and U.S. Pat. No. 6,239,230. Additionally described as suitable further crosslinkers are cyclic carbonates in DE 40 20 780 CI, 2-oxazolidinone and derivatives thereof, such as 2-hydroxyethyl-2- oxazolidinone, in DE 198 07 502 Al, bis- and poly-2-oxazolidinones in DE 198 07 992 CI , 2-oxotetrahydro-l,3-oxazine and derivatives thereof in DE 198 54 573 Al, N -acyl-2- oxazolidinones in DE 198 54 574 Al, cyclic ureas in DE 102 04 93 7 Al, bicyclic amido ace- tals in DE 103 34 584 Al, oxetanes and cyclic ureas in EP 1 199 327 A2 and morpholine-2,3- dione and derivatives thereof in WO 2003/031482 Al. Preferred further crosslinkers are eth- ylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol. Very particularly preferred further crosslinkers are 2-hydroxyethyl-2 -oxazolidinone, 2-oxazolidinone and 1,3- propanediol. In addition, it is also possible to use further crosslinkers which comprise additional polymerizable ethylenically unsaturated groups, as described in DE 3713 601 Al .

The amount of further crosslinker is preferably 0.001 to 2 wt.-%, more preferably 0.02 to 1 wt.-% and most preferably 0.05 to 0.2 wt.-%, based in each case on the treated water- absorbent polymer particles.

In a preferred embodiment of the present invention, polyvalent cations are applied to the surfaces of the treated water-absorbent polymer particles in addition to the crosslinking composition before, during or after the contacting with the crosslinking composition. 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 cati- ons such as the cations of titanium and zirconium. Possible counterions are chloride, bromide, sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphos- phate, dihydrogenphosphate and carboxylate, such as acetate, citrate and lactate. Aluminium sulfate 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 wt.-%, preferably 0.005 to 1 wt.-%, more preferably 0.02 to 0.8 wt.-%, based in each case on the treated water-absorbent polymer particles. The contacting with the crosslinking composition is typically performed in such a way that the crosslinking composition is sprayed as a solution onto the treated water-absorbent polymer particles.

The adding of the crosslinking composition is preferably performed in a second mix- ing device. A preferred second mixing device comprises moving mixing tools, such as a screw mixer, a disc mixer, and a paddle mixer. Therein, a preferred second mixing device is a horizontal mixer or a vertical mixer or both. Particularly preferred is a horizontal mixer. A mixer is horizontal if a mixing tool of the mixer rotates around a horizontal axis. A mixer is vertical if a mixing tool of the mixer rotates around a vertical axis. Suitable mixers are, for example, horizontal Ploughshare® Mixers (Gebruder Lodige Maschinenbau GmbH, Pader- born, Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BY, Doetinchem, the Netherlands), Processall Mixmill mixers (Processall Incorporated, Cincilmati, US), Schugi Flexomix® (Hosokawa Micron BY, Doetinchem, the Netherlands), and particularly preferred are ringlayer mixers CoriMix® by Gebruder Lodige Maschinenbau GmbH, Paderborn, Ger- many. However, it is also possible to perform the contacting by spraying the crosslinking composition in a fluidized bed. The crosslinking composition is typically used in the form of an aqueous solution. The penetration depth of the crosslinking composition into the polymer particles can be adjusted via the content of non-aqueous solvent and total amount of solvent. If exclusively water is used as the solvent, a surfactant is preferably added. This improves the wetting behavior 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. A residence time of the polymer particles in the mixer is preferably from 10 to 120 minutes, more preferably from 10 to 90 minutes, most preferably from 30 to 60 minutes. The fill level of the mixer is preferably from 30 to 80 %, more preferably from 40 to 75 %, most preferably from 50 to 70 %. The fill level of the mixer can be adjusted via a height of an overflow weir.

In process step (xiii) of the process according to the present invention the hot surface- crosslinked polymer particles are cooled in a fluidized bed.

To further improve properties, the surface-crosslinked water-absorbent polymer particles can be coated. Suitable coatings for improving the 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 ten- dency of the polymer particles are, for example, fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20. The surface-crosslinked water-absorbing polymer particles produced by the process according to the invention have a moisture content of preferably 0 to 15 wt.-%, more preferably 0.2 to 10 wt.-% and most preferably 0.5 to 8 wt.-%.

In an embodiment of the invention in process step (xi) the Si-oxide is added to the sized water-absorbent polymer particles in an amount in the range of from 0.1 to 1 wt.-%, preferably from 0.2 to 0.8 wt.-%, more preferably from 0.3 to 0.5 wt.-%, based on the weight of the sized water-absorbent polymer particles.

In an embodiment of the invention in process step (xiii) during the cooling an in-gas- stream enters the cooling device and an out-gas-stream exits the cooling device, wherein the in-gas-stream comprises less Si-oxide than the out-gas-stream. A preferred out-gas-stream is directed upwards. Another preferred out-gas-stream is an air-stream. Preferably, the out-gas- stream comprises the Si-oxide in an amount in the range of from 10 to 30 wt.-%, preferably from 15 to 25 wt.-%, based on the weight of a Si-oxide content of the surface-crosslinked water-absorbent polymer particles prior to the cooling.

In an embodiment of the invention the Si-oxide comprised by the out-gas-stream is comprised by particles, wherein at least 50 wt.-%, preferably at least 60 wt.-%, more preferably at least 70 wt.-%, of the particles have a particle size of less than 1 μηι, preferably less than 0.9 μηι, more preferably less than 0.8 μη , most preferably less than 0.7 μιη.

In an embodiment of the invention in process step (xiii) Si-oxide is removed from the surface-crosslinked water-absorbent polymer particles in an amount in the range of from 10 to 30 wt.-%, preferably from 15 to 25 wt.-%, based on the weight of a Si-oxide content of the surface-crosslinked water-absorbent polymer particles prior to the cooling.

In an embodiment of the invention in process step (xiii) a chelating agent is added to the surface-crosslinked water-absorbent polymer particles in the fluidized bed. A preferred chelating agent is EDTA. Preferably, the chelating agent is added as part of an aqueous solution. Preferably, the chelating agent is added during cooling of the surface-crosslinked water- absorbent polymer particles.

In an embodiment of the invention the chelating agent is added in an amount in the range of from 500 to 3,000 ppm by weight, preferably from 1,000 to 2,000 ppm by weight, based on the weight of the surface-crosslinked water-absorbent polymer particles.

In an embodiment of the invention a molecule of the chelating agent comprises at least two, preferably at least four, active sides.

In an embodiment of the invention the chelating agent is a salt. A preferred salt is an alkali metal salt.

In an embodiment of the invention in process step (xii) after the addition of the cross- linking composition and the heating the surface-crosslinked water-absorbent polymer particles or the treated water-absorbent polymer particles or both are dried. Preferably, the surface- crosslinked water-absorbent polymer particles or the treated water-absorbent polymer parti- cles or both are dried thermally. The surface-crosslinked water-absorbent polymer particles may be obtained during drying or after the drying or both.

In an embodiment of the invention in process step (xi) during the adding of the Si- oxide the water-absorbent polymer particles have a temperature in the range of from 70 to 120°C, preferably from 75 to 1 15°C, more preferably from 80 to 1 10°C, more preferably from 80 to 105°C, most preferably from 80 to 100°C.

In an embodiment of the invention in process step (xii) the treated water-absorbent polymer particles are heated to a temperature in the range of from 100 to 200°C, preferably from 105 to 190°C, more preferably from 1 10 to 180°C, more preferably from 120 to 170°C, most preferably from 130 to 160°C.

In an embodiment of the invention in process step (xiii) the surface-crosslinked water- absorbent polymer particles are cooled to a temperature of 100°C or less, preferably 90°C or less, more preferably 80°C or less, more preferably 75°C or less, more preferably 70°C or less, more preferably 65°C or less, most preferably less than 60°C.

In an embodiment of the invention in process step (xii) during the drying the surface- crosslinked water-absorbent polymer particles or the treated water-absorbent polymer particles or both have a temperature of more than 80°C, preferably more than 85 °C, more preferably more than 90°C, more preferably more than 95°C, more preferably more than 100°C, most preferably in the range of from 130 to 160°C. In an embodiment of the invention in process step (xii) the crosslinking composition further comprises a reducing agent or a poly alkylene glycol or both. A preferred poly al- kylene glycol is a poly ethylene glycol. A preferred poly ethylene glycol has a molecular weight in the range of from 50 to 1000, preferably from 150 to 750, more preferably from 200 to 500, most preferably from 350 to 450.

In an embodiment of the invention the reducing agent is a compound of a chemical formula M _x S0 ₃ , wherein x is either 1 or 2, wherein for x=2 M is Li or Na or both, wherein for x=l M is Mg or Ca or both.

In an embodiment of the invention in process step (xi) the Si-oxide and the water- absorbent polymer particles are charged onto a conveyor belt, preferably prior to charging the Si-oxide and the water-absorbent polymer particles into a first mixing device.

In an embodiment of the invention in process step (xi) the Si-oxide is mixed with the water-absorbent polymer particles in a first mixing device.

In an embodiment of the invention the first mixing device is a disc mixer. A preferred disc mixer comprises at least 5, preferably at least 6, more preferably at least 7, more preferably at least 8, more preferably at least 9, more preferably at least 10, more preferably at least 15, most preferably at least 20, rotating discs.

In an embodiment of the invention in process step (xii) the adding of the crosslinking composition and the heating of the treated water-absorbent polymer particles are performed in a second mixing device. A particularly preferred second mixing device is a High-performance Ringlayer CoriMix® CM 350 by Gebruder Lodige Mascheninenbau GmbH, Paderborn, Germany. In an embodiment of the invention in process step (xii) the adding is a mixing by a plurality of contacts of at least a part of the treated water-absorbent polymer particles to a rotating component, wherein the mixing is performed for a duration in the range of from 0.1 to 60 seconds, preferably from 0.1 to 55 seconds, more preferably from 0.1 to 40 seconds, more preferably from 0.1 to 30 seconds, more preferably from 0.1 to 20 seconds, most preferably from 0.1 to 10 seconds. Therein, the contact of the treated water-absorbent polymer particles to the rotating component is preferably made a plurality of times during the duration of the mixing.

In an embodiment of the invention a mixing shaft of the first mixing device rotates in step (xi) at a first maximum speed and a mixing shaft of the second mixing device rotates in step (xii) at a second maximum speed, wherein the second maximum speed is more, preferably at least 10 rpm, more preferably at least 20 rpm, more preferably at least 30 rpm, more preferably at least 40 rpm, more preferably at least 50 rpm, more preferably at least 100 rpm, more preferably at least 120 rpm, more preferably at least 140 rpm, more preferably at least 160 rpm, more preferably at least 180 rpm, more preferably at least 200 rpm, more preferably at least 220 rpm, more preferably at least 240 rpm, more preferably at least 260 rpm, more preferably at least 280 rpm, more preferably at least 300 rpm, more preferably at least 320 rpm, more preferably at least 340 rpm, more preferably at least 360 rpm, more preferably at least 380 rpm, more preferably at least 400 rpm, more preferably at least 420 rpm, more preferably at least 440 rpm, more preferably at least 460 rpm, more preferably at least 480 rpm, more preferably at least 500 rpm, more preferably at least 520 rpm, more preferably at least 540 rpm, more preferably at least 560 rpm, more preferably at least 580 rpm, more preferably at least 600 rpm, more preferably at least 620 rpm, more preferably at least 640 rpm, more preferably at least 660 rpm, more preferably at least 680 rpm, more preferably at least 700 rpm, more preferably at least 720 rpm, more preferably at least 740 rpm, more preferably at least 760 rpm, more preferably at least780 rpm, more preferably at least 800 rpm, more preferably at least 820 rpm, more preferably at least 840 rpm, more preferably at least 860 rpm, more preferably at least 880 rpm, more preferably at least 900 rpm, more preferably at least 920 rpm, more preferably at least 940 rpm, more preferably at least 960 rpm, more preferably at least 980 rpm, most preferably at least 1000 rpm, than the first maximum speed.

In an embodiment of the invention the second mixing device comprises a rotating shaft and a plurality of mixing tools, wherein the mixing tools are connected to the rotating shaft, wherein in process step (xii) the rotating shaft rotates at a speed in the range of from 500 to 1200 rpm, preferably from 550 to 1200 rpm, more preferably from 600 to 1200 rpm, more preferably from 650 to 1200 rpm, more preferably from 700 to 1200 rpm, more preferably from 750 to 1 150 rpm, more preferably from 800 to 1 100 rpm, more preferably from 850 to 1050 rpm, most preferably from 900 to 1000 rpm.

In an embodiment of the invention the second mixing device comprises a plurality of mixing tools, wherein the mixing tools are rods or paddles or both. A preferred plurality of mixing tools comprises rods and paddles.

In an embodiment of the invention the second mixing device comprises an annular layer of at least a part of the treated water-absorbent polymer particles. A preferred annular layer is shaped by a centrifugal force or a wall of a mixing chamber or preferably both.

In an embodiment of the invention in process step (xii) the drying of the surface- crosslinked water-absorbent polymer particles or the treated water-absorbent polymer particles or both is performed in a drying device, wherein the drying device comprises at least two rotating shafts. A preferred rotating shaft is horizontal. Preferably the least two rotating shafts are horizontal.

In an embodiment of the invention in process step (xiii) the drying of the surface- crosslinked water-absorbent polymer particles is performed in a drying device, wherein the drying device is a paddle mixer or a paddle dryer or both. A particularly preferred paddle dryer and paddle mixer is an Andritz Gouda Paddle Dryer, preferably of type GPWD12W120, by Andritz AG, Graz, Austria.

In an embodiment of the invention in process step (xii) prior to the adding of the crosslinking composition to the treated water-absorbent polymer particles the reducing agent and the further crosslinker are mixed in a third mixing device. A preferred third mixing device is a horizontal mixing device.

In an embodiment of the invention the third mixing device is a line static mixer. Preferably, the line static mixer comprises a particle flow driven by a blower or a pump or both.

In an embodiment of the invention in process step (xiii) the surface-crosslinked water- absorbent polymer particles are contacted with water. A preferred contacting is a spraying water onto or into or both the fluidized bed. Preferably, the surface-crosslinked water- absorbent polymer particles are contacted with the water for remoisturizing. The contacting for 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 surface-crosslinked water- absorbent 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 wt.-%, more preferably from 2 to 8 wt.-% and most preferably from 3 to 5 wt.-%, based on the weight of the surface-crosslinked water- absorbent polymer particles. The remoisturizing preferably increases the mechanical stability of the polymer particles and re- duces their tendency to static charging.

In an embodiment of the invention the Si-oxide added to the sized water-absorbent polymer particles in process step (xi) is comprised by a plurality of particles, wherein the particles are characterized by

a)a specific surface area in the range of from 100 to 300 m ² /g, preferably from 150 to 250 m ² /g, more preferably from 160 to 240 m ² /g, more preferably from 170 to 220 m ² /g, more preferably from 180 to 200 m ² /g, most preferably from 185 to 195 m ² /g, or b) a particle size distribution having a D ₅₀ in the range of from 50 to 200 μιτι, preferably from 60 to 190 μπι, more preferably from 80 to 170 μπι, more preferably from 100 to 150 μηι, most preferably from 1 15 to 125 μιτι, or c) both.

In an embodiment of the invention the polymer gel being discharged in process step

(vii) comprises water in the range of from 40 to 60 wt.-%, preferably from 50 to 60 wt.-%, more preferably from 53 to 56 wt.-%, based on the polymer gel.

In an embodiment of the invention the polymer gel being discharged in process step (vii) is a polymer gel sheet; wherein the polymer gel sheet is characterized by a thickness in the range of from 10 to 200 mm, preferably from 10 to 100 mm, more preferably from 15 to 75 mm, most preferably from 15 to 50 mm..

In an embodiment of the invention the polymer gel being discharged in process step (vii) is a polymer gel sheet; wherein the polymer gel sheet is characterized by a width in the range of from 30 to 300 cm, preferably from 50 to 250 cm, more preferably from 60 to 200 cm, most preferably from 80 to 100 cm.

In an embodiment of the invention the polymerization in step (vi) is performed in presence of a blowing agent. The blowing agent may be added to the aqueous monomer solution in one selected from the group consisting of step (i), step (ii), step (iii), step (iv), step (v), and step (vi), or in a combination of at least two thereof. Preferably, the blowing agent is add- ed to the monomer solution in step (i). The blowing agent should be added prior or immediately after the polymerization in step (vi) is initiated. Particularly preferably, the blowing agent is added to the monomer solution after or simultaneously to adding the initiator or a component of an initiator system. Preferably the blowing agent is added, to the monomer solution in an amount in the range of from 500 to 4000 ppm by weight, preferably from. 1000 to 3500 ppm by weight, more preferably from 1500 to 3200 ppm. by weight, most preferably from 2000 to 3000 ppm by weight, based on the total, weight of the monomer solution.

A blowing agent is a substance which is capable of producing a cellular structure or pores or both via a foaming process during polymerization of the monomers. The foaming process is preferably endothermic. A preferred endothermic foaming process is started by heat from an exothermic polymerisation or crosslinking or both reaction. A preferred blowing agent is a physical blowing agent or a chemical blowing agent or both. A preferred physical blowing agent is one selected from the group consisting of a CFC, a HCFC, a hydrocarbon, and C0 ₂ , or a combination of at least two thereof. A preferred C0 ₂ is liquid C0 ₂ . A preferred hydrocarbon is one selected from the group consisting of pentane, isopentane, and cyclopen- tane, or a combination of at least two thereof. A preferred chemical blowing agent is one selected from the group consisting of a carbonate blowing agent, a nitrite, a peroxide, calcined soda, an oxalic acid derivative, an aromatic azo compound, a hydrazine, an azide, a Ν,Ν'- Dinitrosoamide, and an organic blowing agent, or a combination of at least two thereof.

A very particularly preferred blowing agent is a carbonate blowing agent. Carbonate blowing agents which may be used according to the invention are disclosed in US 5, 1 18, 719 A, and are incorporated herein by reference. A preferred carbonate blowing agent is a carbonate containing salt, or a bicarbonate containing salt, or both. Another preferred carbonate blowing agent comprises one selected from the group consisting of C0 ₂ as a gas, C0 ₂ as a solid, ethylene carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, magnesium carbonate, or magnesium hydroxic carbonate, calcium carbonate, barium carbonate, a bicarbonate, a hydrate of these, other cations, and naturally occurring carbonates, or a combination of at least two thereof. A preferred naturally occurring carbonate is dolomite. The above mentioned carbonate blowing agents release C0 ₂ when being heated while dis- solved or dispersed in the monomer solution. A particularly preferred carbonate blowing agent is MgC0 ₃ , which may also be represented by the formula (MgC0 ₃ )4-Mg(OH) ₂ -5-H ₂ 0. Another preferred carbonate blowing agent is agent is (NH ₄ ) ₂ C0 ₃ . The MgC0 ₃ and (NH ₄ ) ₂ C0 ₃ may also be used in mixtures. Preferred carbonate blowing agents are carbonate salts of multivalent cations, such as Mg, Ca, Zn, and the like. Examples of such carbonate blowing agents are Na ₂ C0 ₃ , K ₂ C0 ₃ , (NH ₄ ) ₂ C0 ₃ , MgC0 ₃ , CaC0 ₃ , NaHC0 ₃ , KHC0 ₃ , NH ₄ HC0 ₃ , Mg(HC0 ₃ ) ₂ , Ca(HC0 ₃ ) ₂ , ZnC0 ₃ , and BaC0 ₃ . Although certain of the multivalent transition metal cations may be used, some of them, such as ferric cation, can cause color staining and may be subject to reduction oxidation reactions or hydrolysis equilibria in water. This may lead to difficulties in quality control of the final polymeric product. Also, other mul- tivalent cations, such as Ni, Ba, Cd, Hg would be unacceptable because of potential toxic or skin sensitizing effects.

A preferred nitrite is ammonium nitrite. A preferred peroxide is hydrogen peroxide. A preferred aromatic azo compound is one selected from the group consisting of a triazene, ar- ylazosulfones. arylazotriarylmethanes, a hydrazo compound, a diazoether, and diazoamino- benzene, or a combination of at least two thereof. A preferred hydrazine is phenylhydrazine. A preferred azide is a carbonyl azide or a sulfonyl azide or both. A preferred Ν,Ν'- Dinitro- so amide is N ,N -dimethy 1-N ,N ' -dinitro soter ephthalainide . A contribution to solving at least one of the above objects is provided by a device for the preparation of surface-crosslinked water-absorbent polymer particles in a process stream, comprising

a) a first container, designed to take an aqueous monomer solution, comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al);

b) a further container, designed to take at least one crosslinker (cc3); c) a fourth mixing device, wherein the fourth mixing device is

i) located down-stream to the first container and the further container,

ii) designed to mix the monomer solution and the at least one crosslinker (a3);

d) a polymerization reactor, wherein the polymerization reactor is i) located down-stream to the first container and the further container,

ii) designed to comprise the aqueous monomer solution and the at least one crosslinker (a3) during polymerizing the monomers in the aqueous monomer solution, thereby obtaining a polymer gel;

e) a comminuting device, wherein the comminuting device is

i) located down-stream to the first container and the further container,

ii) designed to comminute the polymer gel;

f) a belt dryer, wherein the belt dryer is

i) located down-stream to the comminuting device, ii) designed to dry the polymer gel;

g) a grinding device, wherein the grinding device is

i) located down-stream to the belt dryer,

ii) designed to grind the dried polymer gel, thereby obtaining water-absorbent polymer particles;

h) a first sizing device, wherein the first sizing device is

i) located down-stream to the grinding device,

ii) designed to size the water-absorbent polymer particles in a first sizing step;

j) a first mixing device, wherein the first mixing device is i) located down-stream to the first sizing device,

ii) designed to mix a Si-oxide and the sized water-absorbent polymer particles, thereby obtaining treated water- absorbent polymer particles;

k) a second mixing device, wherein the second mixing device is i) located down-stream to the first mixing device, ii) designed to mix a crosslinking composition, comprising a further crosslinker, and the treated water-absorbent polymer particles and heat the treated water-absorbent polymer particles, thereby obtaining surface-crosslinked water-absorbent polymer particles;

1) a cooling device device, wherein the cooling device is

i) located down-stream to the second mixing device, ii) designed to cool the surface-crosslinked water- absorbent polymer particles in a fluidized bed.

Therein, the fourth mixing device may be identical to the polymerization reactor. Moreover, the polymerization reactor may be identical to the comminuting device. Hence, the fourth mixing device, the polymerization reactor, and the comminuting device may be identical. Preferred components or devices or both of the device according to the invention are designed according to the process according to the invention. A preferred first mixing device is the first mixing device according to the process according to the invention. A preferred second mixing device is the second mixing device according to the process according to the invention. A preferred crosslinking composition is the crosslinking composition according to the process according to the invention.

In an embodiment of the invention the device further comprises a) a third mixing device, wherein the third mixing device is

i) located upstream to the second mixing device, ii) designed to mix the further crosslinker and a reducing agent;

b) a drying device, wherein the drying device is

i) located down-stream to the second mixing device, ii) designed to dry the surface-crosslinked water- absorbent polymer particles prior to the cooling in the cooling device;

c) a further sizing device, wherein the further sizing device is

i) located down-stream to the cooling device, ii) designed to separate a portion of the Si-oxide or fine sur- face-crosslinked water-absorbent polymer particles or both from the surface-crosslinked water-absorbent polymer particles in a further sizing step.

A preferred reducing agent is the reducing agent according to the process according to the invention. A preferred third mixing device is the third mixing device according to the process according to the invention. A preferred drying device is the drying device according to the process according to the invention.

A contribution to the solution of at least one of the above objects is provided by a process ibr the preparation of a surface-crosslinked water-absorbent polymer particle in the device according to the invention. Preferably, the process comprises the process steps (i) to (xiii) according to the invention.

A contribution to the solution of at least one of the above objects is provided by a surface-crosslinked water-absorbent polymer particle, obtainable by the process according to the invention.

A contribution to the solution of at least one of the above objects is provided by a plurality of surface-crosslinked water-absorbent polymer particles, comprising

a) a chelating agent, in particular EDTA, in an amount in the range of from 500 to 3,000 ppm by weight, preferably from 1,000 to 2,000 ppm by weight;

b) a poly alkylene glycol, in particular poly ethylene glycol, in an amount in the range of from 500 to 3,000 ppm by weight, preferably from 1,000 to 2,000 ppm by weight; and

c) a Si0 ₂ in an amount in the range of from 500 to 3,000 ppm by weight, preferably from 1,000 to 2,000 ppm by weight;

each based on the weight of the plurality of surface-crosslinked water-absorbent polymer particles. Regarding the chelating agent, reference is made to the details given above in this text. According to a further aspect of this embodiment, the plurality of surface- crosslinked water-absorbent polymer particles further comprises Ag-zeolite, preferably in an amount in the range from 0.0001 to 1 wt.-part, more preferably in the range from 0.001 to 0.5 wt.-part and most preferred in the range of 0.002 to 0.01 wt.-part, each based on the total weight of the plurality of surface-crosslinked water-absorbent polymer particles.

A contribution to the solution of at least one of the above objects is provided by a composite material comprising the surface-crosslinked water-absorbent polymer particle ac- cording to the invention or the plurality of surface-crosslinked water-absorbent polymer particles according to the invention.

In an embodiment of the invention the composite material according to the invention comprises one selected from the group consisting of a foam, a shaped article, a fibre, a foil, a film, a cable, a sealing material, a liquid-absorbing hygiene article, a carrier for plant and fungal growth-regulating agents, a packaging material, a soil additive, and a building material, or a combination of at least two thereof. A preferred cable is a blue water cable. A preferred liquid-absorbing hygiene article is one selected from the group consisting of a diaper, a tampon, and a sanitary towel, or a combination of at least two thereof. A preferred diaper is a baby's diaper or a diaper for incontinent adults or both.

A contribution to the solution of at least one of the above objects is provided by a process for the production of a composite material, wherein the surface-crosslinked water- absorbent polymer particle according to the invention or the plurality of surface-crosslinked water-absorbent polymer particles according to the invention, and a substrate, and optionally an auxiliary substance are brought into contact with one another.

A contribution to the solution of at least one of the above objects is provided by a composite material obtainable by a process according to the invention.

A contribution to the solution of at least one of the above objects is provided by a use of the surface-crosslinked water-absorbent polymer particle according to the invention or the plurality of surface-crosslinked water-absorbent polymer particles according to the invention in a foam, a shaped article, a fibre, a foil, a film, a cable, a sealing material, a liquid-absorbing hygiene article, a carrier for plant and fungal growth-regulating agents, a packaging material, a soil additive, for controlled release of an active compound, or in a building material. Test Methods

The following test methods are used in the invention. In absence of a test method, the ISO test method for the feature to be measured being closest to the earliest filing date of the present application applies. If no ISO test method is available, the ED ANA test method being closest to the earliest filing date of the present application applies. In absence of distinct measuring conditions, standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm) apply. The water content of the water-absorbent polymer particles after drying is determined according to the Karl Fischer method.

The water-soluble polymer content of water-absorbent polymer particles is measured according to a standard test method for superabsorbent materials defined by the EDANA. Said test method is described in EDANA, Harmonized Test Methods Nonwovens and Related Industries, 2012 Edition as "Extractables" under the method number WSP 270.2.R3 (12). Therein, the measurement is performed after aging the water-absorbent polymer particles.

AUL (Absorption Under Load)

The AUL of water-absorbent polymer particles is measured according to a standard test method for superabsorbent materials defined by the EDANA. Said test method is described in EDANA, Harmonized Test Methods Nonwovens and Related Industries, 2012 Edition as "Absorption Under Pressure" under the method number WSP 243.2.R1(12). The water-absorbent polymer particles measured have a particle size range from 200 to 700 μηι.

Aging

Place water-absorbent polymer particles in a mono particle layer on a 400 mesh sieve in order to avoid agglomeration of the water-absorbent polymer particles during aging. Swell water- absorbent polymer particles with water to a water content of 50 wt.-% based on the water-absorent polymer particles and hold the swollen particles at a temperature of 80°C for 3 hours. Then dry the swollen particles to a water content of 5 wt.-% by placing the 400 mesh sieve in an laboratory oven with 100°C for further measurements. specific surface area

The specific surface area is measured using N ₂ according to ISO 9277.

D ₅ o of particles comprising Si-oxide

The particle size distribution and hence the D50 of the particles comprising the Si- oxide are measured by laser diffraction according to ISO 13320-1. Si-oxide dust content

This test provides a process for measuring the amount of unassociated dust having a diameter less than a predetermined size present in a water-absorbent polymer particle composition. In the case of the examples and comparative examples below, the measurement process was used to determine the amount of unassociated matter having a maximum size less than or equal to 10 μηι. The process is applicable, however, to measure the amount of unasso- ciated dust having a diameter less than other predetermined sizes. The process comprises: placing the composition into the sample holder of a pulsed-jet disperser,

- blasting the composition with air dried by passage through dehydration means, and determining the weight of particles in each of a plurality of size increments.

Dehydration means are suitable to reduce the amount of water present in the air to be used to blast the polymer composition. Suitable dehydration means include molecular sieve cartridges, dessicating materials and membranes. An especially preferred dehydration means is a molecular sieve cartridge. The dust levels of the coated samples may be measured as follows. Approximately 0.01 to 0.2 g of the sample to be tested are placed in the sample holder of a pulsed-jet disperser, an AEROSIZER™ available from Amherst Process Instruments, Hadley, Massachusetts. The pulsed-jet disperser is fitted with a 250 μιη screen to prevent particles greater than 250 μπι in diameter from entering the detector. The air delivered to the pulsed-jet disperser is filtered using the factory supplied filter and is dried using a molecular sieve cartridge. The samples are blasted with the dried air, with the exhaust stream being directed to the detector and analyzed until the instantaneous count rate decays to less than 20 particles per second on the low sensitivity setting of the detector. Measurements are made on separate portions of the samples and are averaged. For each portion, a determination is made of the total number of particles in each of 500 size increments evenly distributed logarithmically from 0.2 to 200 μιτι, the instrument exhibiting an approximate lower level of detection of 0.5 μιη. The weight of material each of these size increments is calculated using the following equation:

Weight = (number of particles in an increment) (density) (π) (particle diameter) ³ 16. The aggregate weight of unassociated material less than 10 μηι is designated weight _dU st-

This weight is compared to the weight of sample originally introduced into the disperser, that is weight _sa m _p i _e - The fraction of dust is determined in accordance with the following equation:

Weight percent dust (%) = 100 ^χ (weightdust) / (weight _sam pie)- caking tendency

The test involves placing a desiccator, a Cole-Parmer catalog #G-08904 00, into an oven. The desiccator is maintained at 80 % humidity by placing a straight-sided evaporation dish containing a saturated solution of potassium bromide inside the dessicator. The oven is maintained at 40°C. 5 g-samples of each dried, that is, having a water content of 5 wt.-%, wa- ter-asborbent polymer to be tested are placed in an aluminum foil pan or plastic weigh dish measuring 6 cm in diameter and 1.5 cm in depth. The pan is removed from the dessicator after a duration sufficient for the samples to reach the desired moisture level to test. The samples are weighed to determine the moisturized mass and then allowed to cool to room temperature. While slightly agitating, for example, by lightly tapping, the sample is screened through a No. 8 U.S.A. Standard Testing Sieve (2.36 mm). The amount which passes through the sieve is weighed to determine the mass of the non-caking polymer particles. The fraction of the particles which are non-caking is then determined by the following formula:

Non-caking wt.-% (mass of non-caking polymer particles / moisturized mass) x 100.

The moisture level in wt.-% is determined by the following formula:

[moisturized mass - weight of initial sample) / weight of initial sample] x 100.

[Mode for Invention] Examples

The present invention is now explained in more detail by examples and drawings given by way of example which do not limit it.

A) Preparation of a partially neutralized acrylic acid monomer solution 0.4299 wt.-parts of water are mixed in an adequate container with 0.27 wt.-parts of acrylic acid and 0.0001 wt.-parts of mono methyl ether hydroquinone (MEHQ). 0.2 wt.-parts of an aqueous 48 wt.-% sodium hydroxide solution are added to the mixture. A sodium- acrylate monomer solution with a neutralization ratio of 70 mol-% is achieved.

Optionally the sodium-acrylate monomer solution is degased with nitrogen.

B) Polymerization of the monomer solution

1 wt.-part of the monomer solution prepared in step A) is mixed with 0.001 wt.-parts of trimethylol propane triacrylate as crosslinker, 0.001 wt.-parts of sodium peroxodisulfate as first initiator component, 0.000034 wt.-parts of 2,2-dimethoxy-l ,2-diphenylethan-l-one (Ci- ba ^® Irgacure ^® 651 by Ciba Specialty Chemicals Inc., Basel, Switzerland) as a second initiator component, up to 0.1 wt.-parts of acrylic acid particles (with a particle size of less than 150 μη ) in a container to achieve a mixed solution. As a blowing agent sodium carbonate is added to the mixed solution in an amount of 0.1 wt.-part based on the total amount of the mixed solution.

A sufficient amount of the mixed solution is subjected to further treatment in order to obtain a polymer gel and further downstream water-absorbent polymer particles and further downstream surface-crosslinked water-absorbent polymer particles as well as further downstream a water-absorbent product which is post treated. Details of the further treatment are given below.

Subsequently, the mixed solution is placed on the belt of a conveyer belt reactor and the polymerization is initiated by UV radiation. The conveyor belt has a length of at least 20 m and a width of 0.8 m. The conveyor belt is formed as a trough to keep the solution on the belt prior to and while being polymerized. The dimensions of the conveyor belt and the conveying speed of the conveyer belt are selected in a way that a poly-acrylic acid gel is formed at a downstream end of the belt. At the end of this step a water-absorbent polymer gel is achieved. The polymer gel has a water content of about 52 wt.-%, based on the total weight of the polymer gel.

C) Comminuting and drying of the polymer gel

The polymer gel forms a polymer gel strand which is discharged from the conveyor belt and comminuted in three steps:

The rubbery poly-acrylic acid gel is cut into flat gel strips by a knife. The gel strips have a length in the range of from 10 to 20 cm, a height in the range of from 10 to 20 mm, and a width in the range of from 10 to 200 mm, then

a breaker is used to shred the strips into gel pieces having a length in the range from 5 to 50 mm, a height in the range of from 3 to 20 mm, and a width in the range of from 3 to 20 mm, then

the gel pieces are extruded through a mixer with a grinder to mince the gel pieces obtaining gel pieces having a length in the range of from 3 to 20 mm a height in the range of from 3 to 20 mm and a width in the range of from 3 to less than 20 mm.

The comminuted gel is dried in a belt dryer at a temperature of 180 °C to a water content of 5 wt.-% based on the dried polymer gel. The belt of the belt drier provides orifices, where hot air is pressed into the polymer gel via nozzles. Additionally hot air is blown from above the belt onto the gel.

Milling and sizing The dried polymer gel is ground in three steps. First the dried polymer gel is fed through a Herbold Granulator HGM 60/145 (HERBOLD Meckesheim GmbH) and the achieved parts of the dried polymer gel have a size of less than 7 mm and are then kept for 2.5 hours in a container to equalize the humidity content of the polymer gel parts. The dried polymer gel parts are then milled in a roller mill of Bauermeister Type 350.1 x 1800 (3-stage crusher) (Bauermeister Zerkleinerungstechnik GmbH) to obtain water-absorbent polymer particles having a particle size of less than 1 mm.

The water absorbent polymer particles are sieved with a tumbler sieves having several screens. The mesh sizes of the screens change from 20, 30, 40, 50, 60 to 100 U.S. -mesh. At least 50 wt.-% of the obtained water-absorbent polymer particles have a particles size in the range of from 300 to 600 μηι. Less than 5 wt.-% of the water- absorbent polymer particles of the examples according to the invention are smaller than 150 μηι, less than 5 wt.-% of the water-absorbent polymer particles of the examples according to the invention are have a particle size of more than 850 μηι. The obtained water-absorbent polymer particles are named precursor I.

E) Silicon dioxide treatment

In a treatment step the precursor I is optionally mixed in a disc mixer with silicon dioxide (Si0 ₂ ) in form of Sipernat ^® 22 obtainable from Evonik Industries AG, Essen, Germany. When mixing the precursor I with the Si0 ₂ , the precursor still has a temperature in the range of from 80 °C to 100 °C. A precursor II is obtained. If and how much of the silicon dioxide is added is given below in the table 1 and 2 for the various examples and comparative examples.

F) Surface crosslinking

In a further step 1 wt.-part of the precursor II is mixed with 0.003 wt.-part (+-10 %) of a surface crosslinker, based on the total weight of the mixture of precursor II and crosslinker. The surface crosslinker is composed of 19 wt.-% water, 40 wt.-% ethylene glycol diglycidyl ether, 1 wt.-% Na ₂ S0 ₃ , 40 wt.-% poly ethylene glycol with a molecular weight of 400 g/mol, each based on the total amount of the crosslinker. The ingredients of the crosslinker are mixed in a line static mixer. The crosslinker is mixed in a ringlayer mixer CoriMix ^® CM 350 (Ge- bruder Lodige Mascheninenbau GmbH, Paderborn, Germany) as shown schematically in figures 9a) and 9b) with precursor II. The mixture is heated to a temperature in the range of from 130 to 160 °C. The mixture is then dried in a paddle dryer Andritz Gouda Paddle Dryer, preferably of type GPWD12W120, by Andritz AG, Graz, Austria for 45 minutes at a temperature in the range of from 130 to 160°C. Surface-cross-linked absorbent polymer particles are obtained.

In a cooling device which is specified below in table 1 the temperature of the surface- cross-linked absorbent polymer particles is decreased to below 60 °C, obtaining cooled surface-cross-linked absorbent polymer particles referred as to precursor III. Optionally, a chelating agent is added to the polymer particles during cooling. If and how much of the chelating agent is added is given below in table 2. In the examples and comparative examples in table 1 1 ,500 ppm by weight, based on the precursor III, of Na ₂ -EDTA as chelating agent are added are added.

G) Post treatment

1 wt.-part of precursor III is then subjected to mixing with 0.005 wt.-part Ag-zeolite. Subsequently, the mixture is sieved. The sieve is selected to separate agglomerates of the cooled surface-cross-linked absorbent polymer particle having a particle size of more than 850 μη . At least 50 wt.-% of the surface-crosslined absorbent polymer particles have a particles size in the range of from 300 to 600 μη . Less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples according to the invention are smaller than 150 μηι, less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples according to the invention are have a particle size of more than 850 μη . Post treated crosslinked water-absorbent polymer particles are obtained.

The following scale is used to compare the results of measuring the parameters given in the tables 1 and 2 for the examples and the comparative examples. In the order given in the following the measurement results are getting better from left to right:— , -, +, ++, +++.

Table 1 : Comparison of caking tendency and dust content depending on the cooling device and the amount of Si-oxide added. The cooling device used in example 1 according to the invention is a gas propelled fluidized bed cooler which is commercially available by Daesung Industrial Co. Ltd., Korea. Said fluidized bed cooler comprises an in-gas-stream with a temperature in the range of from 15 to 25°C and an out-gas-stream with a temperature in the range of from 60 to 70°C. Comparing example 1 to the comparative example 2 which is not according to the invention shows that using such a fluidized bed cooler reduces the content of Si0 ₂ -dust, meaning particles comprising Si0 ₂ which have a particle diameter which is less than 1 μηι, with respect to using a water cooled drum dryer for cooling the surface-crosslinked water-absorbent polymer particles. Moreover, comparing the comparative examples 1 and 2 shows that adding Si0 ₂ to the water-absorbent polymer particles (precursor I) prior to surface-crosslinking reduces the caking tendency of the polymer particles. Table 1 gives the amount of Si0 ₂ added to the sized water-absorbent polymer particles prior to surface-crosslinking based on the weight of the sized water-absorbent polymer particles.

Table 2: Water-soluble polymer content, aging behaviour and dust content of surface- crosslinked water-absorbent polymer particles depending on the amounts of chelating agent and Si-oxide added.

In the examples and the comparative example given in table 2 different amounts of a chelating agent which is Na ₂ -EDTA are added to the surface-crosslinked water-absorbent polymer particles during cooling in the fluidized bed cooler described above. The amounts of Na ₂ -EDTA added are given in ppm by weight based on the weight of the surface-crosslinked water-absorbent polymer particles. Moreover, the amount of Si0 ₂ added to the sized water- absorbent polymer particles prior to surface-crosslinking is varied and given in table 2 based on the weight of the sized water-absorbent polymer particles. Comparing the examples 2 to 5 shows that adding the chelating agent reduces the content of water-soluble polymers in the surface-crosslinked polymer particles and increases the AUL (absorption under load) of the surface-crosslinked water-absorbent polymer particles after aging. Therein, the aging is performed as given above under test methods. The caking tendency and the AUL prior to aging and the Si0 ₂ -dust content of the surface-crosslinked water-absorbent polymer particles are unaffected by adding the chelating agent. This means that adding the chelating agent in a sufficient amount clearly improves the quality of the surface-crosslinked water-absorbent polymer particles after storing them in a humid environment with an elevated temperature. The examples 6 to 8 and the comparative example 3 show again that adding the Si-oxide reduces the caking tendency of the surface-crosslinked water- absorbent polymer particles. With re- spect to the amount of Si-oxide added the content of water-soluble polymers and the AUL after aging are not affected. However, the AUL prior to aging becomes worth if an excessively large amount of Si-oxide is added to the sized water-absorbent polymer particles. For Si- oxide amounts in the range of from 0 to 0.7 wt.-% the AUL prior to aging is unaffected. The same holds for the content of Si-oxide dust in the surface-crosslinked water-absorbent poly- mer particles. Only at a high amount of 2 wt.-% of Si-oxide a noticeable increase of the dust content is observed. For Si-oxide amounts in the range of from 0 to 0.7 wt.-% the excessive Si-oxide dust is removed in the fluidized bed cooler. Concluding from table 2, example 5 according to the invention shows the best overall combination of the parameters given in said table.

Figure 1 shows a flow chart diagram depicting the steps 101 to 1 13 of a process 100 for

the preparation of surface-crosslinked water-absorbent polymer particles 505 according to the invention. In a first step 101 an aqueous monomer solution comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al) and at least one crosslinker (a3) is provided. Preferably, the aqueous monomer solution is an aqueous solution of partially neutralized acrylic acid, further comprising crosslinkers. In a second step 102 fine particles of a water-absorbent polymer may be added to the aqueous monomer solution. In a third step 103 a polymerization initiator or at least one component of a polymerization initiator system that comprises two or more components is added to the aqueous monomer solution. In a fourth step 104 the oxygen content of the aqueous monomer solution is decreased by bubbling nitrogen into the aqueous monomer solution. In a fifth step 105 the monomer solution is charged onto a belt of a polymerization belt reactor as a polymerization reactor 704. The belt is an endless conveyor belt. In a sixth step 106 the aqueous monomer solution is polymerized to a polymer gel. In a seventh step 107 the polymer gel is discharged from the belt. Subsequently, the polymer gel is comminuted, whereby polymer gel particles are obtained. In an eighth step 108 the polymer gel particles are charged onto a belt of a belt dryer 706 and subsequently dried at a temperature of about 120 to 150°C. The dried polymer gel particles are discharged from the belt dryer 706 and subsequently in a ninth step 109 ground to obtain water-absorbent polymer particles 501. In a tenth step 1 10 the water-absorbent polymer particles 501 are sized in a first sizing step to obtain water-absorbent polymer particles 501 having a well defined particle size distribution. In an eleventh step 1 1 1 Si0 ₂ as Si-oxide 507 is added to the sized water-absorbent polymer particles 501, thereby obtaining treated water-absorbent polymer particles 503. In a twelfth step 1 12 a crosslinking composition, comprising a further crosslinker 508, is added to the treated water-absorbent polymer particles 503 and the treated water-absorbent polymer particles 503 are heated, thereby obtaining surfaee-crosslinked water-absorbent polymer particles 505. In a thirteenth step 1 13 the surfaee-crosslinked water-absorbent polymer particles 505 are cooled in a fluid- ized bed of the surfaee-crosslinked water-absorbent polymer particles 505.

Figure 2 shows a flow chart diagram depicting the steps 101 to 113 of a process 100 for the preparation of surfaee-crosslinked water-absorbent polymer particles 505 according to the invention. The process 100 shown in figure 2 is the same as the process 100 in figure 1 , wherein the third process step 103 and the fourth process step 104 overlap in time. While the polymerization initiator is added to the aqueous monomer solution, nitrogen is bubbled into the aqueous monomer solution in order to decrease its oxygen content.

Figure 3 shows a flow chart diagram depicting the steps 101, 103, 105 to 1 13 of a process 100 for the preparation of surfaee-crosslinked water-absorbent polymer particles 505 according to the invention. The process 100 shown in figure 3 is the same as the process 100 in figure 1, wherein the second step 102 and the fourth step 104 are not part of the process 100 according to figure 3.

Figure 4 shows a flow chart diagram depicting the steps 101 to 1 14 of a process 100 for the preparation of surfaee-crosslinked water-absorbent polymer particles 505 according to the invention. The process 100 shown in figure 4 is the same as the process 100 in figure 1, wherein the process 100 of figure 4 further comprises a process step (xiv) 1 14 of subjecting the surface-crosslinked water-absorbent polymer particles 505 to a further sizing step; thereby separating a portion of the Si-oxide 507 or fine surface-crosslinked water-absorbent polymer particles 605 or both from the surface-crosslinked water-absorbent polymer particles 505.

Figure 5 shows a flow chart diagram of process steps (xi) 1 11 to (xiii) 1 13 according to the invention. The sized water-absorbent polymer particles 501 are introduced into a first mixing device 502 which is a disc mixer. Therein, the water-absorbent polymer particles 501 are mixed with Si0 ₂ as a Si-oxide 507, thereby obtaining treated water- absorbent polymer particles 503. During said mixing a mixing shaft of the disc mixer rotates at a first maximum speed and the water-absorbent polymer particles 501 have a temperature in the range of from 80 to 100°C. The treated water-absorbent polymer particles 503 are introduced into a second mixing device 504 which is a high-performance ringlayer mixer CoriMix® CM 350 by Ge- briider Lodige Maschinenbau GmbH, Paderborn, Germany. In the second mixing device 504 the treated water-absorbent polymer particles 503 are mixed with a crosslinking composition, comprising a further crosslinker 508, arid heated to a temperature in the range from 120 to 160°C, thereby obtaining surface-crosslinked water- absorbent polymer particles 505. During the mixing in the second mixing device 504 a mixing shaft of the second mixing device 504 rotates at a second maximum speed which is more than the first maximum speed. In a cooling device 506 a fluidized bed of the surface-crosslinked water-absorbent polymer particles 505 is formed and said particles are cooled to a temperature of 60°C or less. Moreover, in the cooling device 506 water 509 is sprayed onto the surface-crosslinked water-absorbent polymer particles 505 for remoisturizing.

Figure 6 shows a flow chart diagram of process steps (xi) 1 1 1 to (xiv) 114 according to the invention. The sized water-absorbent polymer particles 501 are introduced into a first mixing device 502 which is a disc mixer. Therein, the water-absorbent polymer particles 501 are mixed with Si0 ₂ as a Si-oxide 507, thereby obtaining treated water- absorbent polymer particles 503. During said mixing a mixing shaft of the disc mixer rotates at a first maximum speed and the water- absorbent polymer particles 501 have a temperature in the range of from 80 to 100°C. The treated water-absorbent polymer particles 503 are introduced into a second mixing device 504 which is a high-performance ringlayer mixer CoriMix® CM 350 by Ge- briider Lodige Maschinenbau GmbH, Paderborn, Germany. In the second mixing device 504 the treated water-absorbent polymer particles 503 are mixed with a crosslinking composition, comprising a further crosslinker 508, and heated to a temperature in the range from 120 to 160°C, thereby obtaining surface-crosslinked water-absorbent polymer particles 505. Prior to said mixing in the second mixing device 504 the further crosslinker 508 has been mixed with a reducing agent 604, which is Na ₂ S03, in a third mixing device 601, which is a horizontal screw mixer. During the mixing in the second mixing device 504 a mixing shaft of the second mixing device 504 rotates at a second maximum speed which is more than the first maximum speed. Subsequently, the surface-crosslinked water-absorbent polymer particles 505 are dried in a drying device 602 which is an Andritz Gouda Paddle Dryer of type GPWD12W120 by Andritz AG, Graz, Austria comprising two horizontal rotating shafts. The drying is performed at a temperature of more than 100°C. Subsequently, in a cooling device 506 a fluidized bed of the surface-crosslinked water- absorbent polymer particles 505 is formed and said particles are cooled to a temperature of 60°C or less. Moreover, in the cooling device 506 water 509 is sprayed onto the surface-crosslinked water-absorbent polymer particles 505 for remoisturiz- ing. A portion of the Si-oxide 507 is removed from the surface-crosslinked water-absorbent polymer particles 505 for dedusting. Subsequently, the surface-crosslinked water- absorbent polymer particles 505 are subjected to a further sizing step, thereby separating fine surface- crosslinked water-absorbent polymer particles 605 from the surface-crosslinked water- absorbent polymer particles 505. The fine surface-crosslinked water-absorbent polymer particles 605 are recycled back into the process 100 according to the invention.

Figure 7 shows a block diagram of a device 700 for the preparation of surface- crosslinked water- absorbent polymer particles 505 according to the invention. The arrows show a direction of a process stream 709 of the preparation of the surface-crosslinked water- absorbent polymer particles 505. The device 700 comprises a first container 701 , a further ^" container 702, downstream a fourth mixing device 703, downstream a polymerization reactor 704 which is a polymerization belt reactor comprising an endless conveyor belt, downstream a comminuting device 705, downstream a belt dryer 706, downstream a grinding device 707, downstream a first sizing device 708, downstream a first mixing device 502, downstream a second mixing device 504, and downstream a cooling device 506, each according to the invention.

Figure 8 shows a block diagram of another device 700 for the preparation of surface- crosslinked water- absorbent polymer particles 505 according to the invention. The arrows show a direction of a process stream 709 of the preparation of the surface-crosslinked water- absorbent polymer particles 505. The device 700 comprises a first container 701, a further container 702, downstream a fourth mixing device 703, downstream a polymerization reactor 704 which is a polymerization belt reactor comprising an endless conveyor belt, downstream a comminuting device 705, downstream a belt dryer 706, downstream a grinding device 707, downstream a first sizing device 708, downstream a first mixing device 502, downstream a second mixing device 504, downstream a drying device 602, downstream a cooling device 506, downstream a further sizing device 603, and upstream to the second mixing device 504 a third mixing device 601, each according to the invention.

Figure 9a) shows a scheme of a longitudinal cross section of a second mixing device 504 according to the invention. The second mixing device 504 comprises an inlet 901, a mixing chamber 902 which is limited by a mixing chamber wall 903, and an outlet 904. The treated water-absorbent polymer particles 503 are fed via the inlet 901 into the mixing chamber 902. Therein, a rotating shaft 907 with mixing tools 908 (not shown in figure 9a)) rotate at a speed in the range of from 500 to 1200 rpm. Due to a centrifugal force the polymer particles distribute over the mixing chamber wall 903, thereby forming an annular layer 905 of the treated water-absorbent polymer particles 503. A cross section at the axial position 906 of the mixing chamber 902 is shown in figure 9b). The second mixing device 504 is a high- performance ringlayer mixer CoriMix® CM 350 by Gebruder Lodige Maschinenbau GmbH, Paderborn, Germany.

Figure 9b) shows a scheme of a transversal cross section of the second mixing device

504 in figure 9a). The transversal cross section is taken at the axial position 906 is figure 9a). Figure 9b) additionally shows the rotating shaft 907 and one of a plurality of mixing tools 908. The mixing tool 908 is a paddle.