WO2015163512A1

WO2015163512A1 - Multi-stage milling in the production of water-absorbent polymer particles

Info

Publication number: WO2015163512A1
Application number: PCT/KR2014/003670
Authority: WO
Inventors: Jeong Beom Park
Original assignee: Songwon Industrial Co., Ltd.
Priority date: 2014-04-25
Filing date: 2014-04-25
Publication date: 2015-10-29
Also published as: CN106232689B; EA030945B1; KR20160149237A; EA201691522A1; CN106232689A

Abstract

The invention generally relates to a process for the preparation of 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 (α1) and at least one crosslinker (α3); (ii) optionally adding fine particles of a water-absorbent polymer 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; (vii) discharging the polymer gel out of the polymerization reactor and optionally comminuting the polymer gel; (viii) drying the polymer gel; (ix) subjecting the dried polymer gel to a first grinding step in a first grinding de-vice, first ground water absorbent polymer particles to a residence in a residence device, and subsequently at least a first portion to a further grinding step in a further grinding device, thereby obtaining further ground water-absorbent polymer particles; (x) sizing the further ground water-absorbent polymer particles; and (i1) optionally treating the surface of the further ground and sized water-absorbent polymer particles.

Description

[DESCRIPTION]

[Invention Title]

MULTI-STAGE MILLING IN THE PRODUCTION OF WATER-ABSORBENT

POLYMER PARTICLES

[Technical Field]

The invention relates to a process for the preparation of water-absorbent polymer particles; to a water-absorbent polymer particle obtainable by such a process; to a composite material comprising such a 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 water-absorbent polymer particle; to a device for the preparation of water-absorbent polymer particles; and to a process for the preparation of water-absorbent polymer particles using 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 hydro gels, 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 sani- tary 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 condi- tions for the hydrogel obtained after the polymerization (for details see, for example, Modern Superabsorbent Polymer Technology, FL Buchholz, GT Graham, Wiley-VCH, 1998).

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.

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]

In order to obtain water-absorbent polymer particles, the polymer gel is dried and the dried polymer gel is ground. It is known in the prior art that applying a two-stage grinding has advantages. In the first grinding stage the dried polymer gel is coarsely ground and particles are obtained. These particles are fed into the second grinding stage as quick as possible. In result, parts of the water-absorbent material stick to a grinding tool, for example to the teeth of a toothed wheel, and the grinding result is deteriorated. Moreover, deposits of water- absorbent material are accumulated in the grinding device. Such deposits can block moving parts of the grinding device or a passage through the device or both. If grown to a sufficient mass, the deposits can fall down and cause damages to inner parts of the device or affect the grinding process adversely or both.

Performing the invention it has surprisingly been found that storing the particles after the first grinding stage and prior to the second grinding stage improves the result of grinding. This effect has been studied in order to identify dependencies and an optimum storing time with respect to the grinding result and efficiency of the overall production process.

[Technical Solution]

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

A further object is to provide a process for the production of water-absorbent polymers, wherein an operational lifetime or an operational period until maintenance is required or both of at least a part of a device used for grinding a dried polymer gel is increased. It is a further object of the invention to provide a process for the production of water- absorbent polymers, wherein an amount of fine water-absorbent polymer particles produced, preferably by grind- ing the dried polymer gel, is reduced. It is a further object of the invention to provide a continuous process for the production of water-absorbent polymers, wherein a size of a device used in the process is reduced without reducing the yield of the process. It is a further object of the invention to provide a continuous process for the production of water-absorbent poly- mers, wherein costs for a device used in the process are reduced without reducing the yield of the process. It is a further object of the invention to provide a process for the production of water-absorbent polymers, wherein the process shows a balanced combination 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 device for producing water-absorbent polymer particles by a process having at least one of the above advantages. 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.

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]

The present invention provide an effect to at least partly overcome a disadvantage arising from the prior art in the context of the production of superabsorbers.

[Description of Drawings]

The figures show

Figure 1 a flow chart diagram depicting the steps of a process according to the invention; Figure 2 a flow chart diagram depicting the steps of another process according to the invention;

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

Figure 4 a flow chart diagram of a process step (ix) according to the invention;

Figure 5 a flow chart diagram of another process step (ix) according to the invention;

Figure 6 a block diagram of a device for the preparation of water-absorbent polymer particles according to the invention; and

Figure 7 a block diagram of another device for the preparation of water-absorbent polymer particles according to the invention.

LIST OF REFERENCES

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)

110 step (x)

111 step (xi)

401 dried polymer gel

402 first grinding device

403 first ground water-absorbent polymer particles

404 residence device

405 first portion of first ground water-absorbent polymer particles

406 further grinding device

407 further ground water-absorbent polymer particles

408 first residence time

409 second residence time

410 third residence time

501 further sizing device

502 further portion of first ground water-absorbent polymer particles

600 device for the preparation of water-absorbent polymer particles

601 first container

602 further container

603 mixing device

604 polymerization reactor

605 comminuting device

606 belt dryer

607 first sizing device

608 process stream

[Best Mode]

A contribution to the solution of at least one of these objects is made by a process for the preparation of 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 particles of a water-absorbent polymer 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;

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

(viii) drying the polymer gel;

(ix) subjecting

a) the dried polymer gel to a first grinding step in a first grinding device thereby obtaining first ground water-absorbent polymer particles,

b) the first ground water absorbent polymer particles to a residence in a residence device, and

c) subsequently at least a first portion of the first ground water-absorbent polymer particles to a further grinding step in a further grinding device, thereby obtaining further ground water-absorbent polymer particles;

(x) sizing the further ground water-absorbent polymer particles; and

(xi) optionally treating the surface of the further ground and sized water-absorbent polymer particles;

wherein in process step (ix) the dried polymer gel and the first ground water-absorbent polymer particles obtained from the dried polymer gel in total spend a first residence time in the first grinding device; wherein in process step (ix) the first ground water absorbent-polymer particles spend a second residence time in the residence device; wherein in process step (ix) the at least first portion of the first ground water-absorbent polymer particles and the further ground water-absorbent polymer particles obtained from the at least first portion of the first ground water-absorbent polymer particles in total spend a third residence time in the further grinding device; wherein the second residence time is more than the first residence time; wherein the second residence time is more than the third residence time. Therein, the second residence time is preferably more than 5 minutes, more preferably more than 10 minutes, more preferably more than 20 minutes, more preferably more than 30 minutes, more preferably more than 40 minutes, more preferably more than 50 minutes, more preferably more than 60 minutes, more preferably more than 70 minutes, more preferably more than 80 minutes, more preferably more than 90 minutes, more preferably more than 100 minutes, more preferably more than 120 minutes, more preferably more than 130 minutes, most preferably more than 135 minutes, more than the first residence time. The second residence time is preferably more than 5 minutes, more preferably more than 10 minutes, more preferably more than 20 minutes, more preferably more than 30 minutes, more preferably more than 40 minutes, more preferably more than 50 minutes, more preferably more than 60 minutes, more preferably more than 70 minutes, more preferably more than 80 minutes, more preferably more than 90 minutes, more preferably more than 100 minutes, more preferably more than 120 minutes, more preferably more than 130 minutes, most preferably more than 135 minutes, more than the third residence time.

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 hydrogel obtained is continuously discharged out of the polymerization reactor and is continuously optionally comminuted, dried, ground and classified 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 water-absorbent polymer particles with other absorption characteristics have to be prepared.

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, prefera- bly 20 to 2,000 μιη and particularly preferably 150 to 850 μηι. In this context, it is particularly preferable for the content of water-absorbent polymer particles having a particle size in a range of from 300 to 600 μπι 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 water-absorbent poly- mer 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 carbox- ylic 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 functional 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, N" -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, N' -methylene bisacrylamide is even more preferred, and

as compounds of crosslinker class IV, Al₂ (S0₄)₃ and its hydrates are particularly pre- ferred.

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, 1ST -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 ( l),

- to the extent of 0 to 80 wt.-%, preferably to the extent of 0 to 44.99 wt.-% and particularly 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 (ct4), 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 (oc5), 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.

Water-absorbent fine particles are preferably water-absorbent polymer particles the composition of which corresponds to the composition of the above described water-absorbent polymer particles, wherein it is preferred that at least 90 wt.-% of the water-absorbent fine particles, preferably at least 95 wt.-% of the water-absorbent fine particles and most preferred at least 99 wt.-% of the water-absorbent fine particles have a particle size of less than 200 μιη, 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 water-absorbent fine particles which are obtained in process step (x) 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 directions, onto a rotating mixing device. Such a kind of mixing setup can be realised 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 polymerisation 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 peraialeate, tert.-butyl perbenzoate, tert.-butyl-3,5,5-trimethylhexanoate and amyl perneodecanoate. Furthemiore, 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^"5 to 1 mol-% of the reducing component of the redox catalyst and 1 x 10^"5 to 5 mol-% of the oxidising 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 a- 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- 1 ,2-diphenylethan- 1 -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 M₂S₂Og, with M being selected from the group consisting of NH₄, 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 con- tent 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 so- lution.

In process step (v) of the process according to the present invention the aqueous monomer solution is charged into a polymerisation 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 polymerisation 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. Hence, comminuting the polymer gel may be performed prior to discharging the polymer gel out of the polymerization reactor.

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 comminuting device may be the polymerization reactor or a part of the polymerization reactor, or a separate device, or both. Hence, comminuting the polymer gel may be performed before, during, or after discharging the polymer gel out of the polymerization reactor. A preferred polymerization reactor which is the comminuting device is a kneading reactor. If the comminuting is performed in the polymerization reactor, the polymer gel particles obtained are preferably further comminuted after discharging out of the polymerization reactor. If the polymerization reactor is a conveyor belt, the comminuting is preferably performed after discharging the polymer gel as a polymer gel sheet from the conveyor belt in a comminuting device, wherein the 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 three steps:

in a first 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, preferably 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 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 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.

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 polymer gel or the above described polymer gel particles. Rotary tube furnaces, fluidised 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 tlirough, 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 tlirough 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 dryers depend on the product to be processed, the manufacturing 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 en- hancements, such as electropolishing or Teflonizing, are possible.

The 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 between 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 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 pref- erably 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.

In process step (ix) of the process according to the present invention the dried polymer gel or the dried polymer gel particles or both are ground thereby obtaining particulate water- absorbent polymer particles. Said grinding includes the first grinding step in the first grinding device, whereby first ground water-absorbent polymer particles are obtained. Subsequently, the first ground water absorbent polymer particles are fed into the residence device. A pre- ferred residence device is a hopper. The grinding further includes the further grinding step in the further grinding device, whereby further ground water-absorbent polymer particles are obtained.

The first grinding device and the further grinding device can be any device the person skilled in the art considers as appropriate for grinding the above described dried polymer gel or the dried polymer gel particles. As an example for a suitable first grinding device and a suitable further 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. Therein, the first grinding device is preferably chosen to provide a coarse grinding of the dried polymer gel with respect to the further grinding device which is preferably chosen to provide a finer grind- ing. Preferably the first grinding device and the further grinding device are chosen such that a combined grinding of the dried polymer gel in terms of the first grinding step and the further grinding step provides a desired water-absorbent polymer particle size distribution.

In process step (x) of the process according to the present invention the further ground water-absorbent polymer particles are sized, 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 μιιι 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 μιη 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 μπι.

In process step (xi) of the process according to the present invention the surface of the further ground and sized water-absorbent polymer particles are optionally treated. As measures to treat the surface of water-absorbent polymer particles any measure can be used the person skilled in the art considers as appropriate for such a purpose. Examples of surface treatments include, for example, surface crosslinking, the treatment of the surface with water- soluble salts, such as aluminium sulfate or aluminium lactate, the treatment of the surface with inorganic particles, such as silicon dioxide, and the like. Preferably, the components used to treat the surface of the polymer particles (cross-linker, water soluble salts) are added in the form of aqueous solutions to the water-absorbent polymer particles. After the particles have been mixed with the aqueous solutions, they are heated to a temperature in the range of from 150 to 230°C, preferably 160 to 200°C in order to promote the surface-crosslinking reaction.

In an embodiment of the invention the second residence time is in the range of from

120 to 180 minutes, preferably from 120 to 175 minutes, more preferably from 100 to 170 minutes, more preferably from 120 to 165 minutes, more preferably from 130 to 160 minutes, most preferably from 140 to 155 minutes.

In an embodiment of the invention a product of a D₅₀ of a particle diameter of the fur- ther ground water-absorbent polymer particles times the second residence time is in the range of from 40 10³ to 100 · 10³ μην minutes, preferably from 50 · 10³ to 100 · 10³ μπν minutes, more preferably from 50 10³ to 95 10³ μηι·ιηϊηυί68, more preferably from 50 10³ to 90 10³ μm·minutes, more preferably from 50· 10³ to 85 · 10³ μΐΉ-ηώηΐΐεε, more preferably from 50 10³ to 80- 10³ μη ΓηΐηυΐεΒ, more preferably from 50 10³ to 75 - 10³ μm·minutes, most preferably from 54 - 10³ to 72 - 10³ μπν minutes.

In an embodiment of the invention a ratio of a sum of the first residence time and the second residence time to a water content of the dried polymer gel prior to process step (ix) is in the range of from 10 to 100 minutes-wt-%^"1, preferably from 12 to 90 minutes wt-%^"1, more preferably from 14 to 80 minutes wt-%^"1, more preferably from 16 to 70 minutes wt- %^"', more preferably from 18 to 60 minutes wt-%^"1, more preferably from 19 to 50 minutes -wt-%^"1, most preferably from 20 to 45 minutes-wt-%^"1.

In an embodiment of the invention a ratio of a D₅₀ of a particle diameter of the first ground water-absorbent polymer particles to the second residence time is in the range of from 10 to 40 μιτι/minutes, preferably from 15 to 35 μm/minutes, most preferably from 19 to 29 μιη/minutes.

In an embodiment of the invention in process step (ix) subsequently to subjecting the first ground water-absorbent polymer particles to the residence in the residence device and prior to subjecting at least the first portion of the first ground water-absorbent polymer particles to the further grinding step in the further grinding device, the first ground water- absorbent polymer particles are sized, thereby separating the first portion of the first ground water-absorbent polymer particles from a further portion of the first ground water-absorbent polymer particles; wherein the further portion of the first ground water-absorbent polymer particles is not subjected to the further grinding step in the further grinding device. Preferably, the further portion of the first ground water-absorbent polymer particles is at least partly subjected to the process step (x) or (xi) or both. Preferably, the particles of the further portion of the first ground water-absorbent polymer particles are characterised by a particle diameter of more than 800 to 900 μη , preferably more than 850 μιη.

In an embodiment of the invention the first grinding device is an impact mill. A preferred impact mill comprises a rotating component and a plurality of hammers, wherein each hammer is connected to the rotating component. Preferably, the dried polymer gel is ground by multiple impacts of said hammers.

In an embodiment of the invention the first grinding device comprises at least one, preferably at least two, more preferably at least three, more preferably at least 4, more preferably at least 5, more 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 1 1 , more preferably at least 12, more preferably at least 13, most preferably at least 14, rotating knife/knives and at least one, preferably at least two, more preferably at least three, most pref- erably at least 4, static knife/knives. Preferably, the rotating knives are arranged in pairs. Preferably, the static knives are arranged in pairs.

In an embodiment of the invention the residence device is a hopper. A preferred hopper is a storage container used to dispense granular materials through the use of a chute to restrict flow.

In an embodiment of the invention the further grinding device is a roll mill. A preferred roll mill comprises at least 2, preferably at least 3, more preferably at least 4, more preferably at least 5, most preferably at least 6, rolls. Therein, 2 rolls are preferably arranged as a pair, including a roll and a counter roll. Another preferred roll mill is a 3-stage roll mill. A particularly preferred roll mill is a 3-stage roll mill comprising one pair of rolls for each of the 3 stages.

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 solu- tion 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 added 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 endotheraiic. A preferred endothermic foammg 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 C0₂, or a combination of at least two thereof. A preferred C0₂ is liquid C0₂. 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 se- lected 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 C0₂ as a gas, CO? as a solid, ethylene carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, magnesium carbonate, or magnesium hydroxic carbonate, calcium carbonate, barium car- bonate, 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 dolomite. The above mentioned carbonate blowing agents release C0₂ when being heated while dissolved or dispersed in the monomer solution. A particularly preferred carbonate blowing agent is MgC0₃, which may also be represented by the formula (MgC0₃)₄ Mg(OH)2-5-H₂0. Another preferred carbonate blowing agent is agent is (NH₄)₂C0₃. The MgC0₃ and (NH₄)₂C0₃ 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 Na₂C0₃, K₂C0₃, (NH₄)₂C0₃, MgC0₃, CaC0₃, NaHC0₃, KHC0₃, NH₄HC0₃, Mg(HC0₃)₂, Ca(HC0₃)₂, ZnC0₃, and BaC0₃. 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 multivalent 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- soamide is N,N'-dimethyl-N,N'-dinitrosoterephthalamide.

A contribution to solving at least one of the above objects is provided by a device for the preparation of 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 (a3);

c) a mixing device, wherein the 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);

a) 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;

d) 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, thereby obtaining polymer gel particles,

e) a belt dryer, wherein the belt dryer is

i) located down-stream to the comminuting device,

ii) designed to dry the polymer gel particles,

f) a first grinding device, wherein the first grinding device is

i) located down-stream to the belt dryer,

ii) designed to subject the dried polymer gel particles to a first grinding step, thereby obtaining first ground water-absorbent polymer particles, iii) designed to take the dried polymer gel particles and the first ground water-absorbent polymer particles obtained therefrom for a first residence time;

g) a residence device, wherein the residence device is

i) located down-stream to the first grinding device,

ii) designed to take the first ground water-absorbent polymer particles for a second residence time;

h) a further grinding device, wherein the further grinding device is

i) located down-stream to the residence device,

ii) designed to subject at least a first portion of the first ground water- absorbent polymer particles to a further grinding step, thereby obtaining further ground water-absorbent polymer particles;

iii) designed to take at least the first portion of the first ground water- absorbent polymer particles and the further ground water-absorbent polymer particles obtained therefrom for a third residence time;

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

i) located down-stream to the further grinding device,

ii) designed to size the further ground water-absorbent polymer particles; wherein the second residence time is more than the first residence time; wherein the second residence time is more than the third residence time. Therein, the mixing device may be identical to the polymerization reactor. Moreover, the polymerization reactor may be identical to the comminuting device. Hence, the 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 grinding device is the first grinding device according to the process according to the invention. A preferred further grinding device is the further grinding device according to the process according to the invention. A preferred residence device is the residence device according to the process according to the invention. A preferred first residence time is the first residence time according to the process according to the invention. A preferred second residence time is the second residence time according to the process according to the invention. A preferred third residence time is the third residence time according to the process according to the invention. A preferred first sizing device is a sieve which the skilled person deems being appropriate for sieving the further ground water-absorbent polymer particles. In an embodiment of the invention the device further comprises a further sizing device, wherein the further sizing device is

a) located down-stream to the residence device,

b) designed to size the first ground water-absorbent polymer particles, thereby separating the first portion of the first ground water-absorbent polymer particles from a further portion of the first ground water-absorbent polymer particles.

A preferred further sizing device is a sieve which the skilled person deems being appropriate for separating the first portion of the first ground water-absorbent polymer particles from the further portion of the first ground water-absorbent polymer particles. A preferred further sizing device comprises a screen having a mesh size in the range of from 18 to 25 U.S. mesh, preferably 20 U.S. mesh. Another preferred further sizing device comprises a further screen having a mesh size in the range of from 70 to 140 U.S. mesh, preferably 100 U.S. mesh.

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

A contribution to the solution of at least one of the above objects is provided by a water-absorbent polymer particle, obtainable by the process according to the invention. A further aspect of the present invention pertains to 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 Si0₂ 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. 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 a water-absorbent polymer particle 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 liq- uid-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 a water-absorbent polymer parti- cle 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 water-absorbent polymer particle 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. water content The water content of the water-absorbent polymer particles after drying is determined according to the Karl Fischer method.

[MODE FOR INVENTION]

EXAMPLES

The present invention is now explained in more detail by examples and drawings giv- en 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 (Ciba^® 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 μιη) in a container to achieve a mixed solution. If according to the tables 1 to 4 below a blowing agent is added, 0.1 wt.-part, based on the total amount of the mixed solution, of sodi- um carbonate are added to 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.

D) 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) as the first grinding device. The dried polymer gel spends a first residence time in said first grinding de- vice. Subsequently, the obtained parts of the dried polymer gel are kept for a second residence time in a hopper. Subsequently, the dried polymer gel parts are further ground in a roll mill as the further grinding device to obtain water-absorbing polymer particles. The dried polymer gel parts/the water-absorbent polymer particles spend a third residence time in the roll mill. The type, i.e. the number of stages of the roll mill, is given below. Values of the first, second and third residence time are given below for the specific examples and comparative examples.

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 μιη, 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 pre- cursor I.

E) Silicon dioxide treatment

In a treatment step the precursor I is mixed in a disc mixer with about 0.01 wt.-part (+- 10 %) of silicon dioxide (Si0 ), based on the total weight of the precursor I plus Si0₂. The silicon dioxide is used in form of Sipernat^® 22 obtainable from Evonik Industries AG, Essen, Germany. When mixing the precursor I with the Si0₂, the precursor still has a temperature of more than 80 °C to 100 °C, preferably of 100 °C. A precursor II is achieved.

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.-% Na₂S0₃, 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- briider Lodige Mascheninenbau GmbH, Paderborn, Germany) 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 in the form of a fluid bed, 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. 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 μπι. At least 50 wt.-% of the surface-crosslined absorbent polymer particles have a particles size in the range of from 300 to 600 μηι. Less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples according to the invention are smaller than 150 μιτι, 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 μιη. Post treated crosslinked water-absorbent polymer particles are obtained.

The following scale is used to compare the results of measuring the parameters given in tables 1 to 4 for the examples and the comparative example. In the order given in the following the measurement results are getting better from left to right:— , -, 0, +, ++, +++, ++++.

In the comparative example 1 and the examples 1 to 4 the first grinding device is a Herbold Granulator, type SML 60/145-SX7-2 by Herbold Meckesheim GmbH, Meckesheim, Germany. Furthemiore, the first residence time in said first grinding device is kept constant at 1 minute, the third residence time in the further grinding device is kept constant at 1 minute, the D50 of the first ground water-absorbent polymer particles is kept constant, and the water content of the dried polymer gel prior to the first grinding is kept constant at 5 wt.-% based on the dried polymer gel. In each case the further grinding device is a roll mill. The number of rolls and hence the number of grinding stages in the further grinding device (number of rolls divided by 2) are given in table 1. second further blowing operational time amount of fine partiresidence grinding agent of further grindcles produced by time device ing device until grinding

maintenance

comparative 30 s 4-roll none ++

example 1 mill

example 1 30 4-roll none 0 +

minutes mill

example 2 120 4-roll none ++ +

minutes mill example 3 120 4-roll sodium +++

minutes mill carbonate

example 4 120 6-roll sodium +++ +

minutes mill carbonate

* Bauermeister roll crusher, type SWR 350.1 x 1800, 3-stage crusher by Bauermeister Zerkleinerungstechnik GmbH, Norderstedt, Germany

Table 1 : Required maintenance of the further grinding device and amount of fines produced for different further grinding devices and second residence times and depending the the application of a blowing agent.

In the comparative example 1 the second residence time is 30 s, which is less than the first and the third residence time. A roll mil having 4 rolls for 2 milling stages is applied as the further grinding device. In example 1 the same roll mill is applied. However, the second residence time in the hopper is set to 30 minutes, which is more than the first and the third residence time. This results in a severely longer operational period until the further grinding device (4-roll mill) has to be stopped for maintenance. The amount of fine water-absorbent polymer particles having a particle size of less than 150 μηι being produced by the first and further grinding is slightly more than in the comparative example 1. In example 2 the second residence time is further increased to 120 minutes. This leads to a further increase of the operational time. The amount of fines produced is comparable to example 1. In addition, in example 3 a blowing agent (sodium carbonate) is used. This leads to an increase of the amount of fines produced. However, the operational time is further increased. In example 4 a 6-roll mill is used as the further grinding device. This decreases the amount of fines to the level of examples 1 and 2 while keeping the operational time at the level of example 3. Hence, example 4 shows the overall best results.

In the following X is the ratio of the D50 of the first ground water-absorbent polymer particles to the second residence time; Y is the product of the D₅₀ of the further ground water- absorbent polymer particles times the second residence time; and Z is the ratio of the sum of the first residence time and the second residence time to the water content of the dried polymer gel prior to grinding. In the following examples 5 to 13 according to the invention the second residence time is kept constant at 120 minutes, and the first residence time is kept constant at 1 minute. Moreover, first and further grinding devices according to example 2 have been used. X blowing operational required hopper volume

[μηι/minutes] agent time of further

grinding device

until maintenance

example 5 5 none ++ —

example 6 15 none ++ - example 7 20 none + +

example 8 20 sodium car++ +

bonate

Table 2: Operational time of the 4-roll mill and required hopper volume depending on the application of a blowing agent and parameter X.

In example 5 X equals 5. In a continuous process for the production of water- absorbent polymer particles the required volume of the hopper is rather large. Increasing X to 15 in example 6 leads to a decrease of the required hopper volume. The operational time until maintenance of the roll mill stays constant. In example 7 X is 20 which leads to a further decrease of the hopper volume, but also to a slight decrease of the operational time. Using a blowing agent in addition to example 7, as done in example 8, increases the operational time to the level of the examples 5 and 6.

Table 3: Operational time of the 4-roll mill depending on the application of a blowing agent and parameter Y. In table 3 it can be seen that increasing Y from 20 - 10 to 60· 10 increases the operational time until maintenance is required for the 4-roll mill. Moreover, the use of a blowing agent increase said operational time further.

Table 4: Operational time of the 4-roll mill depending on the application of a blowing agent and parameter Z.

Table 4 shows that increasing Z from 4 to 30 increases the operational time until maintenance is required for the 4-roll mill. Moreover, the use of a blowing agent increase said operational time further.

Figure 1 shows a flow chart diagram depicting the steps 101 to 1 1 1 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. In a first step 101 an aqueous monomer solution comprising at least one partially neutralized, mo- noethylenically 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 504.

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 and subsequently dried at a temperature of about 120 to 150°C. The dried polymer gel parti- cles 401 are discharged from the belt dryer and subsequently in a ninth step 109 ground to obtain water-absorbent polymer particles. Said grinding comprises subjecting the dried polymer gel particles 401 to a first grinding step in a first grinding device 402 thereby obtaining first ground water-absorbent polymer particles 403, subjecting the first ground water absor- bent polymer particles 403 to a residence in a residence device 404, and subsequently subjecting a first portion 405 of the first ground water-absorbent polyiner particles 403 to a further grinding step in a further grinding device 406, thereby obtaining further ground water- absorbent polymer particles 406. Therein, the dried polymer gel particles 401 and the first ground water- absorbent polymer particles 403 obtained from the dried polymer gel particles 401 in total spend a first residence time 407 in the first grinding device 402, the first ground water absorbent-polymer particles 403 spend a second residence time 408 in the residence device 404, and the first portion 405 of the first ground water-absorbent polymer particles 403 and the further ground water-absorbent polymer particles 406 obtained from the first ground water-absorbent polymer particles 403 in total spend a third residence time 409 in the further grinding device 405. Therein, the second residence time 408 is more than the first residence time 407, and the second residence time 408 is more than the third residence time 409. In a tenth step 1 10 the further ground water-absorbent polymer particles 406 are sized by a sizing device 507 to obtain further ground and sized water-absorbent polymer particles having a well defined particle size distribution. In an eleventh step 1 1 1 the surface of the ground and sized water-absorbent polymer particles is treated in terms of a surface crosslinking.

Figure 2 shows a flow chart diagram depicting the steps 101 to 1 1 1 of a process 100 for the preparation of water-absorbent polymer particles 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 initia- tor 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 110 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. The process 100 shown in figure 3 is the same as the process 100 in figure 1 , wherein the sec- ond step 102, the fourth step 104, and the eleventh step 1 1 1 are not part of the process 100 according to figure 3.

Figure 4 shows a flow chart diagram of a process step (ix) 109 according to the invention. A dried polymer gel 401 obtained from a belt dryer 506 is fed into a first grinding device 402 for a first grinding step. The first grinding device 402 is a Herbold Granulator (type SML 60/145-SX7-2 by Herbold Meckesheim GmbH, Meckesheim, Germany). Thereby, first ground water-absorbent polymer particles 403 are obtained from the dried polymer gel 401. Said dried polymer gel 401 and said first ground water-absorbent polymer particles 403 in total spend a first residence time 408 in the first grinding device 402. The first residence time 408 is about 3 minutes. Subsequently, the first ground water-absorbent polymer particles 403 are fed into a residence device 404 which is a hopper. The first ground water-absorbent polymer particles 403 spend a second residence time 409 of about 150 minutes in the residence device 404. Subsequently, a first portion 405 of the first ground water-absorbent polymer particles 403 is fed into a further grinding device 406 which is a Bauermeister roll crusher (type SWR 350.1 x 1800, 3-stage crusher by Bauermeister Zerkleinerungstechnik GmbH, Norder- stedt, Germany), wherein the first portion 405 of the first ground water-absorbent polymer particles 403 is further ground to obtain further ground water-absorbent polymer particles 407 having a desired particle size distribution. The first portion 405 of the first ground water- absorbent polymer particles 403 and the further ground water-absorbent polymer particles 407 in total spend a third residence time 410 of about 3 minutes in the further residence device 406.

Figure 5 shows a flow chart diagram of another process step (ix) 109 according to the invention. The process step (ix) 109 is the same as in figure 4, except that subsequently to the residence of the first ground water-absorbent polymer particles 403, said first ground water- absorbent polymer particles 403 are fed into a further sizing device 501 which is a sieve. By sieving the first ground water-absorbent polymer particles 403 the first portion 405 of the first ground water-absorbent polymer particles 403 is separated from a further portion 502 of the first ground water-absorbent polymer particles 403. The further portion 502 is subsequently fed to a further process steps, such as process step (x) or (xi) or both. The further portion 502 of the first ground water- absorbent polymer particles 403 is not subjected to the further grinding step in the further grinding device 406. The first portion 405 of the first ground water- absorbent polymer particles 403 is fed into a further grinding device 406 as described for figure 4.

Figure 6 shows a block diagram of a device 600 for the preparation of water-absorbent polymer particles according to the invention. The arrows show a direction of a process stream 608 of the preparation of water-absorbent polymer particles. The device 600 comprises a first container 601, a further container 602, downstream a mixing device 603, downstream a polymerization belt reactor as a polymerization reactor 604, downstream a comminuting device 605, downstream a belt dryer 606, downstream a first grinding device 402, downstream a residence device 404, downstream a further grinding device 406, and downstream a first sizing device 607, each according to the invention.

Figure 7 shows a block diagram of another device 600 for the preparation of water- absorbent polymer particles according to the invention. The arrows show a direction of a process stream 608 of the preparation of water-absorbent polymer particles. The device 600 comprises a first container 601, a further container 602, downstream a mixing device 603, downstream a polymerization belt reactor as a polymerization reactor 604, downstream a comminuting device 605, downstream a belt dryer 606, downstream a first grinding device 402, downstream a residence device 404, downstream a further sizing device 501, downstream a further grinding device 406, and downstream a first sizing device 607, each according to the invention.

While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

[CLAIMS]

[Claim 1 ]

A process (100) for the preparation of 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 ( 3);

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

(v) charging the aqueous monomer solution into a polymerization reactor (504);

(vi) polymerizing the monomers in the aqueous monomer solution in the polymerization reactor (504);

(vii) discharging the polymer gel out of the polymerization reactor (504) and optionally comminuting the polymer gel;

(viii) drying the polymer gel;

(ix) subjecting

a) the dried polymer gel (401) to a first grinding step in a first grinding device (402) thereby obtaining first ground water-absorbent polymer particles (403), b) the first ground water absorbent polymer particles (403) to a residence in a residence device (404), and

c) subsequently at least a first portion (405) of the first ground water-absorbent polymer particles (403) to a further grinding step in a further grinding device

(406) , thereby obtaining further ground water-absorbent polymer particles

(407) ;

(x) sizing the further ground water-absorbent polymer particles (407); and

(xi) optionally treating the surface of the further ground and sized water-absorbent polymer particles; wherein in process step (ix) the dried polymer gel (401) and the first ground water- absorbent polymer particles (403) obtained from the dried polymer gel (401) in total spend a first residence time (408) in the first grinding device (402);

wherein in process step (ix) the first ground water absorbent-polymer particles (403) spend a second residence time (409) in the residence device (404);

wherein in process step (ix) the at least first portion (405) of the first ground water- absorbent polymer particles (403) and the further ground water- absorbent polymer particles (407) obtained from the at least first portion (405) of the first ground water- absorbent polymer particles (403) in total spend a third residence time (410) in the further grinding device (406);

wherein the second residence time (409) is more than the first residence time (408); wherein the second residence time (409) is more than the third residence time (410).

[Claim 2]

The process (100) according to claim 1, wherein the second residence time (409) is in the range of from 120 to 180 minutes.

[Claim 3]

The process (100) according to claim 1 or 2, wherein a product of a D₅₀ of a particle diameter of the further ground water-absorbent polymer particles (407) times the second residence time (409) is in the range of from 40 · 10³ to 100 · 10³ μην minutes.

[Claim 4]

The process (100) according to any of the preceding claims, wherein a ratio of a sum of the first residence time (408) and the second residence time (409) to a water content of the dried polymer gel (401) prior to process step (ix) is in the range of from 10 to 100

minutes wt.-%^"'.

[Claim 5]

The process (100) according to any of the preceding claims, wherein a ratio of a D₅₀ of a particle diameter of the first ground water-absorbent polymer particles (403) to the second residence time (409) is in the range of from 10 to 40 μηι/minutes. [Claim 6]

The process (100) according to any of the preceding claims, wherein in process step (ix) subsequently to subjecting the first ground water- absorbent polymer particles (403) to the residence in the residence device (404) and prior to subjecting at least the first portion (405) of the first ground water-absorbent polymer particles (403) to the further grinding step in the further grinding device (406), the first ground water- absorbent polymer particles (403) are sized, thereby separating the first portion (405) of the first ground water-absorbent polymer particles (403) from a further portion (502) of the first ground water-absorbent polymer parti- cles (403);

wherein the further portion (502) of the first ground water-absorbent polymer particles (403) is not subjected to the further grinding step in the further grinding device (406). [Claim 7]

The process (100) according to any of the preceding claims, wherein the first grinding device (402) is an impact mill.

[Claim 8] The process (100) according to any of claims 1 to 6, wherein the first grinding device

(402) comprises at least one rotating knife and at least one static knife.

[Claim 9]

The process (100) according to any of the preceding claims, wherein the residence de- vice (404) is a hopper.

[Claim 10]

The process (100) according to any of the preceding claims, wherein the further grinding device (406) is a roll mill.

[Claim 1 1 ] The process (100) according to any of the preceding claims, wherein the polymer gel being discharged in process step (vii) comprises water in the range of from 40 to 60 wt.-%, based on the polymer gel.

[Claim 12]

The process (100) according to any of the preceding claims, wherein 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.

[Claim 13]

wherein the polymer gel sheet is characterized by a width in the range of from 30 to 300 cm.

[Claim 14]

The process (100) according to any of the preceding claims, wherein the polymerization in step (vi) is performed in presence of a blowing agent.

[Claim 15]

A device (600) for the preparation of water-absorbent polymer particles in a process stream (608), comprising

a) a first container (601), 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 (602), designed to take at least one crosslinker (a3);

c) a mixing device (603), wherein the mixing device (603) is

i) located down-stream to the first container (601 ) and the further container (602),

ii) designed to mix the monomer solution and the at least one crosslinker (<x3); b) a polymerization reactor (604), wherein the polymerization reactor (604) is i) located down-stream to the first container (601) and the further container (602),

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;

d) a comminuting device (605), wherein the comminuting device (605) is

i) located down-stream to the first container (601) and the further container (602),

ii) designed to comminute the polymer gel, thereby obtaining polymer gel particles,

e) a belt dryer (606), wherein the belt dryer (606) is

i) located down-stream to the comminuting device (605),

ii) designed to dry the polymer gel particles,

f) a first grinding device (402), wherein the first grinding device (402) is

i) located down-stream to the belt dryer (606),

ii) designed to subject the dried polymer gel particles (401) to a first grinding step, thereby obtaining first ground water-absorbent polymer particles (403),

iii) designed to take the dried polymer gel particles (401 ) and the first ground water-absorbent polymer particles (403) obtained therefrom for a first residence time (408);

g) a residence device (404), wherein the residence device (404) is

i) located down-stream to the first grinding device (402),

ii) designed to take the first ground water-absorbent polymer particles (403) for a second residence time (409);

h) a further grinding device (406), wherein the further grinding device (406) is

i) located down-stream to the residence device (404),

ii) designed to subject at least a first portion (405) of the first ground water-absorbent polymer particles (403) to a further grinding step, thereby obtaining further ground water-absorbent polymer particles (407);

iii) designed to take at least the first portion (405) of the first ground water- absorbent polymer particles (403) and the further ground water- absorbent polymer particles (407) obtained therefrom for a third residence time (410);

j) a first sizing device (607), wherein the first sizing device (607) is

i) located down-stream to the further grinding device (406), ii) designed to size the further ground water-absorbent polymer particles

(407);

wherein the second residence time (409) is more than the first residence time (408); wherein the second residence time (409) is more than the third residence time (410). [Claim 16]

The device (600) according to claim 15, wherein the device (600) further comprises a further sizing device (501), wherein the further sizing device (501) is

a) located down-stream to the residence device (404),

b) designed to size the first ground water- absorbent polymer particles (403), thereby separating the first portion (405) of the first ground water- absorbent polymer particles (403) from a further portion (502) of the first ground water-absorbent polymer particles (403).

[Claim 17] A process for the preparation of water-absorbent polymer particles in the device (600) according to claim 15 or 16.

[Claim 18]

A water-absorbent polymer particle, obtainable by the process (100) according to any of claims 1 to 14, or 17.

[Claim 19]

A composite material comprising a water-absorbent polymer particle according to claim 18.

[Claim 20] The composite material according to claim 19, comprising 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 there- of.

[Claim 21 ]

A process for the production of a composite material, wherein a water-absorbent polymer particle according to claim 18 and a substrate and optionally an auxiliary substance are brought into contact with one another.

[Claim 22]

A composite material obtainable by a process according to claim 21. [Claim 23]

A use of the water-absorbent polymer particle according to claim 18 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.