WO2015163521A1

WO2015163521A1 - Longitudinal shape of a polymerization belt in the production of water-absorbent polymer particles

Info

Publication number: WO2015163521A1
Application number: PCT/KR2014/003679
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: CN106232505B; EA201691517A1; KR102223612B1; EA036072B1; KR20160149236A; CN106232505A

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 onto a belt of a polymerization belt reactor; (vi) polymerizing the monomers in the aqueous monomer solution on the belt, thereby obtaining a polymer gel; (vii) discharging the polymer gel out of the polymerization belt reactor and optionally comminuting the polymer gel; (viii) drying the optionally comminuted polymer gel; (ix) grinding the dried polymer gel thereby obtaining water-absorbent polymer particles; (x) sizing the grinded water-absorbent polymer particles; and (xi) optionally treating the surface of the grinded and sized water-absorbent polymer particles; wherein the belt is shaped as a trough extending longitudinally over at least 30 % of a longitudinal extension of the belt; wherein the trough comprises a first part and a further part.

Description

[DESCRIPTION]

[Invention Title]

LONGITUDINAL SHAPE OF A POLYMERIZATION BELT 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 hydrogels, and to retain such fluids under a certain pressure. By virtue of those characteristic properties, such polymers are chiefly used for incorporation into sanitary articles, such as, for example, baby's nappies/diapers, incontinence products or sanitary towels.

The preparation of superabsorbers is generally carried out by free-radical polymerisation of acid-group-carrying monomers in the presence of crosslinkers, it being possible for poly- mers having different absorber properties to be prepared by the choice of the monomer composition, the crosslinkers and the polymerisation conditions and of the processing conditions for the hydrogel obtained after the polymerisation (for details see, for example, Modem 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 par- tides 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 crosslinkers 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 .

Especially continuous processes of polymerization are commonly performed in a polymerization reactor which is a polymerization belt reactor. A common polymerization belt reactor comprises a conveyor belt being designed to carry an aqueous monomer solution which comprises the acid-group-carrying monomers during polymerization.

A conveyor belt of a polymerization reactor of the prior art comprises a trough to keep the aqueous monomer solution from flowing off the conveyor belt. A shape of said trough in the prior art is such that in a region of the belt where fluid components are present the trough has a longitudinally constant depth in order to keep the fluid components from flow- ing off.

[Disclosure]

[Technical Problem] However, it has been found that having a too deep trough in a polymerization region of the belt is disadvantageous. Moreover, in the prior art a transversal shape of said trough is such that a capacity of the trough for keeping monomer solution from flowing off the belt is high. [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 water-absorbent polymer particles.

A further object is to provide a process for the preparation of water- absorbent polymer par- tides using a polymerization belt reactor, characterized by a favorable balance of the criteria: a suitable fraction of water-soluble polymers in the polymer gel or the water absorbent polymers or both; a belt shape which keeps a suitable amount of aqueous monomer solution from flowing off the belt; long lifetime of a comminuting device for comminuting the polymer gel. A further object is to provide a process for the preparation of water-absorbent polymer particles using a polymerization belt reactor, characterized by a favorable balance of the criteria: low content of residual monomers in the water-absorbent polymer particles, low content of water-soluble polymer in the water-absorbent polymer particles, high capacity of the polymerization belt reactor for monomer solution. It is a further object of the present invention to provide a water-absorbent polymer particle produced by a process hav- ing at least one of the above advantages, wherein the water-absorbent polymer particle shows no reduction of quality. It is a further object of the present invention to provide a composite material comprising a water-absorbent polymer particle produced by a process having at least one of the above advantages, wherein the composite material shows no reduction of quality. A further object is to provide superabsorbent polymer particles which have been produced by a less expensive process. It is a further object of the present invention to provide a composite material comprising a water-absorbent polymer particle produced by a process having at least one of the above advantages, wherein the composite material shows no reduction of quality. It is a further object of the present invention to provide a device for producing water-absorbent polymer particles by a process having at least one of the above advantages. A contribution to the solution of at least one of the above objects is given by the independent claims. The dependent claims provide preferred embodiments of the present invention which also serve solving at least one of the above mentioned objects.

[Advantageous Effects]

Surprisingly, performing the invention it has been found that choosing a specific longitudinal and/or transversal shape of the trough results in a better quality of the water-absorbent polymer particles produced by the overall process while still a sufficient capacity of the trough is maintained.

[Description of Drawings]

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 scheme of a belt shape in a longitudinal cross section according to the invention;

Figure 5 a scheme of a belt shape in a first transversal cross section according to the invention;

Figure 6 a scheme of another belt shape in a first transversal cross section according to the invention;

Figure 7 a scheme of a belt shape in a transversal cross section not according to the invention;

Figure 8 a scheme of another belt shape in a transversal cross section not according to the invention;

Figure 9 a scheme of a basic setup of a polymerization belt reactor according to the invention;

Figure 10 a block diagram of a device for the preparation of water-absorbent polymer particles according to the invention; Figure 1 1 a scheme of a comminuting device according to the invention;

Figure 12a) a scheme of another comminuting device according to the invention in an external view; and

Figure 12b) a scheme of inner parts the comminuting device of figure 12a) in an exploded view.

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)

400 polymerization belt reactor

401 belt

402 trough

403 longitudinally extension of the trough

404 longitudinal extension of the belt

405 first part of the trough

406 further part of the trough

407 first depth

408 electromagnetic waves capable of activating the polymerization initiator or the polymerization initiator system or both

409 mercury vapour lamps 410 guide roller

411 upper run of the belt

412 downstream direction / longitudinal direction

413 upstream direction / longitudinal direction

5 500 first transversal cross section

501 first concave curve

502 horizontal width of the first concave curve

503 a vertical height of the first concave curve

504 horizontally aligned portion

I o 505 tangent

506 horizontal line in the transversal cross section

507 first curve angle

508 trough section area

700 transversal cross section

15 701 concave curve

702 horizontal width

703 a vertical height

704 tangent

705 horizontal line

20 706 maximum angle

901 direction of movement of the polymer gel on the belt

902 counter direction to a direction of movement of the polymer gel on the belt

903 longitudinal directions

904 transversal directions

25 404 longitudinal extension of the belt

1000 device for the preparation of water-absorbent polymer particles

1001 first container

1002 further container

1003 mixing device

30 1004 comminuting device 1005 belt dryer

1006 grinding device

1007 sizing device

1008 process stream

1100 crusher

1101 axis of rotation

1102 toothed wheel

1103 polymer gel particles

1200 mincer

1201 static hole plate

1202 screw

1203 feed unit

1204 rotating hole plate

1205 CIRCULAR CUTTING EDGE

[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 onto a belt of a polymerization belt reactor;

(vi) polymerizing the monomers in the aqueous monomer solution on the belt, thereby obtaining a polymer gel; (νϋ) discharging the polymer gel out of the polymerization belt reactor and optionally comminuting the polymer gel;

(viii) drying the optionally comminuted polymer gel;

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

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

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

wherein the belt is shaped as a trough extending longitudinally over at least 30 %, prefera- bly at least 35 %, more preferably at least 40 %, of a longitudinal extension of the belt; wherein the trough comprises a first part of the trough and a further part of the trough; wherein at least a part of the first part of the trough is irradiated by electromagnetic waves (405) capable of activating the polymerization initiator or the polymerization initiator system or both; wherein a first depth of the first part of the trough is more than a further depth of the further part of the trough; wherein the first depth refers to a maximum depth in a transversal direction of the belt, at each longitudinal position of the first part of the trough; wherein the further depth refers to a maximum depth in the transversal direction of the belt, at each longitudinal position of the further part of the trough. 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).

If the belt is an endless conveyor, the first longitudinal section is preferably a longitudinal section of an upper run of the endless conveyor belt. If the belt is an endless conveyor, the further longitudinal section is preferably a longitudinal section of an upper run of the endless conveyor belt. Preferred electromagnetic waves capable of activating the polymerization initiator or the polymerization initiator system or both are electromagnetic waves in the ultraviolet range. A preferred ultraviolet range extends from 10 to 400 nm, preferably from 100 to 400 nm, more preferably from 100 to 350 nm. Preferably, the first longitudinal sec- tion is irradiated by the electromagnetic waves in an intensity which is high enough to activate a polymerization initiator or a polymerization initiator system or both on the first longitudinal section of the belt. Preferably, the further longitudinal section is irradiated by the electromagnetic waves in an intensity which is high enough to activate a polymerization initiator or a polymerization initiator system or both on the further longitudinal section of the belt. If the belt is an endless conveyor, each maximum vertical extension of the belt is preferably a maximum vertical extension of an upper run of the endless conveyor belt. A preferred further longitudinal section follows the first longitudinal section directly in a downstream direction of the belt.

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 belt reactor, preferably onto the belt of the polymerization belt reactor. The polymer gel obtained is continuously discharged out of the polymerization belt reactor and is continuously optionally comminuted, dried, grinded and sized in the subsequent process steps. This continuous process may, however, be interrupted in order to, for example, to substitute certain parts of the process equipment, like the belt material of the polymerization belt 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, preferably 20 to 2,000 μηι 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 μηι to be at least 30 wt.-%, particularly preferably at least 40 w - % and most preferably at least 50 wt.-%, based on the total weight of the water-absorbent polymer particles.

In process step (i) of the process according to the present invention an aqueous monomer solution containing at least one partially neutralized, monoethylenically unsaturated mono* mer bearing carboxylic acid groups ( l) 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 par- ticularly preferred.

It is preferred according to the present invention that the water-absorbent polymer produced by the process according to the invention comprises monomers bearing carboxylic acid groups to at least 50 wt.-%, preferably to at least 70 wt.-% and further preferably to at least 90 wt.-%, based on the dry weight. It is particularly preferred according to the invention, that the water-absorbent polymer produced by the process according to the invention is formed from at least 50 wt.-%, preferably at least 70 wt.-% of acrylic acid, which is preferably neutralized to at least 20 mol-%, particularly preferably to at least 50 mol-%. The concentration of the partially neutralized, monoethylenically unsaturated monomers bearing carboxylic acid groups (al) in the aqueous monomer solution that is provided in process step (i) is preferably in the range between 10 to 60 wt.-%, preferably 30 to 55 wt.-% and most preferably between 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 acryla- mide 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 ( l) 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 polymerisation 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 polymerisation 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, IST -methylene bisacrylamide, polyethylene- glycol di(meth)acrylates, triallylmethylammonium chloride, tetraallylammonium chloride and allylnonaethyleneglycol acrylate produced with 9 mol ethylene oxide per mol acrylic acid are particularly preferred, wherein N, 1ST -methylene bisacrylamide is even more preferred, and as compounds of crosslinker class IV, Al₂ (S0₄)₃ and its hydrates are particularly preferred.

Preferred water-absorbent polymers produced by the process according to the invention are polymers which are crosslinked by crosslinkers of the following crosslinker classes or by crosslinkers of the following combinations of crosslinker classes respectively: I, II, III, IV, I II, I III, I IV, I II III, I II IV, I III IV, II III IV, II IV or III IV.

Further preferred water-absorbent polymers produced by the process according to the inven- tion 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 bis- acrylamide, polyethyleneglycol di(meth)acrylates, triallyl-methylammonium chloride, tetraallylammonium chloride and allylnonaethylene-glycol acrylate produced from 9 mol ethylene oxide per mol acrylic acid are particularly preferred as crosslinkers of crosslinker class I, wherein N, N'-methylene bisacrylamide is even more preferred.

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

The aqueous monomer solution can furthermore also comprise auxiliary substances (a5), these auxiliary substances including, in particular, complexing agents, such as, for example, EDTA. The relative amount of monomers (al) and (a2) and of crosslinking agents (a3) and water- soluble polymers (cc4) and auxiliary substances (a5) in the aqueous monomer solution is preferably chosen such that the water-absorbent polymer structure obtained after drying the optionally comminuted polymer gel is based to the extent of 20 to 99.999 wt.-%, preferably to the extent of 55 to 98.99 wt.-% and particularly preferably to the extent of 70 to 98.79 wt.-% on monomers (al), to the extent of 0 to 80 wt.-%, preferably to the extent of 0 to 44.99 wt.-% and 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 (a4), to the extent of 0 to 20 wt.-%, preferably to the extent of 0 to 10 wt.-% and particularly preferably to the extent of 0.1 to 8 wt.-% on the auxiliary substances (a5), and to the extent of 0.5 to 25 wt.-%, preferably to the extent of 1 to 10 wt.-% and particularly preferably to the extent of 3 to 7 wt.-% on water (a6) the sum of the amounts by weight (al) to (a6) being 100 wt.-%.

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

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

Water-absorbent fine particles are preferably water-absorbent polymer particles the compo- sition 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 μη , 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 mono- mer 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 5 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°. i o 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,

15 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 Kinematika AG, Luttau, Switzerland, for example under the trademark 0 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 5 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.

0 As polymerization initiators for initiation of the polymerisation all initiators forming radicals under the polymerisation 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 irradiation with energetic electromagnetic waves, preferably UV radiation. The polymerisation initiators may be dissolved or dispersed in the aqueous monomer solution. The use of water-soluble catalysts is preferred.

As thermal initiators may be used all compounds known to the person skilled in the art that decompose under the effect of an increased temperature to form radicals. Particularly preferred are thermal polymerisation initiators with a half life of less than 10 seconds, more preferably less than 5 seconds at less than 180°C, more preferably at less than 140°C. Peroxides, hydroperoxides, hydrogen peroxide, persulfates and azo compounds are particularly preferred thermal polymerization initiators. In some cases it is advantageous to use mixtures of various thermal polymerization initiators. Among such mixtures, those consisting of hydrogen peroxide and sodium or potassium peroxodisulfate are preferred, which may be used in any desired quantitative ratio. Suitable organic peroxides are preferably acetylacetone peroxide, methyl ethyl ketone peroxide, benzoyl peroxide, lauroyl peroxide, acetyl peroxide, capryl peroxide, isopropyl peroxidicarbonate,2-ethylhexyle peroxidicarbonate, tert.-butyl hydroperoxide, cumene hydroperoxide, and peroxides of tert.- amyl perpivalate, tert.-butyl perpivalate, tert.-butyl perneohexonate, tert.-butyl isobutyrate, tert.-butyl per-2-ethylhexenoate, tert.-butyl perisononanoate, tert.-butyl permaleate, tert.- butyl perbenzoate, tert.-butyl-3,5,5-trimethylhexanoate and amyl perneodecanoate. Furthermore, the following thermal polymerisation initiators are preferred: azo compounds such as azo-bis-isobutyronitril, azo-bis-dimethylvaleronitril, azo-bis-ami-dinopropane dihydrochloride, 2,2'-azobis-(N,N-dimethylene)isobutyramidine di-hydrochloride, 2- (carbamoylazo)isobutyronitrile and 4,4'-azobis-(4-cyano-valeric acid). The aforementioned compounds are used in conventional amounts, preferably in a range from 0.01 to 5 mol-%, more preferably 0.1 to 2 mol-%, respectively based on the amount of the monomers to be polymerized. Redox catalysts comprise two or more components, usually one or more of the peroxo compounds listed above, and at least one reducing component, preferably ascorbic acid, glucose, sorbose, mannose, ammonium or alkali metal hydrogen sulfite, sulfate, thiosulfate, hyposulfite or sulfide, metal salts such as iron II ions or silver ions or sodium hydroxymethyl sulfoxylate. Preferably ascorbic acid or sodium pyrosulfite is used as reducing component of the redox catalyst. 1 ^χ 10^"5 to 1 mol-% of the reducing component of the redox catalyst and 1 ^χ 10^"5 to 5 mol-% of the oxidizing component of the redox catalyst are used, in each case referred to the amount of monomers used in the polymerization. Instead of the oxidizing 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 benzophe- none derivatives such as Michlers ketone, phenanthrene derivatives, fluorine derivatives, anthraquinone derivatives, thioxanthone derivatives, cumarin derivatives, benzoinether and derivatives thereof, azo compounds such as the above-mentioned radical formers, substituted hexaarylbisimidazoles or acylphosphine oxides. Examples of azides are: 2-(N,N- dimethylamino)ethyl-4-azidocinnamate, 2-(N,N-dimethylamino)ethyl-4- azidonaphthylketone, 2-(N,N-di-methylamino)ethyl-4-azidobenzoate, 5-azido-l-naphthyl- 2'-(N,N-dimethylami-no)ethylsulfone, N-(4-sulfonylazidophenyl)maleinimide, N-acetyl-4- sulfonyl-azidoaniline, 4-sulfonylazidoaniline, 4-azidoaniline, 4-azidophenacyl bromide, p- azidobenzoic acid, 2,6-bis(p-azidobenzylidene)cyclohexanone and 2,6-bis(p- azidobenzylidene)-4-methylcyclohexanone. A further group of photo-initiators are di- alkoxy ketales such as 2,2-dimethoxy-l,2-diphenylethan-l-one. The photo-initiators, when used, are generally employed in quantities from 0.0001 to 5 wt.-% based on the monomers to be polymerized.

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

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

wherein the initiator comprises the peroxodisulfate and the organic initiator molecule in a molar ratio in the range of from 20: 1 to 50: 1. In one aspect of this embodiment it is preferred that the concentration of the initiator component iiia. is in the range from 0.05 to 2 wt.-%, based on the amount of monomers to be polymerized. In another aspect of this embodiment it is preferred that the organic initiator molecule is selected from the group consisting of 2,2-dimethoxy-l ,2-diphenylethan-l -one, 2,2-azobis-(2- amidinopropane)dihydrochloride, 2,2-azobis-(cyano valeric acid) or a combination of at least two thereof. In a further aspect of this embodiment it is preferred that the peroxodisulfate is of the general formula M₂S₂0₈, 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 content of the aqueous monomer solution is decreased after the fine particles have been added in process step (ii).

Whenever the oxygen content of the aqueous monomer solution is decreased, this may be realized by bringing the aqueous monomer solution into contact with an inert gas, such as nitrogen. The phase of the inert gas being in contact with the aqueous monomer solution is free of oxygen and is thus characterized by a very low oxygen partial pressure. As a conse- quence 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 mix- ing can be achieved, for example, with nozzles, static or dynamic mixers or bubble columns. The oxygen content of the monomer solution before the polymerization is preferably lowered to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, based on the monomer solution. In process step (v) of the process according to the present invention the aqueous monomer solution is charged onto the belt of the polymerization belt reactor. Preferably, the aqueous monomer solution is charged onto the belt at an upstream position of the belt. A preferred belt 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 polymerization belt reactor 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 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 that is arranged in the vicinity of the guide roller for the return run of the conveyor belt. A preferred irradiating means irradiates UV-light, preferably comprises a UV-lamp.

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 may be 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 becomes 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 that is obtained on the belt is discharged from the belt. Preferably, the polymer gel is removed from the belt as a continuous strand that is of a soft semi-solid consistency and is then passed on for further processing optionally comprising comminuting. By comminuting the polymer gel polymer gel particles are obtained.

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

The size and design of the 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, mul- ti-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, cer- tain surface enhancements, such as electropolishing or Teflonizing, are possible.

The polymer gel to be dried is 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 to be dried is preferably not less than 2 cm, more preferably not less than 5 cm and most preferably not less than 8 cm and preferably not more than 20 cm, more preferably not more than 15 cm and most preferably not more than 12 cm. The belt speed of the belt dryer is preferably not less than 0.005 m/s, more preferably not less than 0.01 m/s and most preferably not less than 0.015 m/s and preferably up to 0.05 m/s, more preferably up to 0.03 m/s and most preferably up to 0.025 m/s. Furthermore, it is preferable according to the invention that the polymer gel is dried to a water content in the range of from 0.5 to 25 wt.-%, preferably from 1 to 10 wt.-% and particularly preferably from 3 to 7 wt.-%, based on the dried polymer gel.

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

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

In process step (x) of the process according to the present invention the ground water- absorbent polymer particles are sized, 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 μηι 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 μηι is also less than 10 wt.-%, preferably less than 8 wt.-% and particularly preferably less than 6 wt.-%, each based on the total weight of the water-absorbent polymer particles. 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.-%, based on the total weight of the water-absorbent polymer particles, of the water-absorbent polymer 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 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 treat- ments 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 parti- cles 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 first depth is in the range of from 100 to 500 mm, preferably from 100 to 480 mm, more preferably from 140 to 460 mm, more preferably from 100 to 440 mm, more preferably from 100 to 420, more preferably from 100 to 400 mm, more preferably from 100 to 380 mm, more preferably from 100 to 360 mm, more preferably from 120 to 340 mm, more preferably from 140 to 320 mm, more preferably from 160 to 300 mm, more preferably from 180 to 280 mm, more preferably from 200 to 260 mm, more preferably from 220 to 260 mm, more preferably from 240 to 260 mm, most preferably from 245 to 255 mm.

In an embodiment of the invention the further depth is in the range of from 5 to 100 mm, preferably from 5 to 90 mm, more preferably from 5 to 80 mm, more preferably from 5 to 70 mm, more preferably from 5 to 60 mm, more preferably from 5 to 50 mm, more preferably from 5 to 40 mm, more preferably from 5 to 30 mm, more preferably from 5 to 20 mm, most preferably from 5 to 10 mm.

In an embodiment of the invention the first part of the trough longitudinally extends over a length in the range of from 25 to 45 %, preferably from 25 to 40 %, more preferably from 25 to 35 %, most preferably from 28 to 33 %, of the longitudinal extension of the belt.

In an embodiment of the invention the further part of the trough longitudinally extends over a length in the range of from 3 to 70 %, preferably from 3 to 60 %, more preferably from 3 to 50 %, more preferably from 3 to 40 %, more preferably from 5 to 30 %, more preferably from 5 to 20 %, most perefrably from 10 to 20 %, of the longitudinal extension of the belt.

In an embodiment of the invention the further part of the trough directly follows the first part of the trough in a downstream longitudinal direction of the belt.

In an embodiment of the invention in a first transversal cross section of the belt the belt has a shape of a first concave curve; wherein R is in the range of from 2 to 30, preferably from 2.5 to 25, more preferably from 3 to 25, more preferably from 3.5 to 25, more preferably from 4 to 25, more preferably from 4.5 to 25, more preferably from 5 to 25, more preferably from 5.5 to 25, more preferably from 6 to 20, most preferably from 6.5 to 20; wherein R is a ratio of a horizontal width of the first concave curve to a vertical height of the first concave curve. For the use throughout this document a concave curve is a curved line comprising a point, wherein going away from that point along the line includes going monotonically upwards. Going monotonically upwards means, going upwards or keeping a constant level of height. Going monotonically upwards excludes any downward movement. This means that the concave curve is curved away from the belt of the polymerization belt reactor. A preferred first transversal cross section of the belt is a transversal cross section of an upper run the belt.

In an embodiment of the invention the polymerizing in step (vi) is initiated by an irradiation, preferably of the aqueous monomer solution, with UV radiation. For the use throughout this document UV radiation is electromagnetic radiation having a wavelength in the range of from 100 to 380 nm.

In an embodiment of the invention a longitudinal position of the first transversal cross section can be every longitudinal position within a first longitudinally extending portion of the belt; wherein the first longitudinally extending portion of the belt extends longitudinally over 25 to 40 %, preferably over 25 to 38 %, more preferably over 25 to 36 %, more pref- erably over 25 to 34 %, more preferably over 25 to 32 %, more preferably over 25 to 30 %, most preferably over 25 to 28 %, of a longitudinal extension of the belt. Preferably, the longitudinally extending portion is a longitudinally connected portion. In an embodiment of the invention the first concave curve comprises a horizontally aligned portion; wherein the horizontally aligned portion extends over at least 70 %, preferably at least 75 %, more preferably at least 80 %, more preferably at least 85 %, most preferably at least 90 %, of the horizontal width of the first concave curve. In an embodiment of the invention a tangent on any position of the first concave curve inclines a first curve angle with a horizontal line in the first transversal cross section; wherein a maximum of the first curve angle (407) is in the range of from 45 to 80°, preferably from 50 to 75°, more preferably from 55 to 75°, more preferably from 60 to 75°, more preferably from 65 to 75°, most preferably from 68 to 72°. Preferably the tangent is a tangent on an endpoint of the first concave curve. A preferred first curve angle is an external angle with respect to the first concave curve.

In an embodiment of the invention downstream to the first longitudinally extending portion of the belt the belt comprises a further longitudinally extending portion; wherein in a further transversal cross section of the belt the belt has a shape of a further concave curve; wherein a longitudinal position of the further transversal cross section can be every longitudinal position within the further longitudinally extending portion of the belt; wherein a tangent on any position of the further concave curve inclines a further curve angle with a horizontal line in the further transversal cross section; wherein a maximum of the further curve angle is in the range of from 0 to 45°, preferably from 0 to 40°, more preferably from 0 to 35°, more preferably from 0 to 30°, more preferably from 0 to 25°, more preferably from 0 to 20°, more preferably from 0 tol 5°, most preferably from 0 to 10°. A preferred further longitudinally extending portion follows the first longitudinally extending portion directly in a downstream direction. A preferred further curve angle is an external angle with respect to the further concave curve. A preferred further transversal cross section of the belt is a transversal cross section of an upper run the belt.

In an embodiment of the invention wherein the further longitudinally extending portion of the belt extends longitudinally over 30 % or less, preferably over 25 % or less, more preferably over 20 % or less, most preferably over 15 % or less, of the longitudinal extension of the belt.

In an embodiment of the invention the first concave curve forms a trough section; wherein the trough section has a trough section area; wherein the trough section area is trapezoidally shaped. Therein, the trough section is preferably formed by the belt, preferably by a horizontally aligned portion of the belt and by sidewalls. A preferred trapezoidal shape comprises rounded corners. Preferably, the trough section is a section of the trough. Therein, the trough is the trough which comprises the first part and the further part of the trough. Pref- erably, the trough section is a section of the first part of the trough. Therein, a trough section is a longitudinally limited part of the trough.

In an embodiment of the invention the first concave curve forms a trough section; wherein the trough section has a trough section area; wherein the trough section area has an area of in the range of from 1 ,000 to 3,000 cm², preferably from 1,200 to 2,800 cm², more preferably from 1,500 to 2,500 cm².

In an embodiment of the invention the first concave curve has a vertical height of 500 mm or less, preferably of 450 mm or less, more preferably of 400 mm or less, more preferably of 350 mm or less, more preferably of 300 mm or less, more preferably 250 mm or less, more preferably 200 mm or less, most preferably of 150 mm or less.

In an embodiment of the invention the polymer gel being discharged in process step (vii) comprises water in the range of from 40 to 60 wt.-%, preferably from 50 to 60 wt.-%, more preferably from 53 to 56 wt.-%, based on the polymer gel. In an embodiment of the invention the polymer gel being discharged in process step (vii) is a polymer gel sheet; wherein the polymer gel sheet is characterized by a thickness in the range of from 10 to 200 mm, preferably from 10 to 100 mm, more preferably from 15 to 75 mm, most preferably from 15 to 50 mm.

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

In an embodiment of the invention the polymerization in step (vi) is performed in presence of a blowing agent. The blowing agent may be added to the aqueous monomer solution in one selected from the group consisting of step (i), step (ii), step (iii), step (iv), step (v), and step (vi), or in a combination of at least two thereof. Preferably, the blowing agent is 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 endothermic. A preferred endothermic foaming process is started by heat from an exothermic polymerisation or crosslinking or both reaction. A preferred blowing agent is a physical blowing agent or a chemical blowing agent or both. A preferred physical blowing agent is one selected from the group consisting of a CFC, a HCFC, a hydrocarbon, and 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 cyclopentane, or a combination of at least two thereof. A preferred chemical blowing agent is one selected from the group consisting of a carbonate blowing agent, a nitrite, a peroxide, calcined soda, an oxalic acid derivative, an aromatic azo compound, a hydrazine, an azide, a Ν,Ν'- Dinitro- soamide, 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, C0₂ as a solid, ethylene carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, magnesium carbonate, or magnesium hydroxic carbonate, calcium carbonate, barium carbonate, a bicar- bonate, 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 car- bonate 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, ary- lazosulfones, 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-NjN'-dinitrosoterephthalamide.

A contribution to the solution of at least one of the above objects is provided by 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 ( l);

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);

d) a polymerization belt reactor, wherein the polymerization belt reactor

i) is located down-stream to the mixing device,

ii) is 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 iii) comprises a belt;

e) a comminuting device, wherein the comminuting device is

i) located down-stream to the polymerization belt reactor, ii) designed to comminute the polymer gel, thereby obtaining polymer gel particles,

f) a belt dryer, wherein the belt dryer is

i) located down-stream to the comminuting device,

ii) designed to dry the polymer gel particles, g) a grinding device, wherein the grinding device is

i) located down-stream to the belt dryer,

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

h) a sizing device, wherein the sizing device is

i) located down-stream to the grinding device,

ii) designed to size the grinded water-absorbent polymer particles;

wherein the belt is shaped as a trough extending longitudinally over at least 30 %, preferably at least 35 %, more preferably at least 40 %, of a longitudinal extension of the belt; wherein the trough comprises a first part of the trough and a further part of the trough; wherein at least a part of the first part of the trough is irradiated by electromagnetic waves (405) capable of activating the polymerization initiator or the polymerization initiator system or both; wherein a first depth of the first part of the trough is more than a further depth of the further part of the trough; wherein the first depth refers to a maximum depth in a transversal direction of the belt, at each longitudinal position of the first part of the trough; wherein the further depth refers to a maximum depth in the transversal direction of the belt, at each longitudinal position of the further part of the trough. Therein, preferred components or devices or both of the device for the preparation of water-absorbent polymer particles according to the invention are designed according to the process according to the invention. Preferred electromagnetic waves capable of activating the polymerization initiator or the polymerization initiator system or both are UV radiation. Preferably, the first longitudinal section is irradiated by the electromagnetic waves in an intensity which is high enough to activate a polymerization initiator or a polymerization initiator system or both on the first longitudinal section of the belt. Preferably, the further longitudinal section is irradiated by the electromagnetic waves in an intensity which is high enough to activate a polymerization initiator or a polymerization initiator system or both on the further longitudinal section of the belt.

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 in- vention. 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 mate- rial, or a combination of at least two thereof. A preferred cable is a blue water cable. A pre- ferred liquid-absorbing hygiene article is one selected from the group consisting of a diaper, a tampon, and a sanitary towel, or a combination of at least two thereof. A preferred diaper is a baby's diaper or a diaper for incontinent adults or both. A contribution to the solution of at least one of the above objects is provided by a process for the production of a composite material, wherein a water-absorbent polymer particle 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. water-soluble polymer content

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

EXAMPLES

The present invention is now explained in more detail by examples and drawings given by way of example which do not limit it. A) Preparation of a partially neutralized acrylic acid monomer solution

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

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

B) Polymerization of the monomer solution

1 wt.-part of the monomer solution prepared in step A) is mixed with 0.001 wt.-parts of trimethylol propane triacrylate as crosslinker, 0.001 wt.-parts of sodium peroxodisulfate as first initiator component, 0.000034 wt.-parts of 2,2-dimethoxy-l ,2-diphenylethan-l-one (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 μη ) in a container to achieve a mixed solution. As a blowing agent sodium carbonate is added to the mixed solution in an amount of 0.1 wt.-part based on the total amount of the mixed solution. A sufficient amount of the mixed solution is subjected to further treatment in order to obtain a polymer gel and further downstream water-absorbent polymer particles and further downstream surface-crosslinked water-absorbent polymer particles as well as further downstream a water-absorbent product which is post treated. Details of the further treatment are given below.

Subsequently, the mixed solution is placed on the belt of a conveyer belt reactor and the polymerization is initiated by UV radiation which. The UV radiation is provided by mercury vapor lamps which are located above the conveyor belt and irradiate at a power of 1.5 mW. The conveyor belt has a length of 20 m and a width of 2 m. The conveyor belt is formed as a trough to keep the solution on the belt while polymerized. The trough is 12 m long. At least the deepest part of the trough is irradiated by the UV radiation. Details of the trough are given in the tables 1 and 2 below for the examples and the comparative examples. 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 the following steps. The polymer gel is cut into semi-endless spaghetti-like gel strips by a crusher as shown in figure 1 1. Then a mincer according to figure 12a) and 12b) is used to shred the strips into gel pieces in the range from 5 to 10 mm, wherein the above given comminuting devices according to the invention are used in the examples. Dif- ferent comminuting devices are used in the comparative examples. 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) and the achieved parts of the dried polymer gel have a size of less than 7 mm and are then kept for 2.5 hours in a container to equalize the humidity content of the polymer gel parts. The dried polymer gel parts are then milled in a roller mill of Bauermeister Type 350.1 x 1800 (3-stage crusher) (Bauermeister Zerkleinerungstechnik GmbH) to obtain water-absorbent polymer particles having a particle size of less than 1 mm.

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

E) Silicon dioxide treatment

In a treatment step the precursor I is mixed in a disc mixer with about 0.01 wt.-part (+- · 10 %) of silicon dioxide (Si0₂), 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, Es- sen, 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 (Gebruder 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. Sub- sequently, 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 μηι. At least 50 wt.-% of the surface-crosslined absorbent polymer particles have a particles size in the range of from 300 to 600 μπι. Less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples according to the invention are smaller than 150 μ^ηι, less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples accord- ing to the invention are have a particle size of more than 850 μηι. 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 and 2 for the examples and the comparative example. In the order given in the following the measurement results are getting better from left to right:— , -, +, ++, +++.

Table 1 : Belt capacity and water-soluble polymer content of the surface-crosslinked water- absorbent polymer particles depending on the longitudinal shape of the trough of the belt.

In table 1 the term first part refers to a longitudinally extending part of the belt which is characterized by a depth of 250 mm over the whole first part. As well in table 1 the term further part refers to a longitudinally extending part of the belt which follows the first part in downstream direction and which is characterized by a depth of 75 mm over the whole further part. In total the trough is 12 m long. Together or, if only one of the first and the further part is present in the corresponding comparative example, said part alone account(s) to 11 m.

In the comparative example 1 the trough of the belt comprises only the first part, which hence is 1 1 m long. This leads to a high capacity of the belt for taking aqueous monomer solution. However, the content of water-soluble polymer in the surface-crosslinked water- absorbent polymer particles produced by the process using such a belt is rather high. In the comparative example 2 the trough of the belt comprises only the further part, which hence is 1 1 m long. This leads to a low capacity of the belt for taking aqueous monomer solution. However, the content of water-soluble polymer in the surface-crosslinked water-absorbent polymer particles produced by the process using such a belt is rather low. None of said comparative examples provides a desirable balanced combination of the parameters capacity and water-soluble polymer content.

In the examples 1 to 3 the trough comprises a first and a further part, wherein the length of the first part (and hence also of the further part) is different for the 3 examples. It can be seen from table 1 that a length of the first part of 6 m in example 3 gives a balanced combination of the parameters studied. In example 1 the first part is 10 m long and the water- soluble content is more than in example 3. In example 2 the first part is only 1 m long and hence the belt capacity is less than in example 3.

Table 2: Characteristics of example processes for different longitudinal belt shapes regarding the depth of the belt.

In the examples and comparative examples in table 2 the following trough is used. In total the trough is 12 m long. The trough comprises a first and a further longitudinally extending section. The further longitudinally extending section follows the first section in a downstream direction of the belt. The first section is 6 m long and the further section is 5 m long. Throughout its longitudinal extension the first section has a first depth as given in table 2 and throughout its longitudinal extension the further section has a further depth as given in table 2 as well.

In the comparative example 3 the first depth and the further depth are 300 mm. Throughout the first and the further section the trough has a constant depth. This leads to a good capacity of the belt for taking aqueous monomer solution. However, the content of water-soluble polymer of the surface-crosslinked water-absorbent polymer particles produced by the process using such a belt is rather high. Surprisingly, it has been found that comminuting the polymer gel obtained on the belt in comparative example 3 in the crusher shown in figure 1 1 , a power consumption of the crusher showed a rather large variation in time. In the comparative example 4 the first depth is 50 mm. The further depth is 0 mm. Hence, there is no trough in the further section. Accordingly, the belt capacity is much lower than in comparative example 3. However, the water-soluble polymer content of the surface-crosslinked water-absorbent polymer particles produced is less and the variation of the power consumption of the crusher is less as well. In the example 4 according to the invention the first depth is

100 mm. The further depth is 50 mm. The belt capacity is better than in comparative example 4 and worse than in comparative example 3. The water-soluble polymer content is less than in both comparative examples of table 2. The variation of the power consumption of the crusher is similar to comparative example 4 which holds also for the examples 5 and 6. Therein, different values for the first and the further depth are applied. It shows that example 5 having a first depth of 250 mm and a further depth of 50 mm gives the most balanced combination of the observed parameters.

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, monoethy- lenically unsaturated monomer bearing carboxylic acid groups (al) and at least one cros- slinker (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 401 of a polymerization belt reactor 400. The belt 401 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 401. 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 particles are discharged from the belt dryer and subsequently in a ninth step 109 grinded to obtain water-absorbent polymer particles. In a tenth step 1 10 the water-absorbent polymer particles are sized to obtain water-absorbent polymer particles having a well defined particle size distribution. In an eleventh step 1 1 1 the surface of the water-absorbent polymer par- tides 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 initiator is added to the aqueous monomer solution, nitrogen is bubbled into the aqueous monomer solution in order to decrease its oxygen content.

Figure 3 shows a flow chart diagram depicting the steps 101 , 103, 105 to 1 10 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 second 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 scheme of a shape of a belt 401 of a polymerization belt reactor 400 in a · longitudinal cross section according to the invention. The polymerization belt reactor 400 comprises the belt 401 , at least two guide rollers 410 and mercury vapor lamps 409. One of the guide rollers 410 drives the belt 401 which is an endless conveyor belt such that an upper run 41 1 of the belt 401 moves in a downstream direction 412. Consequently, a lower run 41 1 of the belt 401 moves in an upstream direction 413. The downstream direction 412 and the upstream direction 413 are longitudinal directions of the belt 401. The upper run 41 1 of the belt 401 comprises a trough 402; wherein the trough 402 longitudinally extends by a longitudinal extension 403 of the trough 402 over about 60 % of a longitudinal extension 404 of the belt 401. The trough 402 comprises a first part 405 and a further part 406. The mercury vapour lamps 409 irradiate a part of the first part 405 of the trough 402 with elec- tromagnetic waves 408 capable of activating the polymerization initiator or the polymerization initiator system or both, wherein the electromagnetic waves 408 comprise UV radiation. The first part 405 is characterized by a first depth 407, wherein the first depth 407 refers to a maximum depth in a transversal direction 904 of the belt 401 , at each longitudinal position of the first part 405 of the trough 402. This means any depth of the trough 402 within a lon- gitudinal extension of the first part 405 fulfils the criteria of the first depth 407. Therein, the depths have to be chosen to be the maximum depth in the transversal direction 904 at the corresponding longitudinal position of the belt 401. The further part 406 is characterized by a further depth, wherein the further depth refers to a maximum depth in a transversal direction 904 of the belt 401 , at each longitudinal position of the further part 406 of the trough 402. The first depth 407 is more than the further depth. This means that at each longitudinal position the first part 405 of the trough 402 is deeper than the further part 406 of the trough 402. Here, the first depth 407 is 250 mm throughout the first part 405. The further depth ranges from more than 0 to less than 250 mm. The first part 405 longitudinally extends over 25 %, which are a first longitudinally extending portion of the belt 401 according to the invention, of the longitudinal extension 404 of the belt 401. The further part 406 longitudinally extends over 30 %, which are a further longitudinally extending portion of the belt 401 according to the invention, of the longitudinal extension 404 of the belt 401. The further part 406 directly follows the first part 405 in a downstream longitudinal direction 412 of the belt 401. In each transversal cross section of the first part 405 the belt 401 has a shape of a first concave curve 501. Each transversal cross section of the first part 405 is a first trans- versal cross section 500 according to the invention. The first concave curve 501 has a horizontal width 502 and a vertical height 503 which determine an R of 80. Furthermore, the first concave curve 501 comprises a horizontally aligned portion 504 which extends over 90 % of the horizontal width 502 of the first concave curve 501. A tangent 505 on any posi- tion of the first concave curve 501 inclines a first curve angle 507 with a horizontal line 506 in the first transversal cross section 500. The maximum first curve angle 507 is 45°. Furthermore, the first concave curve 501 forms a trough section having a trough section area 508. The trough section area 508 is trapezoidally shaped. The first concave curve 501 has a vertical height 503 of 250 mm. The further part 406 is a further longitudinally extending portion of the belt 401 according to the invention. Throughout the further part 406, that means in each transversal cross section of the further part 406, the belt 401 has a shape of a further concave curve. These transversal cross sections are further transversal cross sections according to the invention. A tangent on any position of the further concave curve inclines a further curve angle 507 with a horizontal line in the further transversal cross section. The maximum further curve angle is in the range of from more than 0 to less than 45°. The polymerization belt reactor 400 may comprise further components which are not shown in the figure, such as support elements, a supply and metering system, cooling and heating devices, and a removal system. Figure 5 shows a scheme of a shape of a belt of 401 a polymerization belt reactor 400 in a first transversal cross section 500 according to the invention. The first transversal cross section 500 is a transversal cross section of an upper run 41 1 of the belt 401 which is an endless conveyor belt. The belt 401 shown has the shape of a first concave curve 501. The first concave curve 501 has a horizontal width 502 and a vertical height 503 which determine an R of 4. Furthermore, the first concave curve 501 comprises a horizontally aligned portion 504 which extends over 70 % of the horizontal width 502 of the first concave curve 501. A tangent 505 on any position of the first concave curve 501 inclines a first curve angle 507 with a horizontal line 506 in the first transversal cross section 500. The maximum first curve angle 507 is 50°. Furthermore, the first concave curve 501 forms a trough section having a trough section area 508. The trough section area 508 is trapezoidally shaped. Figure 6 shows a scheme of another shape of a belt 401 of a polymerization belt reactor 400 in a first transversal cross section 500 according to the invention. The first transversal cross section 500 is a transversal cross section of an upper run 41 1 of the belt 401 which is an endless conveyor belt. The belt 401 shown has the shape of a first concave curve 501. The first concave curve 501 has a horizontal width 502 and a vertical height 503 which determine an R of 20. Furthermore, the first concave curve 501 comprises a horizontally aligned portion 504 which extends over 90 % of the horizontal width 502 of the first concave curve 501. A tangent 505 on any position of the first concave curve 501 inclines a first curve angle 507 with a horizontal line 506 in the first transversal cross section 500. The maximum first curve angle 507 is 70°.

Figure 7 shows a scheme of a shape of a belt of a polymerization belt reactor in a transversal cross section 700 not according to the invention. The transversal cross section 700 is a transversal cross section of an upper run of the belt which is an endless conveyor belt. The belt shown has the shape of a concave curve 701. The concave curve 701 has a horizontal width 702 and a vertical height 703 which determine an R of 1.5. A tangent 704 on any position of the concave curve 701 inclines a maximum angle 706 with a horizontal line 705 in the transversal cross section 700. The maximum angle 706 is 87°. Figure 8 shows a scheme of another shape of a belt of a polymerization belt reactor in a transversal cross section 700 not according to the invention. The transversal cross section 700 is a transversal cross section of an upper run of the belt which is an endless conveyor belt. The belt shown has the shape of a concave curve 701. The concave curve 701 has a horizontal width 702 and a vertical height 703 which determine an R of 40. A tangent 604 on any position of the concave curve 701 inclines a maximum angle 706 with a horizontal line 705 in the transversal cross section 700. The maximum angle 706 is 87°.

Figure 9 shows a scheme of a basic setup of a polymerization belt reactor 400 according to the invention. The polymerization belt reactor 400 comprises a belt 401. The belt 401 is an endless conveyor belt. The belt 401 passes over two guide rollers 410 such that an upper run 41 1 of the belt 401 moves downstream. The downstream movement of the upper run 41 1 of the belt 401 determines the direction of movement 901 of the polymer gel on the belt 401 , indicated by an arrow. Another arrow indicates a counter direction 902 to a direction of movement 901 of the polymer gel on the belt 401. Another arrow indicates both longitudi- nal directions 903 of the belt 401 and yet another arrow the transversal directions 904 of the belt 401. The belt 401 extends in the longitudinal directions 903 over a length which is a longitudinal extension 404 of the belt 401. The polymerization belt reactor 400 may comprise further components which are not shown in the figure, such as support elements, a supply and metering system, irradiating means, cooling and heating devices, and a removal system.

Figure 10 shows a block diagram of a device 1000 for the preparation of water-absorbent polymer particles according to the invention. The arrows show a direction of a process stream 1008 of the preparation of water-absorbent polymer particles. The device 1000 com- prises a first container 1001 , a further container 1002, downstream a mixing device 1003, downstream a polymerization belt reactor 400, downstream a comminuting device 1004, downstream a belt dryer 1005, downstream a grinding device 1006, and downstream a sizing device 1007, each according to the invention. Figure 1 1 shows a scheme of a comminuting device according to the invention. The comminuting device, a crusher 1 100, comprises a plurality of toothed wheels 1 101. A first portion of the toothed wheels 1 102 rotates around a first axis of rotation 1 101 and a further portion of the toothed wheels 1 102 rotates around a further axis of rotation 1 101. The toothed wheels 1 102 of the first portion rotate in counter direction and towards the toothed wheels 1 102 of the further portion. A polymer gel being fed between the rotating toothed wheels 1 102 is comminuting by the crusher 1 100 and polymer gel particles 1 103 are obtained.

Figure 12a) shows a scheme of another comminuting device according to the invention in an external view. The comminuting device is a mincer 1200 ("meat grinder") comprising a static hole plate 1201 , a rotating screw 1202, and a feed unit 1203 for feeding polymer gel particles, preferably polymer gel strands, into the mincer 1200. The polymer gel strands obtained in the crusher 1 100 in figure 1 1 may be further comminuted by the mincer 1200. Figure 12b) shows a scheme of inner parts the further comminuting device, the mincer 1200, of figure 12a) in an exploded view. The mincer 1200 comprises a screw 1202 which rotates together with a rotating hole plate 1204. Thereby, the screw 1202 conveys the polymer gel particles, preferably the polymer gel strands, towards the static hole plate 1201 and through holes of the static hole plate 1201. As the rotating hole plate 1204 rotates with respect to the static hole plate 1201 circular cutting edges 1205 of holes of the rotating hole plate 1204 comminute the polymer gel obtaining polymer gel particles (not shown). The circular cutting edges 1205 are planar cutting edges. The further comminuting device 1200 comprises no non-planar cutting edges.

Claims

[CLAIMS]

[Claim 1 ]

A process (100) for the preparation of water-absorbent polymer particles, comprising the process steps of

. (ii) optionally adding fine particles of a water-absorbent polymer to the aqueous monomer solution;

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

(v) charging the aqueous monomer solution onto a belt (401) of a polymerization belt reactor (400);

(vi) polymerizing the monomers in the aqueous monomer solution on the belt (401), thereby obtaining a polymer gel;

(vii) discharging the polymer gel out of the polymerization belt reactor (400) and optionally comminuting the polymer gel;

(viii) drying the optionally comminuted polymer gel;

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

wherein the belt (401 ) is shaped as a trough (402) extending longitudinally over at least 30 % of a longitudinal extension (404) of the belt (401);

wherein the trough (402) comprises a first part (405) of the trough (402) and a further part (406) of the trough (402); wherein at least a part of the first part (405) of the trough (402) is irradiated by electromagnetic waves (408) capable of activating the polymerization initiator or the polymerization initiator system or both;

wherein a first depth (407) of the first part (405) of the trough (402) is more than a further depth of the further part (406) of the trough (402);

wherein the first depth (407) refers to a maximum depth in a transversal direction (904) of the belt (401), at each longitudinal position of the first part (405) of the trough (402);

wherein the further depth refers to a maximum depth in the transversal direction (904) of the belt (401), at each longitudinal position of the further part (406) of the trough (402).

[Claim 2]

The process (100) according to claim 1 , wherein the first depth (407) is in the range of from 100 to 500 mm.

[Claim 3]

The process (100) according to claim 1 or 2, wherein the further depth is in the range of from 5 to 100 mm.

[Claim 4]

The process (100) according to any of the preceding claims, wherein the first part (405) of the trough (402) longitudinally extends over a length in the range of from 25 to 45 % of the longitudinal extension (404) of the belt (401).

[Claim 5]

The process (100) according to any of the preceding claims, wherein the further part (406) of the trough (402) longitudinally extends over a length in the range of from 3 to 70 % of the longitudinal extension (404) of the belt (401). [Claim 6]

The process (100) according to any of the preceding claims, wherein the further part (406) of the trough (402) directly follows the first part (405) of the trough (402) in a downstream longitudinal direction (412) of the belt (401).

[Claim 7]

The process (100) according to any of the preceding claims, wherein in a first transversal cross section (500) of the belt (401) the belt (401) has a shape of a first concave curve (501);

wherein R is in the range of from 2 to 30;

wherein R is a ratio of a horizontal width (502) of the first concave curve (501) to a vertical height (503) of the first concave curve (501).

[Claim 8]

The process (100) according to any of the preceding claims, wherein the polymerizing in step (vi) is initiated by an irradiation with UV radiation.

[Claim 9]

The process (100) according to claim 7 or 8, wherein a longitudinal position of the first transversal cross section (500) can be every longitudinal position within a first longitudinally extending portion of the belt;

wherein the first longitudinally extending portion of the belt extends longitudinally over 25 to 40 % of a longitudinal extension of the belt. [Claim 10]

The process (100) according to any of claims 7 to 9, wherein the first concave curve (401) comprises a horizontally aligned portion (504);

wherein the horizontally aligned portion (504) extends over at least 70 % of the horizontal width (502) of the first concave curve (501). [Claim 1 1 ]

The process (100) according to any of claims 7 to 10, wherein a tangent (505) on any position of the first concave curve (501) inclines a first curve angle (507) with a horizontal line (506) in the first transversal cross section (500);

wherein a maximum of the first curve angle (507) is in the range of from 45 to 80°.

I Claim 12]

The process (100) according to any of claims 7 to 1 1 , wherein downstream to the first longitudinally extending portion of the belt (401) the belt (401) comprises a further longitudinally extending portion;

wherein in a further transversal cross section of the belt (401) the belt (401) has a shape of a further concave curve;

wherein a longitudinal position of the further transversal cross section can be every longitudinal position within the further longitudinally extending portion of the belt; wherein a tangent on any position of the further concave curve inclines a further curve angle with a horizontal line in the further transversal cross section;

wherein a maximum of the further curve angle is in the range of from 0 to 45°.

[Claim 13]

The process (100) according to claim 12, wherein the further longitudinally extending portion of the belt (401) extends longitudinally over 30 % or less of the longitudinal extension (404) of the belt (401).

[Claim 14]

The process (100) according to any of claims 7 to 13, wherein the first concave curve (501) forms a trough section;

wherein the trough section has a trough section area (508);

wherein the trough section area (508) is trapezoidally shaped.

[Claim 15] The process (100) according to any of claims 7 to 14, wherein the first concave curve (501) forms a trough section;

wherein the trough has a trough section area (508);

wherein the trough section area (508) has an area of in the range of from 1 ,000 to 3,000 cm².

[Claim 16]

The process (100) according to any of claims 7 to 15, wherein the first concave curve (501) has a vertical height (503) of 500 mm or less.

[Claim 17]

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 18]

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 19]

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

[Claim 20]

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 21 ]

A device (1000) for the preparation of water-absorbent polymer particles in a process stream (1008), comprising

a) a first container (1001), 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 (1002), designed to take at least one crosslinker (a3);

c) a mixing device (1003), wherein the mixing device (1003) is

i) located down-stream to the first container (1001) and the further container (1002),

ii) designed to mix the monomer solution and the at least one crosslinker (cc3);

d) a polymerization belt reactor (400), wherein the polymerization belt reactor (400)

i) is located down-stream to the mixing device (1003),

ii) is 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 iii) comprises a belt (401 );

e) a comminuting device (1004), wherein the comminuting device (1004) is

i) located down-stream to the polymerization belt reactor (400), ii) designed to comminute the polymer gel, thereby obtaining polymer gel particles,

f) a belt dryer (1005), wherein the belt dryer (1005) is

i) located down-stream to the comminuting device (1004), ii) designed to dry the polymer gel particles,

g) a grinding device (1006), wherein the grinding device (1006) is

i) located down-stream to the belt dryer (1005), ii) designed to grind the dried polymer gel particles, thereby obtaining water-absorbent polymer particles;

h) a sizing device (1007), wherein the sizing device ( 007) is

i) located down-stream to the grinding device (1006),

ii) designed to size the grinded water-absorbent polymer particles;

wherein the belt (401) is shaped as a trough (402) extending longitudinally over at least 30 % of a longitudinal extension (404) of the belt (401);

wherein the trough (402) comprises a first part (405) of the trough (402) and a further part (406) of the trough (402);

wherein at least a part of the first part (405) of the trough (402) is irradiated by electromagnetic waves (408) capable of activating the polymerization initiator or the polymerization initiator system or both;

[Claim 22]

A process for the preparation of water-absorbent polymer particles in the device (1000) according to claim 21.

[Claim 23]

A water-absorbent polymer particle, obtainable by the process (100) according to any of claims 1 to 20, or 22.

[Claim 24] A composite material comprising a water-absorbent polymer particle according to claim 23.

[Claim 25]

The composite material according to claim 24, 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 thereof.

[Claim 26]

A process for the production of a composite material, wherein a water-absorbent polymer particle according to claim 23 and a substrate and optionally an auxiliary substance are brought into contact with one another.

[Claim 27]

A composite material obtainable by a process according to claim 26. [Claim 28]

A use of the water-absorbent polymer particle according to claim 23 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.