WO2015163518A1

WO2015163518A1 - Initiator system for preparation of a water-absorbent polymer by radical polymerization

Info

Publication number: WO2015163518A1
Application number: PCT/KR2014/003676
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

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 system to the aqueous monomer solution; (iv) optionally decreasing the oxygen content of the aqueous monomer solution; (v) charging the aqueous monomer solution into a polymerization reactor; (vi) polymerizing the monomers; thereby obtaining a polymer gel; (vii) discharging the polymer gel out of the polymerization 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) surface-crosslinking the grinded and sized water-absorbent polymer particles; wherein in process step (iii) the polymerization initiator system comprises the fol-lowing initiator components iiia. a peroxodisulfate; and iiib. an organic initiator molecule comprising at least three oxygen atoms or at least three nitrogen atoms; wherein the polymerization initiator system comprises the peroxodisulfate and the organic initiator molecule in a molar ratio in the range of from 20:1 to 50:1.

Description

[DESCRIPTION]

[Invention Title]

INITIATOR SYSTEM FOR PREPARATION OF A WATER-ABSORBENT POLYMER BY RADICAL POLYMERIZATION

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

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

The polymer gel, also called hydrogel, obtained after the polymerization is usually comminut- ed, dried and classified in order to obtain a particulate superabsorber with a well defined particle size distribution. In a further process step these superabsorbent particles are often surface crosslinked in order to improve the absorption behavior. For this purpose the particles are mixed with an aqueous solution containing a surface crosslinking agent and optionally further additives and the thus obtained mixture is heat treated in order to promote the crosslinking reaction. The acid-group-carrying monomers can be polymerized in the presence of the 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 2003/05141 5 A l , EP 1 470 905 A l , WO 2007/02875 1 Al , WO 2007/028746 Al and WO 2007/028747 A l .

Continuous processes of polymerization are commonly performed in a polymerization reactor under specific conditions. Often the polymerization reactor is a polymerization belt reactor. A common polymerization belt reactor comprises a conveyor belt which is designed to carry an aqueous monomer solution comprising acid-group-carrying monomers during polymerization. Such a conveyor belt is disclosed in EP 0 955 086 B 1 .

[Disclosure] [Technical Problem]

Disadvantages of the processes for the preparation of water-absorbent polymer particles using a polymerization belt reactor comprising a conveyor belt arise when the acid-group-carrying monomers are mixed with the crosslinker and initiator. One issue of the use of a belt reactor can be that the polymerization reaction can often not be kept under control. It might occur that the polymerization proceeds too fast or too slow and thus an unsatisfactory product are obtained.

[Technical Solution]

Generally, it is an object of the present invention to at least partly overcome a disadvantage arising form the prior art in the context of the production of water-absorbent polymer particles. It is a further object of the present invention to provide a process for the production of water- absorbent polymer particles, wherein the provided monomer solution is easily manageable. It is a further object of the present invention to provide a process for the production of water- absorbent polymer particles, wherein the quality of the product is improved. It is a further object of the present invention to provide a process for the production of water-absorbent polymer particles, wherein the reaction time of the polymerization process can be easily controlled. It is a further object of the present invention to provide a process for the production of water-absorbent polymer particles, wherein the reaction time of the polymerization process can be reduced. Further, it IS an object of the invention to provide a process for the production of water-absorbent polymer particles, which is both easily managed and cost efficient using a polymerization belt reactor. It is a further object of the present invention to provide surface-crossl inked water-absorbent polymer particles which are characterized by a low re- sidual monomer content or as little yellowing as possible or preferably both.

It is a further object of the present invention to provide a water-absorbent polymer particle produced by a process having 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 pn duced 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]

The present invention provide a process for the production of water-absorbent polymer particles, wherein the provided monomer solution is easily manageable.

[ 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 basic setup of a polymerization belt reactor according to the invention; and

Figure 5 a block diagram of a device for the preparation of water-absorbent polymer particles according to the invention.

List of references

100 process according to the invention

101 step (i)

102 step (ii)

103 step (iii)

1 03a first sub-step of step (iii)

103b further sub-step of step (iii)

104 step (iv)

105 step (v)

106 step (vi)

107 step (vii)

108 step (viii)

109 step (ix)

1 10 step (x)

1 1 1 step (xi)

1 12 step (xii)

1 14 step (xiii)

1 16 step (xiv)

400 polymerization reactor; polymerization belt reactor

401 belt

402 guide roller

403 direction of movement of the polymer gel on the belt

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

405 longitudinal directions 406 transversal directions

500 device for the preparation of water-absorbent polymer particles

501 first container

502 further container

503 mixing device

504 comminuting device

505 belt dryer

506 grinding device

507 sizing device

508 process stream

[ Best Mode]

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

(i) preparing an aqueous monomer solution comprising at least one partially neutralized, monoethylenically unsaturated monomers 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 system to the aqueous monomer solution;

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

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

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

(vii) discharging the polymer gel from the polymerization 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) surface-crosslinking of the grinded and sized water-absorbent polymer particles; wherein in process step (iii) the polymerization initiator system comprises the following initiator components

iiia. a peroxodisulfate, preferably Na₂S₂0₈; and

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

wherein the polymerization initiator system comprises the peroxodisulfate and the organic initiator molecule in a molar ratio in the range of from 20: 1 to 50: 1 , preferred in the range form 25 : 1 to 45: 1 , more preferred in the range from 30: 1 to 40: l and most preferred in the range from 27: 1 to 34: 1.

Therein, subsequent steps of the process according to the invention may be performed simultaneously or may overlap in time or both. This holds particularly for the steps (i) to (iv), especially particularly for the steps (iii) and (iv).

The process according to the present invention is preferably a continuous process in which the aqueous monomer solution is continuously provided and is continuously fed into the polymerization reactor. Preferably the polymerization reactor is a polymerization belt reactor. Preferably, the aqueous monomer solution is continuously provided and is continuously fed onto the belt of the polymerization belt reactor. The polymer gel obtained is continuously discharged out of the polymerization 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, substitute certain parts of the process equipment, like the belt material of the conveyor belt if a conveyor belt is used as the polymerization reactor,

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

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

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

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

Preferred monoethylenically unsaturated monomers bearing carboxylic acid groups (al ) are those cited in DE 102 23 060 Al as preferred monomers (al), whereby acrylic acid is particularly preferred. Preferred monoethylenically unsaturated monomers bearing carboxylic acid groups (al) are acrylic acid, methacrylic acid, ethacrylic acid, a-chloro-acrylic acid, a-cyano- acrylic acid, β-methylacrylic acid (Crotonic acid), a-phenyl-acrylic acid, β-acryloxypropionic acid, sorbic acid, a-chlorosorbic acid, 2'-methylisocrotonic acid, cinamic acid, p-chloro cin- amic acid, β-stearylic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fumaric acid, tri-carboxy-ethylen- and maleic anhydride, wherein acrylic acid as well as methacrylic acid are preferred and acrylic acid is particularly preferred.

It is preferred according to the present invention that the water-absorbent polymer produced by the process according to the invention comprises monomers bearing carboxylic acid groups to at least 50 wt.-%, preferably to at least 70 wt.-% and further preferably to at least 90 wt.-%, based on the dry weight of the polymer. 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 from 10 to 60 wt.-%, preferably from 30 to 55 wt.-% and most preferably from 40 to 50 wt.-%, based on the total weight of the aqueous monomer solution.

In a preferred embodiment of the process, the preparing of an aqueous monomer solution, comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al ), comprises a neutralization of a monomer solution, comprising at least one monoethylenically unsaturated monomer bearing carboxylic acid groups. Preferably, in step (i) of the process according to the invention, preparation of the aqueous monomer solution further comprises

11. providing a first portion of acrylic acid comprising a polymerization inhibitor, preferably mono methyl ether hydroquinone (MEHQ) or hydroquinone (HQ) or both; and

12. contacting the first portion of acrylic acid with a in a first contacting step in order to obtain a first aery late phase,

wherein the first aery late phase has a pH of 10 or more,

wherein in the first acrylate phase at least 95 mol-%, of the acrylic acid is an acry- late;

13. contacting the first acrylate phase with a further portion of acrylic acid to obtain the monomer solution,

wherein the further portion of acrylic acid has a smaller water content than the first acrylate phase,

wherein the monomer solution has a pH smaller than the pH of the first acrylate phase,

wherein 70 to 80 mol-% of the acrylic acid in the monomer solution is an acrylate a is

wherein the content of acrylate and acrylic acid in the aqueous monomer solution is in the range of 30 to 50 wt.-%, based on the total weight of the aqueous monomer solution.

The neutralization of the monomers bearing carboxylic acid groups is preferably established by addition of sodium hydroxide to at least a part of the aqueous monomer solution at the beginning of step (i). Preferably, a part of the aqueous monomer solution comprises acrylic acid as the monomer bearing carboxylic acid groups, this is the first portion of acrylic acid. In step (i l ) of the preferred process, the first portion of acrylic acid preferably comprises mono methyl ether hydroquinone (MEHQ) or hydroquinone (HQ). The first portion of acrylic acid is preferably contacted with sodium hydroxide (NaOH) as hydroxide, wherein a pH of 10 or more is obtained resulting in a first aqueous Na-acrylate comprising phase. However, potassium hydroxide or lithium hydroxide or a mixture of these with itself or with sodium hydroxide may be used as hydroxide The first portion of acrylic acid preferably comprises the hydroxide in a ratio to the acrylic acid from 0.1 : 1 to 1.5: 1 , or preferably in a ratio from 0.2: 1 to 1.3 : 1 , or preferably in a ratio from 0.3: 1 to 1 : 1. The preferred addition of sodium hydroxide to the acrylic acid results in a conversion of at least part of the acrylic acid to sodium acrylate.

The first portion of acrylic acid preferably comprises the mono methyl ether hydroquinone (MEHQ) or hydroquinone (HQ) in an amount from 0.1 to 10 wt.-%, or preferably in an amount from 0.3 to 7 wt.-%, or preferably in an amount from 0.5 to 5 wt.-%, based on the total weight of the first portion of acrylic acid. Preferably, the first portion of acrylic acid comprises mono methyl ether hydroquinone (MEHQ).

In step (i) the preparation of the aqueous monomer solution preferably comprises several further steps: a. addition of the first portion of acrylic acid comprising sodium acrylate and MEHQ or HQ and;

β. addition of the acrylic acid monomer;

χ. addition of further monoethylenically unsaturated monomers (a2); The steps β to χ can be performed in any order and in any combination with step a. In one preferred embodiment only step a is performed. In a further preferred embodiment steps a and β are performed. In yet a further preferred embodiment steps a and χ are performed. Also the order can be varied in step (i). In a preferred embodiment the crosslinker (a3) is added first and then step a and optionally one of steps β and/or χ can be added. In a further pre- ferred embodiment step a alone or in a combination with one of steps β or χ is carried out first and afterwards the crosslinker (ct3) is added.

The acrylate and acrylic acid content of the aqueous monomer solution is less than 55 wt.-%, preferably less than 50 wt.-%, or preferably less than 45 wt.-%, related to the total weight of the aqueous monomer solution. It is furthermore preferred that the acrylate and acrylic acid content of the aqueous monomer solution is not below 30 wt.-%. The aqueous monomer solution may also comprise monoethylenically unsaturated monomers (a2) which are copolymerizable with (al ). Preferred monomers (a2) are those monomers which are cited in DE 102 23 060 Al as preferred monomers (a2), whereby acrylamide is particularly preferred.

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

Preferred crosslinkers (a3) are all those compounds which are cited in DE 102 23 060 Al as crosslinkers (a3) of the crosslinker classes I, II, III and IV, whereby as compounds of crosslinker class I, N, N^'-methylene bisacrylamide, polyethylenegly- col di(meth)acrylates, triallylmethylammonium chloride, tetraallylammonium chloride and allylnonaethyleneglycol acrylate produced with 9 mol ethylene oxide per mol acrylic acid are particularly preferred, wherein N, N" -methylene bisacrylamide is even more preferred, and as compounds of crosslinker class IV, Al₂ (S0₄)₃ and its hydrates are particularly 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 fo llowing combinations of crosslinker classes respectively: I, II, III, IV, I II, I I II, 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 A l as crosslinkers of crosslinker classes I, whereby Ν,Ν' -methylene bisacrylamide, polyethyleneglycol di(meth)acrylates, triallyl-methylammonium chloride, tetraallylammonium chloride and allylnonaethylene-glycol acrylate produced from 9 mol ethylene oxide per mol acrylic acid are particularly preferred as crosslinkers of crosslinker class I, wherein N, 1ST -methylene bisacrylamide is even more preferred.

The aqueous monomer solution may further comprise water-soluble polymers (a4). Preferred water-soluble polymers (a4) include partly or completely saponified polyvinyl alcohol, polyvinylpyrrolidone, starch or starch derivatives, polyglycols or polyacrylic acid. The molecular weight of these polymers is not critical, as long as they are water-soluble. Preferred water- soluble polymers (a4) are starch or starch derivatives or polyvinyl alcohol. The water-soluble polymers (ot4), 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 (ot3) and water- soluble polymers (a4) and auxiliary substances (a5) in the aqueous monomer solution is preferably chosen such that the water-absorbent polymer structure obtained after drying the optionally comminuted polymer gel is based to the extent of from 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 from 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 from 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

(cx3),

to the extent of from 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 from 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 from 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 A l , US 4,076,663, DE 35 03 458 Al , DE 40 20 780 C I , DE 42 44 548 Al , DE 43 33 056 A l and DE 44 18 818 Al .

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

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

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

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

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

Such a kind of mixing set up can, for example, be realized by means of mixing devices which are disclosed in DE-A-25 20 788 and DE-A-26 17 612. Specific 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 I A^® Werke GmbH & Co. KG, Staufen, Germany, under designations MHD 2000/4, MHD 2000/05, MHD 2000/10, MDH 2000/20, MHD 2000/30 und MHD 2000/50, wherein the mixing device MHD 2000/20 is particularly preferred. Further mixing devices which can be used are those offered by ystral GmbH, Ballrechten-Dottingen, Germany, for example under designation „Conti TDS", or by Kinematika AG, Luttau, Switzerland, for example under the trademark Megatron^®.

The amount of fine particles that may be added to the aqueous monomer solution in process step (ii) is preferably in the range from 0.1 to 15 wt.-%, even more preferred in the range from 0.5 to 10 wt.-% and most preferred in the range from 3 to 8 wt.-%, based on the weight of the aqueous monomer solution.

In process step (iii) of the process according to the present invention a polymerization initiator system is added to the aqueous monomer solution. The polymerization initiator system com- prises at least two initiator components. The polymerization initiator system may also comprise three or more components.

As initiator components of the polymerization initiator system for initiation of the polymerisation all initiators forming radicals under the polymerization conditions can be used, which are commonly used in the production of superabsorbers. Among these belong thermal catalysts, redox catalysts and photo-initiators, whose activation occurs by energetic irradiation.

The polymerization initiators may be dissolved or dispersed in the aqueous monomer solution.

The use of water-soluble catalysts is preferred. According to the invention at least a peroxodisulfate, preferably, sodium peroxodisulfate (Na₂S208) and an organic initiator molecule are added in step (iii) of the process. However, further initiator components might be added.

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 amy! perneodecanoate. Furthermore, the fo llowing thermal po lymerisation 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 mo l-%, 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 x 10^"5 to 1 mol-% of the reducing component of the redox catalyst and 1 x 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.

If the polymerization is initiated by action of energetic radiation, so-called photo-initiators are generally used as an initiator component of the polymerization initiator system. These can comprise for example so-called a-splitters, H-abstracting systems or also azides. Examples of such initiators are benzophenone derivatives such as Michlers ketone, phenanthrene derivatives, fluorine derivatives, 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)ethyI-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. The photo-initiators, when used, are generally employed in quantities from 0.01 to 5 wt.-% based on the total amount of monomers to be polymerized.

A particularly preferred redox system that is used in the process according to the present invention is a redox system comprising hydrogen peroxide, sodium peroxodisulfate and ascorbic acid.

In this context it should also be noted that step (iii), adding the polymerization initiator system, 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. As a polymerization initiator system is used that comprises two or more components, for example, the preferred initiator system that comprises hydrogen peroxide, sodium peroxodisulfate and ascorbic acid, one or more of the initiator components of such a polymerization initiator system may, for example, be added before process step (iv), whereas the remaining initiator component or the remaining initiator 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).

In process step (iv) of the process according to the present invention the oxygen content of the aqueous monomer solution can be optionally decreased. Alternative or additional decreasing the oxygen content of the aqueous monomer solution in step (iv), may also be performed before, during or after process step (ii). Preferably, the oxygen content of the aqueous monomer solution is decreased after the fine particles have been added in process step (ii). Decreasing of the oxygen content in the aqueous monomer solution may be realized by bringing the aqueous monomer solution into contact with an inert gas, such as nitrogen. The phase of the inert gas being in contact with the aqueous monomer solution is free of oxygen and is thus characterized by a very low oxygen partial pressure. As a consequence oxygen converts from the aqueous monomer solution into the phase of the inert gas until the oxygen partial pressures in the phase of the inert gas and the aqueous monomer solution are equal. Bringing the aqueous monomer phase into contact with a phase of an inert gas can be accomplished, for example, by introducing bubbles of the inert gas into the monomer solution in concurrent, countercurrent or intermediate angles of entry. Good mixing can be achieved, for example, with nozzles, static or dynamic mixers or bubble columns. The oxygen content of the monomer solution before the polymerization is preferably lowered to less than 1 ppm, more preferably to less than 0.5 ppm, based on the total amount of monomers in the solution.

In process step (v) of the process according to the present invention the aqueous monomer solution is charged into a polymerization reactor, preferably onto a conveyor belt, especially preferred at an upstream position of the conveyor belt and in process step (vi) the monomers in the aqueous monomer solution are polymerized in the polymerization reactor, thereby obtaining a polymer gel. If polymerization is performed on a polymerization belt as the polymerization reactor, a polymer gel strand is obtained in a downstream portion of the conveyor belt, which, before drying, is preferably comminuted in order to obtain gel particles.

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

Another example of a preferred polymerization reactor is a conveyor belt. As a conveyor belt that is useful for the process according to the present invention any conveyor belt can be used which the person skilled in the art considers to be useful as a support material onto which the above described aqueous monomer solution can be charged and subsequently polymerized to form a hydrogel. Examples of conveyor belts which can be used in the process according to the present invention are disclosed in DE-A-35 44 770, EP-A-0 955 086 and EP-A- 1 683 813.

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

In process step (vii) of the process according to the present invention the particulate polymer gel that is obtained in the polymerization reactor, preferably the polymer gel particles ob- tained in the kneading reactor or the polymer gel strand obtained in the downstream portion of the conveyor belt, is/are discharged out of the reactor and is/are, especially in the case of the polymer gel strand obtained on the conveyor belt, (further) comminuted, thereby obtaining polymer gel particles. Preferably, the resulting polymer gel strand is removed from the con- veyor belt as a continuous strand that is of a soft semi-solid consistency and is then passed on for further processing such as comminution.

Comminution of the polymer gel or gel strand 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 f om 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, prefera- bly 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 "airability" 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, fluidised bed dryers, plate dryers, paddle dryers and infrared dryers may be mentioned by way of example.

Especially preferred are belt dryers. A belt dryer is a convective system of drying, for the particularly gentle treatment of through-airable products. The product to be dried is placed onto a continuous conveyor belt which allows the passage of gas, and is subjected to the flow of a heated gas stream, preferably air. The drying gas preferably 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 1 5 % 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, multistage 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 poly- mers are individually determined as a function of the processing parameters chosen. The hole size and mesh size of the belt is conformed to the product. Similarly, certain surface enhancements, such as electropolishing or Teflonizing, are possible.

The polymer gel 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 in the range from 1 to 10 wt.-%, or preferably in the range 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 con- siders 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 μιτι 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 μιτι 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.-% of the water-absorbent polymer particles have a particle size in a range of from 300 to 600 μιη.

In process step (xi) of the process according to the present invention the surface of the ground and sized water-absorbent polymer particles is optionally treated. As measures to treat the surface of water-absorbent polymer particles any measure can be used the person skilled in the art considers as appropriate for such a purpose. Examples of surface treatments include, for example, surface crosslinking, the treatment of the surface with water-soluble salts, such as aluminium sulfate or aluminium lactate, the treatment of the surface with inorganic particles, such as silicon dioxide, and the like. Preferably, the components used to treat the surface of the polymer particles (cross-linker, water soluble salts) are added in the form of aqueous solutions to the water-absorbent polymer particles. After the particles have been mixed with the aqueous solutions, they are heated to a temperature in the range from 150 to 230°C, preferably 160 to 200°C in order to promote the surface-crosslinking reaction. A further preferable treatment of the water-absorbent polymer particles or the surface-crosslinked water- absorbent particles is the mixing of the particles in one, two or more fractions with zeolites. The zeolites preferably comprise one of the elements selected from the group consisting of Ag, Na, Zn or a mixture of at least two thereof. Preferably, the zeolites are mixed with the water- absorbent particles in a range from 0.05 to 5 wt.-%, or preferably in the range from 0.1 to 1 3 wt.-%, or preferably in the range from 0.2 to 10 wt.-%, based on the total weight of the water-absorbent polymer particles. By this kind of treatment post-treated water-absorbent polymer particles are achieved or post-treated cross-linked water-absorbent polymer particles.

The process (100) according to claim 1 , wherein the concentration of the initiator component iiia. is in the range from 0.05 to 2 wt.-%, preferred in the range from 0.1 to 1 wt.-%, more preferred in the range from 0.15 to 0.7 wt.-% and most preferred in the range from 0.2 to 0.4 wt.-%, each based on the amount of monomers to be polymerized

In a preferred embodiment of the process, 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 preferred embodiment of the process, the peroxodisulfate is of the general formula M2S2O8, with M being selected from the group consisting of NH₄, Li, Na, Ka or at least two thereof. Preferably, the peroxodisulfate is sodium peroxodisulfate. In a preferred embodiment of the process, 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 immedi- ately after the polymerization in step (vi) is initiated. Particularly preferably, the blowing agent is added to the monomer so lution after or simultaneously to adding the polymerization initiator system or an initiator component of the polymerization 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'- Dinitrosoamide, and an organic blowing agent, or a combination of at least two thereof.

A very particularly preferred blowing agent is a carbonate blowing agent. Carbonate blowing agents which may be used according to the invention are disclosed in US 5, 1 18, 719 A, and are incorporated herein by reference. A preferred carbonate blowing agent is a carbonate con- taining 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 bicarbonate, a hydrate of these, other cations, and naturally occurring carbonates, or a combination of at least two thereof. A preferred naturally occurring carbonate is dolomite. The above mentioned carbonate blowing agents release C0₂ when being heated while dissolved or dispersed in the monomer solution. A further preferred carbonate blowing agent is MgC0₃, which may also be represented by the formula (MgC0₃) Mg(OH)₂- 5-H₂0. Another preferred carbonate blowing agent is agent is (NH₄)₂C03. The MgCC and (NH₄)₂C03 may also be used in mix- tures. Preferred carbonate blowing agents are carbonate salts of multivalent cations, such as Mg, Ca, Zn, and the like. Examples of such carbonate blowing agents are Na C0₃, K₂C0₃, (NH₄)₂C0₃, MgC0₃, CaC0₃, NaHC0₃, KHC0₃, NH₄HC0₃, Mg(HC0₃)₂, Ca(HC0₃)₂, ZnC0₃, and BaC0₃. Although certain of the multivalent transition metal cations may be used, some of them, such as ferric cation, can cause color staining and may be subject to reduction oxidation reactions or hydrolysis equilibria in water. This may lead to difficulties in quality control of the final polymeric product. Also, other multivalent cations, such as Ni, Ba, Cd, Hg would be unacceptable because of potential toxic or skin sensitizing effects. A preferred nitrite is ammonium nitrite. A preferred peroxide is hydrogen peroxide. A preferred aromatic azo compound is one selected from the group consisting of a triazene, ar- ylazosulfones, arylazotriarylmethanes, a hydrazo compound, a diazoether, and d iazoamino- 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.

In a preferred embodiment of the process the blowing agent is C0₂ or a carbonate which is provided to the monomer solution in at least one of the steps (i) to (v). Preferably, the blowing agent is a carbonate containing salt, or a bicarbonate containing salt, or both.

In a preferred embodiment of the process the blowing agent is selected from the group consisting of C0₂ as a gas, C0₂ as a solid, ethylene carbonate, sodium carbonate, potassium car- bonate, ammonium carbonate, magnesium carbonate, or magnesium hydroxic carbonate, calcium carbonate, barium carbonate, a bicarbonate, a hydrate of these, other cations, and naturally occurring carbonates, or a combination of at least two thereof.

Preferably, the oxygen gas content of the monomer solution is in the range from 3 to 15 ppm, or preferably in a range from 5 to 12 ppm, or preferably in a range from 6 to 10 ppm. Preferably, the monomer solution is saturated by oxygen gas. In a preferred embodiment of the process, in step (v) the polymerization reactor is a polymerization belt reactor. The polymerization belt reactor provides a belt to hold the monomer solution while polymerized.

In a preferred embodiment of the process the polymer gel being discharged in process step (vii) comprises water in the range of from 40 to 60 wt.-%, preferably in the range from 50 to 60 wt.-%, or preferably in the range from 53 to 56 wt.-%, based on the polymer gel.

In a preferred embodiment of the process 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 a preferred embodiment of the process 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.

A contribution to the solution of at least one of the above objects is provided by a device for the preparation of water-absorbent polymer particles in a process stream, comprising

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

b) a further container, designed to take at least one crosslinker (oc3);

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);

iii) designed to comprise a polymerization initiator system, comprising as initiator components at least

iiia. sodium peroxodisulfate and

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

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 (<x3) 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) is located down-stream to the polymerization belt reactor,

ii) is designed to comminute the polymer gel;

f) a belt dryer, wherein the belt dryer

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

ii) designed to dry the polymer gel,

g) a grinding device, wherein the grinding device is

i) located down-stream to the belt dryer,

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

h) a 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.

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.

In a preferred embodiment of the device the concentration of the initiator component iiia. is in the range from 0.05 to 0.3 mol-% and the concentration of the initiator component iiib. is in the range from 0.001 to 0.02 mol-%, based on the amount of monomers to be polymerized.

A contribution to the so lution of at least one of the above objects is provided by a process for the preparation of water-absorbent polymer particles in the device according to the invention. Preferably, the process comprises the process steps (i) to (xi) according to the invention. A contribution to the solution of at least one of the above objects is provided by a water- absorbent polymer particle, obtainable by the process according to the invention. A further aspect of this invention is 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 a preferred 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 fun- gal growth-regulating agents, a packaging material, a soil additive, and a building material, or a combination of at least two thereof. A preferred cable is a blue water cable. A preferred liquid-absorbing hygiene article is one selected from the group consisting of a diaper, a tampon, and a sanitary towel, or a combination of at least two thereof. A preferred diaper is a baby's diaper or a diaper for incontinent adults or both.

A contribution to the solution of at least one of the above objects is provided by a process for the production of a composite material, wherein 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.

Polymerization belt reactor

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

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 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.

Another means of providing a trough-like configuration is arranging weir elements on an ap- proximately flat belt. Therein, at least one weir element is arranged in longitudinal direction at each lateral edge of the belt. At least two further weir elements are arranged in transversal direction on different longitudinal positions of the belt, wherein the trough longitudinally extends between those two positions. The height of the weir elements determines the deepness of the trough and hence the amount of reaction components which can be fed into the trough. The longitudinal weir elements have to be designed to be longitudinally bent with the belt at turning sections of the belt. Therefor, the longitudinal weir elements may be made of a flexible material or they may consist of a plurality of shorter longitudinal weir elements which are arranged close to each other. A trough-like configuration of the polymerization belt can also be achieved by a combination of transversally oriented weir elements and shaping the belt in the transversal direction by supporting elements, or by a combination of longitudinally oriented weir elements and shaping the belt in the longitudinal direction by supporting elements. 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 EDANA test method being closest to the earliest filing date of the present application applies. In absence of distinct measuring conditions, standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm) apply. The water content after drying is determined according to the Karl Fischer method. Residual monomer content

The residual monomer content of the water-ansorbent 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 "Residual Monomers" under the method number WSP 210.2.R3 (12). Yellowing

The yellowing was determined by comparing the color of the dried polymer gel to a white sheet of commercially avai lable Xerox printing paper.

[ 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 trime- thylol propane triacrylate as crosslinker, 0.001 wt.-parts of sodium peroxodisulfate as first initiator component, amounts of 2,2-dimethoxy-l ,2-diphenylethan-l -one (Ciba^® Irgacure^® 651 by Ciba Specialty Chemicals Inc., Basel, Switzerland) as shown in table 1 and 2 as a second initiator component, up to 0.1 wt.-parts of acrylic acid particles (with a particle size of less than 150 μηι) in a container to achieve a mixed solution. If according to table 1 below a blowing agent is added, 0.1 wt.-part, based on the total amount of the mixed solution, of sodi- um carbonate are added to the mixed solution.

A sufficient amount of the mixed solution is subjected to further treatment in order to obtain a polymer gel and further downstream water-absorbent polymer particles and further down- stream 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, as shown in figure 4, and the polymerization is initiated by UV radiation. The conveyor belt has a length of at least 20 m. The conveyor belt is formed as a trough to keep the solution on the belt prior and during polymerization. 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 poly- mer gel has a water content of about 52 wt.-%, based on the total weight of the polymer gel.

C) Comminuting and drying of the polymer gel

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

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

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

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

The comminuted gel is dried in a belt dryer at a temperature of 1 80 °C to a water content of 5 wt.-% based on the dried polymer gel. The belt of the belt drier provides orifices, where hot air is pressed into the polymer gel via nozzles. Additionally hot air is blown from above the belt onto the gel.

Milling and sizing The dried polymer gel is ground in three steps. First the dried polymer gel is fed through a Herbold Granulator HGM 60/145 (HERBOLD Meckesheim GmbH) and the achieved parts of the dried polymer gel have a size of less than 7 mm and are then kept for 2.5 hours in a container to equalize the humidity content of the polymer gel parts. The dried polymer gel parts are then milled in a roller mill of Bauermeister Type 350.1 x 1800 (3-stage crusher) (Bau- ermeister 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 μιη. Less than 5 wt.-% of the water-absorbent polymer particles of the examples according to the invention are smaller than 150 μπι, 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 μιτι. 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, Essen, Germany. When mixing the precursor I with the Si0₂, the precursor still has a temperature of more than 80 °C to 100 °C, preferably of 100 °C. A precursor II is achieved. F) Surface crosslinking

In a further step 1 wt.-part of the precursor II is mixed with 0.003 wt.-part (+- 10 %) of a surface crosslinker, based on the total weight of the mixture of precursor II and crosslinker. The surface crosslinker is composed of 19 wt.-% water, 40 wt.-% ethylene glycol diglycidyl ether, 1 wt.-% Na₂S0₃, 40 wt.-% poly ethylene glycol with a molecular weight of 400 g/mol, each based on the total amount of the crosslinker. The ingredients of the crosslinker are mixed in a line static mixer. The crosslinker is mixed in a ringlayer mixer CoriMix® CM 350 (Gebriider 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 GPWD 12W120, by Andritz AG, Graz, Austria for 45 minutes at a temperature in the range of from 130 to 160°C. Surface-cross- linked absorbent polymer particles are obtained. In a cooling device in the form of a fluid bed, the temperature of the surface-cross-linked absorbent polymer particles is decreased to below 60 °C, obtaining cooled surface-cross-linked absorbent polymer particles referred as to precursor III.

G) Post treatment

1 wt.-part of precursor III is then subjected to mixing with 0.005 wt.-part Ag-zeolite. Subsequently, the mixture is sieved. The sieve is selected to separate agglomerates of the cooled surface-cross- linked absorbent polymer particle having a particle size of more than 850 μιη. 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 μιη, less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples according to the invention are have a particle size of more than 850 μπι. Post treated crosslinked water- absorbent polymer particles are obtained.

Table 1 : Residual monomer content of surface-crosslinked water-absorbent polymer particles depending on mol ratio of Na peroxodisulfate to Irgarcure^®-651 (2,2-dimethoxy-l ,2- diphenylethan- l -one ) in the polymerization initiator system Examples UV Intitators Yellowing after drying polymer gel

Example 2 2,2-dimethoxy- 1 ,2-diphenylethan- 1- no

one (Irgarcure^®-651)

Example 4 2,2-azobis-(2- no

amidinopropane)dihydrochloride

Example 5 2,2-azobis-(cyano valeric acid) no

Comparative example 5 1 -Hydro xy-cyclohexy-phenyl-ketone yes

Table 2: Results for polymerization initiator system Na peroxodisulfate and UV Initiator in a mol ratio of 31.6: 1 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: --, -, +, ++, +++.

It is evident from the above tables that the initiator combination according to the invention yields a low residual monomer content with no yellowing after drying.

Figure 1 shows a flow chart diagram depicting the steps (101 to 1 1 1 ) provided after steps 1 12, 1 14 and 1 16 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. The first step (il ) 1 12 comprises: providing of a first portion of acrylic acid comprising mono methyl ether hydroquinone (MEHQ) or hydroquinone (HQ) or both is achieved. The further step (i2) 1 14 comprises: contacting the first portion of acrylic acid with sodium hydroxide (NaOH) in a first contacting step, wherein a pH of 10 or more is obtained resulting in a first aqueous sodium-acrylate comprising phase. In step (i) 101 the aqueous monomer solution comprising the at least one partially neutralized, monoethylenically unsatu- rated monomer bearing carboxylic acid groups (al) of steps (il) 1 12 and (i2) 1 14 and at least one crosslinker (<x3) is provided. Preferably, the aqueous monomer solution is an aqueous solution of partially neutralized acrylic acid, further comprising crosslinkers. The partially neutralized acrylic acid is provided as a solution in form of a first portion of an acrylic acid comprising MEHQ and NaOH. The aqueous monomer solution can also comprise a further monomer like acrylic acid or acrylamide. In a second step 102 fine particles of a water- absorbent polymer may be added to the aqueous monomer solution. Preferably, these particles comprise poly acrylic acid particles. In a third step 103, divided into a first sub-step 103a and a further sub-step 103b, a polymerization initiator system according to the invention is added to the aqueous monomer solution. Therein, in the first sub-step 103a Na peroxodisulfate as an initiator component is added to the aqueous monomer solution. Subsequently, in the further sub-step 103b Irgarcure^®-651 as an initiator component is added to the aqueous monomer solution. Therein, a molar ratio of Na peroxodisulfate : Irgarcure^®-651 is 32: 1 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 601 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 11 the surface of the water-absorbent polymer particles is treated in terms of a surface crosslinking.

Figure 2 shows a flow chart diagram depicting the steps (101 to 1 1 1 ) as well as steps 1 12, 1 14 and 1 16 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, divided into the first sub-step 103a and the further sub- step 103b, and the fourth process step 104 overlap in time. Here, the first sub-step 103a is performed overlapping in time with the fourth process step 104, and subsequently the further sub-step 103b. Hence, between adding the initiator components of the polymerization initiator system to the aqueous monomer solution, nitrogen is bubbled into the aqueous monomer solution in order to decrease its oxygen content, wherein the bubbling is started while the Na peroxodisulfate is added.

Figure 3 shows a flow chart diagram depicting the steps (1 12, 1 14, 1 16 101 , 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 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 402 such that an upper run of the belt 401 moves downstream. The downstream movement of the upper run of the belt 401 determines the direction of movement 403 of the polymer gel (not shown here) on the belt 401 , indicated by an arrow. Another arrow indicates a counter direction 404 to a direction of movement of the polymer gel on the belt. Another arrow indicates both longitudinal directions 405 of the belt 401 and yet another arrow the transversal directions 406 of the belt. 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 5 shows a block diagram of a device 500 for the preparation of water-absorbent polymer particles according to the invention. The arrows show a direction of a process stream 508 of the preparation of water-absorbent polymer particles. The device 500 comprises a first container 501 , a further container 502, downstream a mixing device 503, downstream a polymerization belt reactor 400, downstream a comminuting device 504,

downstream a belt dryer 505, downstream a grinding device 506, and downstream a sizing device 507, each according to the invention.

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

Claims

[CLAIMS]

[Claim 1 ]

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

(i) preparing an aqueous monomer solution comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al ) and at least one crosslinker (oc3);

(iii) adding a polymerization initiator system to the aqueous monomer solution;

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

(v) charging the aqueous monomer solution into a polymerization reactor (400);

(vi) polymerizing the monomers in the aqueous monomer solution in the polymerization reactor (400); thereby obtaining a polymer gel;

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

(viii) drying the optionally comminuted polymer gel;

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

(xi) surface-crosslinking the grinded and sized water-absorbent polymer particles; wherein in process step (iii) the polymerization initiator system comprises the following initiator components

iiia. a peroxodisulfate; and

wherein the polymerization initiator system comprises the peroxodisulfate and the organic initiator molecule in a molar ratio in the range of from 20: 1 to 50: 1.

[Claim 2]

The process (100) according to claim 1 , wherein 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.

[Claim 3]

The process (100) according to claim 1 or 2, wherein 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.

[Claim 4]

The process (100) according to any of the preceding claims, wherein the peroxodisul- fate is of the general formula M2S2O8, with M being selected from the group consisting of NH₄, Li, Na, Ka or at least two thereof.

[Claim 5]

The process ( 00) according to any of the preceding claims, wherein the polymerization in step (vi) is performed in presence of a blowing agent.

[Claim 6]

The process (100) according to claim 5, wherein the blowing agent is C0₂ or a carbonate which is provided to the monomer solution in at least one of the steps (i) to (v).

[Claim 7]

The process ( 100) according to any of the claims 5 or 6, wherein the blowing agent is selected from the group consisting of CO2 as a gas, CO2 as a solid, ethylene carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, magnesium carbonate, or magnesium hydroxic carbonate, calcium carbonate, barium carbonate, a bicarbonate, a hydrate of these, other cations, and naturally occurring carbonates, or a combination of at least two thereof.

[Claim 8] The process (100) according to any of the preceding claims, wherein in step (v) the polymerization reactor (400) is a polymerization belt reactor (400).

[Claim 9]

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 10]

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 1 1 ]

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 width in the range of from 30 to 300 cm.

[Claim 12]

A device (500) for the preparation of water-absorbent polymer particles in a process stream (508), comprising

a) a first container (501 ), 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 (502), designed to take at least one crosslinker (a3);

c) a mixing device (503), wherein the mixing device (503) is

i) located down-stream to the first container (501 ) and the further container (502),

ii) designed to mix the monomer solution and the at least one crosslinker (<x3), iii) designed to comprise a polymerization initiator system, comprising as initiator components at least

iiia. a peroxodisulfate and

d) a polymerization belt reactor (400), wherein the polymerization belt reactor (400) i) is located down-stream to the mixing device (503),

ii) is designed to comprise the aqueous monomer solution and the at least one crosslinker ( 3) during polymerizing the monomers in the aqueous monomer solution, thereby obtaining a polymer gel, iii) comprises a belt (401);

e) a comminuting device (504), wherein the comminuting device (504) is

i) is located down-stream to the polymerization belt reactor (400), ii) is designed to comminute the polymer gel;

f) a belt dryer (505), wherein the belt dryer (505)

i) is located down-stream to the comminuting device (504), ii) designed to dry the polymer gel,

g) a grinding device (506), wherein the grinding device (506) is

i) located down-stream to the belt dryer (505),

h) a sizing device (507), wherein the sizing device (507) is

i) located down-stream to the grinding device (506),

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

[Claim 13]

The device (500) according to claim 12, wherein the concentration of the initiator component iiia. is in the range from 0.05 to 0.3 mol-% and the concentration of the initiator component iiib. is in the range from 0.001 to 0.02 mol-%, based on the amount of monomers to be polymerized.

[Claim 14] A process for the preparation of water-absorbent polymer particles in the device (500) according to claim 12 or 13.

[Claim 15]

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

[Claim 16]

A composite material comprising a water-absorbent polymer particle according to claim 15.

[Claim 1 7]

The composite material according to claim 16, comprising one selected from the group consisting of a foam, a shaped article, a fibre, a fo il, 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 18]

A process for the production of a composite material, wherein a water-absorbent polymer particle according to claim 15 and a substrate and optionally an auxiliary substance are brought into contact with one another.

[Claim 19]

A composite material obtainable by a process according to claim 18. [Claim 20]

A use of the water-absorbent polymer particle according to claim 15 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.