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Title:
RECYCLING OF FINE PARTICLES IN THE PRODUCTION OF WATER-ABSORBENT POLYMER PARTICLES
Document Type and Number:
WIPO Patent Application WO/2015/163519
Kind Code:
A1
Abstract:
The invention generally relates to a process for the preparation of surface-crosslinked water-absorbent polymer particles, comprising a sequence of 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) 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; (iii) optionally decreasing the oxygen content of the aqueous monomer solution; (iv) charging the aqueous monomer solution into a polymerization reactor; (v) polymerizing the monomers in the aqueous monomer solution in the polymerization reactor; (vi) discharging the polymer gel out of the polymerization reactor and comminuting the polymer gel, thereby obtaining polymer gel particles; (vii) drying the polymer gel particles; (viii) grinding the dried polymer gel particles thereby obtaining water-absorbent polymer particles; (ix) contacting at least a part of the water-absorbent polymer particles with a crosslinking composition, comprising a further crosslinker, thereby obtaining surface-crosslinked water-absorbent polymer particles; wherein in process step (vi) a plurality of fine surface-crosslinked water-absorbent polymer particles is added to the polymer gel or the polymer gel particles or both.

Inventors:
PARK JEONG BEOM (KR)
Application Number:
PCT/KR2014/003677
Publication Date:
October 29, 2015
Filing Date:
April 25, 2014
Export Citation:
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Assignee:
SONGWON IND CO LTD (KR)
International Classes:
C08J3/12; A61L15/60; C08F2/01; C08F2/10
Domestic Patent References:
WO2013139673A12013-09-26
Foreign References:
US20110118430A12011-05-19
US20140114035A12014-04-24
US20130210947A12013-08-15
US20090312184A12009-12-17
Attorney, Agent or Firm:
YOU ME PATENT AND LAW FIRM (Gangnam-gu, Seoul 135-912, KR)
Download PDF:
Claims:
[CLAIMS]

[Claim 1 ]

A process (100) for the preparation of surface-crosslinked water- absorbent polymer particles (610), comprising a sequence of 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) adding a polymerization initiator or a at least one component of a polymeriza- tion initiator system that comprises two or more components to the aqueous monomer solution;

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

(iv) charging the aqueous monomer solution into a polymerization reactor (904);

(v) polymerizing the monomers in the aqueous monomer solution in the polymer- ization reactor (904), thereby obtaining a polymer gel (601);

(vi) discharging the polymer gel (601 ) out of the polymerization reactor (904) and comminuting the polymer gel (601), thereby obtaining polymer gel particles (604);

(vii) drying the polymer gel particles (604);

(viii) " grinding the dried polymer gel particles (606) thereby obtaining water- absorbent polymer particles (608);

(ix) contacting at least a part of the water-absorbent polymer particles (608) with a crosslinking composition (612), comprising a further crosslinker, thereby obtaining surface- crosslinked water- absorbent polymer particles (610);

wherein in process step (vi) a plurality of fine surface-crosslinked water-absorbent polymer particles (61 1) is added to the polymer gel (601) or the polymer gel particles (604) or both.

[Claim 2]

The process (100) according to claim 1 , wherein the sequence of process steps (i) to (ix) is repeated at least one time.

[Claim 3] The process (100) according to claim 1 or 2, wherein in process step (vi) comminuting the polymer gel (601) comprises a first comminuting step in a first comminuting device (602) and subsequently a further comminuting step in a further comminuting device (603);

wherein the plurality of the fine surface-crosslinked water-absorbent polymer particles (61 1) is fed into the further comminuting device (603) in process step (vi).

[Claim 4]

The process (100) according to claim 2 or 3, wherein the process (100) comprises a process step (x) of subjecting the surface-crosslinked water-absorbent polymer particles (610) to a first sizing step; thereby obtaining the plurality of the fine surface- crosslinked water-absorbent polymer particles (61 1).

[Claim 5]

The process (100) according to any of the preceding claims, wherein prior to adding the plurality of the fine surface-crosslinked water-absorbent polymer particles (61 1) to the polymer gel (601) or the polymer gel particles (604) or both in process step (vi) the fine surface-crosslinked water-absorbent polymer particles of the plurality (61 1) are contacted with water in an amount in the range of from 65 to 150 wt.-% based on the weight of the plurality of the fine surface-crosslinked water-absorbent polymer particles (61 1) in a fines mixing device.

[Claim 6]

The process (100) according to any of the preceding claims, wherein in process step (vi) a plurality of fine non- surface-crosslinked water-absorbent polymer particles

(802) is added to the polymer gel (601) or the polymer gel particles (604) or both.

[Claim 7]

The process (100) according to claim 6, wherein in process step (vi) comminuting the polymer gel (601) comprises a first comminuting step in a first comminuting de- vice (602) and subsequently a further comminuting step in a further comminuting device (603);

wherein the plurality of the fine non- surface-crosslinked water-absorbent polymer particles (802) is fed into the further comminuting device (603) in process step (vi).

[Claim 8] The process (100) according to claim 6 or 7, wherein in process step (via) the water- absorbent polymer particles (608) are subjected to a further sizing step, thereby obtaining the plurality of the fine non-surface-crosslinked water-absorbent polymer particles (802).

[Claim 9]

The process (100) according to any of claims claim 6 to 8, wherein prior to adding the plurality of the fine non-surface-crosslinked water-absorbent polymer particles (802) to the polymer gel (601) or the polymer gel particles (604) or both in process step (vi) the plurality of the fine non-surface-crosslinked water-absorbent polymer particles (802) is contacted with water in an amount of 65 to 150 wt.-% based on the weight of the plurality of the fine non-surface-crosslinked water-absorbent polymer particles (802) in a fines mixing device.

[Claim 10]

The process (100) according to any of claims 5 to 9, wherein the fines mixing device comprises a fluidized bed of the fine non-surface-crosslinked water-absorbent polymer particles (802) or the fine surface-crosslinked water-absorbent polymer particles (61 1) or both.

[Claim 1 1 ]

The process (100) according to any of claims 6 to 10, wherein a weight ratio of the plurality of the fine surface-crosslinked water-absorbent polymer particles (61 1) to the plurality of the fine non-surface-crosslinked water-absorbent polymer particles (802) being added to the polymer gel (601) or the polymer gel particles (604) or both in process step (vi) is in the range of from 0.3 : 0.7 to 0.7 : 0.3.

[Claim 12]

The process (100) according to any of the preceding claims, wherein in process step (ix) the crosslinking composition (612) further comprises a reducing agent or a poly alkylene glycol or both.

[Claim 13]

The process (100) according to claim 12, wherein the reducing agent is a compound of a chemical formula MXS03, wherein x is either 1 or 2,

wherein for x=2 M is Li or Na or both, wherein for x=l M is Mg or Ca or both.

[Claim 14]

The process (100) according to any of the preceding claims, wherein in process step (ix) the contacting is performed in a first mixing device (609),

wherein the first mixing device (609) comprises a rotating shaft (1 107) and a plurality of mixing tools (1 108),

wherein the mixing tools (1 108) are connected to the rotating shaft (1 107), wherein in process step (ix) the rotating shaft (1 107) rotates at a speed in the range of from 500 to 1200 rpm.

[Claim 15]

The process (100) according to any of the preceding claims, wherein in process step (ix) the contacting is performed in a first mixing device (609),

wherein the first mixing device (609) comprises a plurality of mixing tools (1 108), wherein the mixing tools (1 108) are rods or paddles or both.

[Claim 16]

The process (100) according to any of the preceding claims, wherein in process step (ix) the contacting is performed in a first mixing device (609),

wherein the first mixing device (609) comprises an annular layer (1 105) of the at least part of the water- absorbent polymer particles (608).

[Claim 17]

The process (100) according to any of the preceding claims, wherein in process step (ix) the contacting is a mixing by a plurality of contacts of the at least part of the water-absorbent polymer particles (608) to a rotating component,

wherein the mixing is performed for a duration in the range of from 0.1 to 60 seconds. [Claim 18]

The process (100) according to any of the preceding claims, wherein the polymer gel (601) being discharged in process step (vi) comprises water in the range of from 40 to 60 wt.-%, based on the polymer gel (601).

[Claim 19] The process (100) according to any of the preceding claims, wherein the polymer gel

(601) being discharged in process step (vi) is a polymer gel sheet; wherein the polymer gel sheet is characterized by a thickness in the range ot trom 10 to 200 mm.

[Claim 20]

The process (100) according to any of the preceding claims, wherein the polymer gel (601) being discharged in process step (vi) 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 21 ]

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

[Claim 22]

A device (900) for the preparation of surface-crosslinked water- absorbent polymer particles (610) in a process stream (905), comprising

a) a first container (901), 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 (902), designed to take at least one crosslinker (cx3);

c) a further mixing device (903), wherein the further mixing device (903) is

i) located down-stream to the first container (901) and the further container (902),

ii) designed to mix the monomer solution and the at least one cross- linker (a3);

d) a polymerization reactor (904), wherein the polymerization reactor (904) is i) located down-stream to the first container (901) and the further container (902),

ii) 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 (601); e) a first comminuting device (602), wherein the first comminuting device (602) i) is located down-stream to the first container (901) and the further container (902),

ii) is designed to comminute the polymer gel (601); f) a further comminuting device (603), wherein the further comminuting device (603)

i) is located down-stream to the first comminuting device (602), ii) is designed to comminute the polymer gel (601) to which a plurality of fine surface-cross linked water-absorbent polymer particles (61 1) has been added, thereby obtaining polymer gel particles (604); g) a belt dryer (605), wherein the belt dryer (605) is

i) located down-stream to the further comminuting device (603), ii) designed to dry the polymer gel particles (604);

h) a grinding device (607), wherein the grinding device (607) is

i) located down-stream to the belt dryer (605),

ii) designed to grind the dried polymer gel particles (606), thereby obtaining water-absorbent polymer particles (608);

j) a first mixing device (609), wherein the first mixing device (609) is

i) located down-stream to the grinding device (607), ii) designed to contact at least a part of the water-absorbent polymer particles (608) with a crosslinking composition (612), comprising a further crosslinker, thereby obtaining surface-crosslinked water- absorbent polymer particles (610);

k)a first sizing device (701), wherein the first sizing device (701) is

i) located down-stream to the first mixing device (609), ii) designed to size the surface-crosslinked water-absorbent polymer particles (610), thereby obtaining the plurality of the fine surface- crosslinked water-absorbent polymer particles (61 1).

3]

The device (900) according to claim 22, wherein the device (900) further comprises a further sizing device (801), wherein the further sizing device (801) is

a) located down-stream to the grinding device (607),

b) designed to size the water-absorbent polymer particles (608), thereby obtaining a plurality of fine non-surface-crosslinked water-absorbent polymer particles (802);

wherein the further comminuting device (603) is further designed to comminute the polymer gel (601) to which the plurality of the fine surface-crosslinked water- absorbent polymer particles (61 1) and the plurality oi me nne non-suriace- crosslinked water-absorbent polymer particles (802) have been added, thereby obtaining polymer gel particles (604).

[Claim 24]

A process for the preparation of surface-crosslinked water-absorbent polymer particles (610) in the device (900) according to claim 22 or 23.

[Claim 25]

A surface-crosslinked water-absorbent polymer particle (610), obtainable by the process (100) according to any of claims 1 to 21 , or 24.

[Claim 26]

A composite material comprising a surface-crosslinked water-absorbent polymer particle (610) according to claim 25.

[Claim 27]

The composite material according to claim 26, 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 28]

A process for the production of a composite material, wherein a surface-crosslinked water-absorbent polymer particle (610) according to claim 25 and a substrate and optionally an auxiliary substance are brought into contact with one another.

[Claim 29]

A composite material obtainable by a process according to claim 28.

[Claim 30]

A use of the surface-crosslinked water-absorbent polymer particle (610) according to claim 25 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.

Description:
[DESCRIPTION]

[Invention Title]

RECYCLING OF FINE PARTICLES IN THE PRODUCTION OF WATER-ABSORBENT POLYMER PARTICLES

[Technical Field]

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

[ B ackgro und Art ]

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

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

The polymer gel, also called hydrogel, obtained after the polymerization is usually comminuted, dried and classified in order to obtain a particulate superabsorber with a well defined particles size distribution. In a further process step these superabsorbent particles are often surface crosslinked in order to improve the absorption behavior. For this purpose the particles are mixed with an aqueous solution containing a surface crosslinking agent and optionally further additives and the thus obtained mixture is heat treated in order to promote the crosslinking reaction. The acid-group-carrying monomers can be polymerized in the presence of the cross- linkers in a batch process or in a continuous process. Both in continuous and in batchwise polymerization, partially neutralized acrylic acid is typically used as the monomer. Suitable neutralization processes are described, for example, in EP 0 372 706 A2, EP 0 574 260 Al, WO 2003/051415 Al , EP 1 470 905 Al, WO 2007/028751 Al , WO 2007/028746 Al and WO 2007/028747 Al .

[Disclosure] [Technical Problem]

In the prior art water-absorbent polymer particles are prepared by grinding the dried hydrogel. During such a grinding not only water-absorbent polymer particles of desired particle sizes are produced, but also water-absorbent polymer particles having smaller particle sizes - the fine water-absorbent polymer particles or fines. In order to increase the efficiency of the overall production process for the preparation of water-absorbent polymer particles said fines are separated and recycled back into the process in the prior art. In the prior art the fines are separated from the larger water-absorbent polymer particles by sieving the polymer particles prior to surface-crosslinking. However, prior to surface-crosslinking the polymer particles still have a relatively high water content and hence sieving is rather ineffective. Thus, separating the fines from the larger water-absorbent polymer particles prior to surface cross- linking is rather ineffective. Moreover, in the prior art the separated fines are recycled into the monomer composition which is a monomer solution. This means in the prior art the fines are fed into the polymerization reactor in which polymerization is performed or into a mixing device or container which is located upstream to the polymerization reactor in the production process. In consequence, less monomer solution can be fed into the polymerization reactor or the mixing device or container. By recycling the fines into the polymerization reactor or a mixing device or container upstream to the reactor, the reactor capacity for taking the monomer solution is decreased. This is particularly severe if the polymerization reactor is a polymerization belt.

[Technical Solution]

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

A further object is to provide a process for the production of surface-crosslinked water-absorbent polymers, being characterized by an increased polymerization reactor capacity for taking monomer solution. A further object is to provide a process for the production of surface-crosslinked water-absorbent polymers, being characterized by a larger volume of the polymerization reactor being available for monomer solution. A further object is to provide a process for the production of surface-crosslinked water- absorbent polymers, being characterized by reduced amount of fine particles being produced. A further object is to provide a process for the production of surface-crosslinked water- absorbent polymers, being characterized by an improved yield. It is a further object of the present invention to provide water-absorbent polymer particles being a process showing one or more of the above advantages and showing similar overall physical properties, in particular referring to retention capacity of the water- absorbent polymer particles. It is a further object of the invention to provide a process for the production of superabsorbent polymer particles, wherein the process shows a balanced combination of at least two, preferably at least three, of the above advantages. A further object is to provide 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 superabsorbent 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]

An increased polymerization reactor capacity for taking monomer solution and a larger volume of the polymerization reactor being available for monomer solution may be provided. Further reduced amount of fine particles may be produced and an improved yield may be provided.

[Description of Drawings]

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

Figure 2 is a flow chart diagram depicting the steps of another process

according to the invention;

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

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

Figure 5 is a flow chart diagram depicting the steps of another process according to the invention;

Figure 6 is a flow chart diagram of a sequence of process steps (vi) to (ix) according to the invention;

Figure 7 is a flow chart diagram of another sequence of process steps (vi) to (x) according to the invention;

Figure 8 is a flow chart diagram of another sequence of process steps (vi) to (x) according to the invention;

Figure 9 is a block diagram of a device for the preparation of surface- crosslinked water-absorbent polymer particles according to the invention;

Figure 10 is a block diagram of another device for the preparation of sur- face-crosslinked water-absorbent polymer particles according to the invention;

Figure 1 1a is a scheme of a longitudinal cross section of a first mixing device according to the invention;

Figure 1 lb is a scheme of a transversal cross section of the first mixing device in figure 1 la);

Figure 12 is a scheme of a first comminuting device according to the invention;

Figure 13a is a scheme of a further comminuting device according to the invention; and

Figure 13b is a scheme of inner parts the further comminuting device of figure 13 a) in an exploded view.

100 process according to the invention

101 step (i)

102 step (ii)

103 step (iii)

104 step (iv)

105 step (v)

106 step (vi)

107 step (vii) 108 step (viii)

109 step (ix)

1 10 step (x)

601 polymer gel

602 first comminuting device

603 further comminuting device

604 polymer gel particles

605 belt dryer

606 dried polymer gel particles

607 grinding device

608 water-absorbent polymer particles

609 first mixing device

610 surface-crosslinked water-absorbent polymer particles

61 1 plurality of fine surface-crosslinked water-absorbent polymer par- tides

612 crosslinking composition

701 first sizing device

801 further sizing device

802 fine non-surface-crosslinked water-absorbent polymer particles

900 device for the preparation of surface-crosslinked water-absorbent mer particles

901 first container

902 further container

903 further mixing device

904 polymerization reactor

905 process stream

1 101 inlet

1 102 mixing chamber

1 103 mixing chamber wall

1 104 outlet

1 105 annular layer of at least a part of the water-absorbent polymer particles

1 106 axial position 1 107 rotating shaft

1 108 mixing tool

1201 axis of rotation

1202 toothed wheel

[Best Mode]

A contribution to the solution of at least one of these objects is made by a process for the preparation of surface-crosslinked water-absorbent polymer particles, comprising a sequence of 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) ■ 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 solu- tion;

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

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

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

(vi) discharging the polymer gel out of the polymerization reactor and comminuting the polymer gel, thereby obtaining polymer gel particles;

(vii) drying the polymer gel particles;

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

(ix) contacting at least a part of the water-absorbent polymer particles with a crosslinking composition, comprising a further crosslinker, thereby obtaining surface- crosslinked water-absorbent polymer particles;

wherein in process step (vi) a plurality of fine surface-crosslinked water-absorbent polymer particles is added to the polymer gel or the polymer gel particles or both.

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 (iii), especially particularly for the steps (ii) and (iii).

The process according to the present invention is preferably a continuous process in which the aqueous monomer solution is continuously provided and is continuously fed into the polymerization reactor. The hydrogel obtained is continuously discharged out of the polymerization reactor and is continuously comminuted, dried and ground in the subsequent process steps. This continuous process may, however, be interrupted in order to, for example, substitute certain parts of the process equipment, like the belt material of the conveyor belt if a conveyor belt is used as the polymerization reactor,

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

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

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

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

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

It is preferred according to the present invention that the water- absorbent polymer produced by the process according to the invention comprises monomers bearing carboxylic acid groups to at least 50 wt.-%, preferably to at least 70 wt.-% and further preferably to at least 90 wt.-%, based on the dry weight. It is particularly preferred according to the invention, that the water-absorbent polymer produced by the process according to the invention is formed from at least 50 wt.-%, preferably at least 70 wt.-% of acrylic acid, which is prefera- bly neutralized to at least 20 mol-%, particularly preferably to at least 50 mol-%. The concentration of the partially neutralized, monoethylenically unsaturated monomers bearing carboxylic acid groups (al) in the aqueous monomer solution that is provided in process step (i) is preferably in the range of from 10 to 60 wt.-%, preferably from 30 to 55 wt.-% and most preferably from 40 to 50 wt.-%, based on the total weight of the aqueous monomer solution. The aqueous monomer solution may also comprise monoethylenically unsaturated monomers (a2) which are copolymerizable with (al). Preferred monomers (al) are those monomers which are cited in DE 102 23 060 Al as preferred monomers (a2), whereby acrylamide is particularly preferred.

Preferred crosslinkers (a3) according to the present invention are compounds which have at least two ethylenically unsaturated groups in one molecule (crosslinker class I), compounds which have at least two functional groups which can react with functional groups of the monomers (al) or (a2) in a condensation reaction (= condensation crosslinkers), in an addition reaction or a ring-opening reaction (cross-linker class II), compounds which have at least one ethylenically unsaturated group and at least one functional group which can react with functional groups of the monomers (al) or (a2) in a condensation reaction, an addition reaction or a ring-opening reaction (crosslinker class III), or polyvalent metal cations (cross- linker class IV). Thus with the compounds of crosslinker class I a crosslinking of the polymer is achieved by radical polymerization of the ethylenically unsaturated groups of the crosslink- er 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 com- pounds 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, Al 2 (S0 4 ) 3 and its hydrates are particularly preferred.

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

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

The aqueous monomer solution may further comprise water-soluble polymers (oc4).

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 ( 4), 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 ex- ample, EDTA.

The relative amount of monomers (al) and (oc2) and of crosslinking agents (a3) and water-soluble polymers (a4) and auxiliary substances (a5) in the aqueous monomer solution is preferably chosen such that the water-absorbent polymer structure obtained after drying the 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 particu- larly 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) of the process according to the present invention a polymerization initiator or at least one component of a polymerization initiator system that comprises two or more components is added to the aqueous monomer solution.

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

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

Redox catalysts comprise two or more components, usually one or more of the peroxo compounds listed above, and at least one reducing component, preferably ascorbic acid, glucose, sorbose, mannose, ammonium or alkali metal hydrogen sulfite, sulfate, thiosulfate, hyposulfite or sulfide, metal salts such as iron II ions or silver ions or sodium hydroxymethyl sulfoxylate. Preferably ascorbic acid or sodium pyrosulfite is used as reducing component of the redox catalyst. 1 χ 10 "5 to 1 mol-% of the reducing component of the redox catalyst and 1 x 10 "5 to 5 mol-% of the 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 oc- splitters, H-abstracting systems or also azides. Examples of such initiators are benzophenone derivatives such as Michlers ketone, phenanthrene derivatives, fluorine derivatives, anthra- quinone derivatives, thioxanthone derivatives, cumarin derivatives, benzoinether and deriva- tives thereof, azo compounds such as the above-mentioned radical formers, substituted hex- aarylbisimidazoles or acylphosphine oxides. Examples of azides are: 2-(N,N- dimethylamino)ethyl-4-azidocinnamate, 2-(N,N-dimethylamino)ethyl-4-azidonaphthylketone, 2-(N,N-di-methylamino)ethyl-4-azidobenzoate, 5-azido- 1 -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 embodi- ment it is preferred that the organic initiator molecule is selected from the group consisting of 2,2-dimethoxy-l ,2-diphenylethan-l-one, 2,2-azobis-(2-amidinopropane)dihydrochloride, 2,2- azobis-(cyano valeric acid) or a combination of at least two thereof. In a further aspect of this embodiment it is preferred that the peroxodisulfate is of the general formula M 2 S 2 0 8 , with M being selected from the group consisting of NH 4 , Li, Na, Ka or at least two thereof. The above described components are in particular suitable for UV initiation of the polymerization in step (vi) of the process of the present invention. Employing this composition further yields low residual monomer and reduced yellowing in the water-absorbent polymer particle, obtainable by the process according to the present invention.

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

In process step (iii) of the process according to the present invention the oxygen con- tent of the aqueous monomer solution is optionally decreased.

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 mixing can be achieved, for example, with nozzles, static or dynamic mixers or bubble columns. The oxygen content of the monomer solution before the polymerization is preferably lowered to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, based on the monomer solution.

In process step (iv) 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 (v) the monomers in the aqueous monomer solution are polymerized in the polymerization reactor, thereby obtaining a polymer gel. If polymerization is performed on a coneyor belt as the polymerization reactor, a polymer gel sheet is obtained in a downstream portion of the conveyor belt, which, before drying, is comminuted in order to obtain polymer gel particles.

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

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

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

The belt can be made of various materials, although these preferably have to meet the requirements of good tensile strength and flexibility, good fatigue strength under repeating bending stresses, good deformability and chemical resistance to the individual reaction components under the conditions of the polymerization. These demands are usually not met by a single material. Therefore, a multi-layer material is commonly used as belt of the present invention. The mechanical requirements can be satisfied by a carcass of, for example, fabric inserts of natural and/or synthetic fibers or glass fibers or steel cords. The chemical resistance can be achieved by a cover of, for example, polyethylene, polypropylene, polyisobutylene, halogenated polyolefines such as polyvinyl chloride or polytetrafluorethylene, polyamides, natural or synthetic rubbers, polyester resins or epoxy resins. The preferred cover material is silicone rubber. In process step (vi) of the process according to the present invention the polymer gel obtained in the polymerization reactor is comminuted, thereby obtaining polymer gel particles. Preferred polymer gel particles are one selected from the group consisting of polymer gel strands, polymer gel flakes, and polymer gel nuggets, or a combination of at least two thereof. The comminuting according to the invention comprises preferably at least two comminuting steps. Therein, a first comminuting step is performed in a first comminuting device and a further comminuting step in a further comminuting device. The first comminuting device may be the polymerization reactor or a part of the polymerization reactor, or a separate device, or both. Hence, the first comminuting step may be performed before, during, or after discharging the polymer gel out of the polymerization reactor. A preferred polymerization reactor which is the first comminuting device is a kneading reactor. If the first comminuting step is performed in the polymerization reactor, the polymer gel particles obtained are preferably further comminuted in the further comminuting step after discharging out of the polymerization reactor. If the polymerization reactor is a conveyor belt, the first comminuting step is preferably per- formed after discharging the polymer gel as a polymer gel sheet from the conveyor belt in a first comminuting device, wherein the first comminuting device is a separate device. Preferably, the polymer gel sheet is discharged from the conveyor belt as a continuous sheet that is of a soft semi-solid consistency and is then passed on for further processing such as comminuting.

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

in the first comminuting step, a cutting unit of the first comminuting device, 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, prefera- bly from 10 to 300 mm and particularly preferably from 100 to 200 mm, a height within the range of from 1 to 30 mm, preferably from 5 to 25 mm and particularly preferably from 10 to 20 mm as well as a width within the range of from 1 to 500 mm, preferably from 5 to 250 mm and particularly preferably from 10 to 200 mm; - in a second step, a shredding unit, preferably a breaker, is used for shredding the gel strips into gel pieces, preferably with a length within the range of 3 to 100 mm, preferably from 5 to 50 mm, a height within the range from 1 to 25 mm, preferably from 3 to 20 mm as well as a width within the range from 1 to 100 mm, preferably from 3 to 20 mm and in a third step, which is preferably the further comminuting step according to the invention 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. The mincer is a preferred further comminuing device according to the invention.

An optimal surface-volume ratio is achieved by comminuting, which has an advantageous effect on the drying behaviour in process step (vii). A polymer gel which has been comminuted in the above 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 ker- nels.

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

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

In process step (vi) a plurality of fine surface-crosslinked water-absorbent polymer particles is added to the polymer gel or the polymer gel particles or both. Hence, the fine surface-crosslinked water-absorbent polymer particles may be added prior to the first comminuting step, during the first comminuting step, or after the first comminuting step. Moreover, the fine surface-crosslinked water-absorbent polymer particles may be added prior to the further comminuting step, during the further comminuting step, or after the further comminuting step. In general, the fine surface-crosslinked water- absorbent polymer particles may be added prior to the comminuting, during, or after the comminuting.

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

The amount of fine surface-crosslinked water-absorbent polymer particles and fine non-surface-crosslinked water-absorbent polymer particles that may be added in process step (vi) 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 polymer gel or the polymer gel particles or both to which the fines are added.

In process step (vii) of the process according to the present invention the polymer gel particles are dried.

The drying of the polymer gel particles can be effected in any dryer or oven the person skilled in the art considers as appropriate for drying 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 dryer depends on the product to be processed, the manufacturing capacity and the drying duty. A belt dryer can be embodied as a single-belt, multi-belt, multi-stage or multistory system. The present invention is preferably practiced using a belt dryer having at least one belt. One-belt dryers are very particularly preferred. To ensure opti- mum performance of the belt-drying operation, the drying properties of the water-absorbent polymers are individually determined as a function of the processing parameters chosen. The hole size and mesh size of the belt is conformed to the product. Similarly, certain surface enhancements, such as electropolishing or Teflonizing, are possible. The polymer gel particles to be dried are preferably applied to the belt ot the belt dryer by means of a swivel belt. The feed height, i.e. the vertical distance between the swivel belt and the belt of the belt dryer, is preferably not less than 10 cm, more preferably not less than 20 cm and most preferably not less than 30 cm and preferably up to 200 cm, more preferably up to 120 cm and most preferably up to 40 cm. The thickness on the belt dryer of the polymer gel particles to be dried is preferably not less than 2 cm, more preferably not less than 5 cm and most preferably not less than 8 cm and preferably not more than 20 cm, more preferably not more than 15 cm and most preferably not more than 12 cm. The belt speed of the belt dryer is preferably not less than 0.005 m/s, more preferably not less than 0.01 m/s and most pref- erably not less than 0.015 m/s and preferably up to 0.05 m/s, more preferably up to 0.03 m/s and most preferably up to 0.025 m/s.

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

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

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

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

A preferred further crosslinker is a surface crosslinker. Preferred further crosslinkers are disclosed in US 201 1/0204288 Al and incorporated herein by reference and in the following. Preferred further crosslinkers are compounds which comprise groups which can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional amido amines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 Al and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 Al , DE 35 23 617 Al and EP 0 450 922A2, or β-hydroxyalkylamides, as described in DE 102 04 938 Al and U.S. Pat. No. 6,239,230. Additionally described as suitable further crosslinkers are cyclic carbonates in DE 40 20 780 CI , 2-oxazolidinone and derivatives thereof, such as 2-hydroxyethyl-2- oxazolidinone, in DE 198 07 502 Al, bis- and poly-2-oxazolidinones in DE 198 07 992 CI , 2-oxotetrahydro-l ,3-oxazine and derivatives thereof in DE 198 54 573 Al, N -acyl-2- oxazolidinones in DE 198 54 574 Al, cyclic ureas in DE 102 04 93 7 Al , bicyclic amido ace- tals in DE 103 34 584 Al , oxetanes and cyclic ureas in EP 1 199 327 A2 and morpholine-2,3- dione and derivatives thereof in WO 2003/031482 Al. Preferred further crosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1 ,4-butanediol. Very particularly preferred further crosslinkers are 2-hydroxyethyl-2 -oxazolidinone, 2-oxazolidinone and 1,3- propanediol. In addition, it is also possible to use further crosslinkers which comprise addi- tional polymerizable ethylenically unsaturated groups, as described in DE 3713 601 Al .

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

In a preferred embodiment of the present invention, polyvalent cations are applied to the surfaces of the ground water-absorbent polymer particles in addition to the crosslinking composition before, during or after the contacting with the crosslinking composition. The polyvalent cations usable in the process according to the invention are, for example, divalent cations such as the cations of zinc, magnesium, calcium, iron and strontium, trivalent cations such as the cations of aluminum, iron, chromium, rare earths and manganese, tetravalent cati- ons such as the cations of titanium and zirconium. Possible counterions are chloride, bromide, sulfate, hydro gensulfate, carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphos- phate, dihydrogenphosphate and carboxylate, such as acetate, citrate and lactate. Aluminium sulfate and aluminum lactate are preferred. Apart from metal salts, it is also possible to use polyamines as polyvalent cations. The amount of polyvalent cation used is, for example, 0.001 to 1.5 wt.-%, preferably 0.005 to 1 wt.-%, more preferably 0.02 to 0.8 wt.-%, based in each case on the ground water-absorbent polymer particles. The contacting with the crosslinking composition is typically performed in such a way that the crosslinking composition is sprayed as a solution onto the ground water-absorbent polymer particles. In process step (ix) after the contacting, the polymer particles with the crosslinking composition are preferably dried thermally. The surface-crosslinked water-absorbent polymer particles may be obtained during drying or after the drying or both. The contacting with the crosslinking composition is preferably performed in a mixer with moving mixing tools, such as a screw mixer, a disc mixer, and a paddle mixer. Therein, a preferred mixer is a horizontal mixer or a vertical mixer or both. Particularly preferred is a horizontal mixer. A mixer is horizontal if a mixing tool of the mixer rotates around a horizontal axis. A mixer is vertical it a mixing tool ot the mixer rotates around a vertical axis. Suitable mixers are, for example, horizontal Ploughshare® Mixers (Gebriider Lodige Maschinenbau GmbH, Paderborn, Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BY, Doetinchem, the Netherlands), Processall Mixmill mixers (Processall Incorporated, Cincilmati, US), Schugi Flexomix® (Hosokawa Micron BY, Doetinchem, the Netherlands), and particularly preferred a high-performance ringlayer mixer CoriMix® CM 350 by Gebriider Lodige Maschinenbau GmbH, Paderborn, Germany. However, it is also possible to perform the contacting by spraying the crosslinking composition in a fluidized bed. The crosslinking composition is typically used in the form of an aqueous so- lution. The penetration depth of the crosslinking composition into the polymer particles can be adjusted via the content of non-aqueous solvent and total amount of solvent. If exclusively water is used as the solvent, a surfactant is preferably added. This improves the wetting behavior and reduces the tendency to form lumps. However, preference is given to using solvent mixtures, for example isopropanol/water, 1 ,3-propanediol/water and propylene glycol/water, where the mixing ratio in terms of mass is preferably from 20:80 to 40:60.

The temperature of the water-absorbent polymer particles in the mixer is preferably from 100 to 250°C, more preferably from 1 10 to 220°C, most preferably from 120 to 160°C. The residence time in the mixer is preferably from 10 to 120 minutes, more preferably from 10 to 90 minutes, most preferably from 30 to 60 minutes. The fill level of the mixer is prefer- ably from 30 to 80 %, more preferably from 40 to 75 %, most preferably from 50 to 70 %. The fill level of the mixer can be adjusted via a height of an overflow weir. Subsequently, the surface-crosslinked water-absorbent polymer particles are preferably sized again, excessively small and/or excessively large polymer particles being separated.

To further improve properties, the surface-crosslinked water-absorbent polymer parti- cles can be coated or remoisturized. The remoisturizing is preferably performed at 30 to 80°C, more preferably at 35 to 70°C, most preferably at 40 to 60°C. At excessively low temperatures, the surface-crosslinked water-absorbent polymer particles tend to form lumps, and at higher temperatures water already evaporates to a noticeable degree. The amount of water used for remoisturizing is preferably from 1 to 10 wt.-%, more preferably from 2 to 8 wt.-% and most preferably from 3 to 5 wt.-%, based on the weight of the surface-crosslinked water- absorbent polymer particles. The remoisturizing increases the mechanical stability of the polymer particles and reduces their tendency to static charging. Suitable coatings for improving the swell rate and the permeability (SFC) are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers and di- or polyvalent metal cations. Suitable coatings for dust binding are, for example, polyols. Suitable coatings for counteracting the undesired caking tendency of the polymer particles are, for example, fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20. The surface-crosslinked water-absorbing polymer particles produced by the process according to the invention have a moisture content of preferably 0 to 15 wt.-%, more preferably 0.2 to 10 wt.-% and most preferably 0.5 to 8 wt.-%.

In am embodiment of the invention the sequence of process steps (i) to (ix) is repeated at least one time, preferably at least two times, more preferably at least three times, more preferably at least 5 times, more preferably at least 10 times, most preferably the process is a continuous process, wherein the sequence of the process steps (i) to (ix) or (i) to (x) is repeated as cycles of the continuous process.

In an embodiment of the invention in process step (vi) comminuting the polymer gel comprises a first comminuting step in a first comminuting device and subsequently a further comminuting step in a further comminuting device; wherein the plurality of the fine surface- crosslinked water-absorbent polymer particles is fed into the further comminuting device in process step (vi). A preferred further comminuting device is a mincer, such as a meat grinder.

In an embodiment of the invention the process comprises a process step (x) of subjecting the surface-crosslinked water-absorbent polymer particles to a first sizing step; thereby obtaining the plurality of the fine surface-crosslinked water-absorbent polymer particles. Preferably, the first sizing step is performed by a first sizing device. A preferred first sizing device is an appropriate sieve, such as for example a tumbler sieve.

In process step (x) of the process according to the present invention the surface- crosslinked water-absorbent polymer particles are sized, preferably using appropriate sieves. In this context it is particularly preferred that after sizing the surface-crosslinked water- absorbent polymer particles the content of polymer particles, excluding the plurality of fine surface-crosslinked water-absorbent polymer particles, that have a particle size of less than 150 μιη is less than 10 wt.-%, preferably less than 8 wt.-% and particularly less than 6 wt.-% and that the content of polymer particles having a particle size of more than 850 μιη is also less than 10 wt.-%, preferably less than 8 wt.-% and particularly preferably less than 6 wt.-%. It is also preferred that after sizing the surface-crosslinked water-absorbent polymer particles at least 30 wt.-%, more preferred at least 40 wt.-% and most preferred at least 50 wt.-% of the particles have a particle size in a range of from 300 to 600 μιτι.

In an embodiment of the invention prior to adding the plurality of the fine surface- crosslinked water-absorbent polymer particles to the polymer gel or the polymer gel particles or both in process step (vi) the fine surface-crosslinked water-absorbent polymer particles of the plurality are contacted with water in an amount in the range of from 65 to 150 wt.-%, preferably from 70 to 140 wt.-%. more preferably from 75 to 130 wt.-%, more preferably from 80 to 120 wt.-%, more preferably from 85 to 1 10 wt.-%, most preferably from 90 to 100 wt.-%, based on the weight of the plurality of the fine surface-crosslinked water-absorbent polymer particles in a fines mixing device.

In an embodiment of the invention in process step (vi) a plurality of fine non-surface- crosslinked water-absorbent polymer particles is added to the polymer gel or the polymer gel particles or both. Fine non-surface-crossjinked water-absorbent particles are preferably water- absorbent polymer particles, wherein it is preferred that at least 90 wt.-% of the fine non- surface-crosslinked water-absorbent particles, preferably at least 95 wt.-% of the fine non- surface-crosslinked water-absorbent particles and most preferred at least 99 wt.-% of the fine non-surface-crosslinked water-absorbent particles have a particle size of less than 200 μπι, preferably less than 150 μιη and particular preferably less than 100 μιη.

In an embodiment of the invention in process step (vi) comminuting the polymer gel comprises a first comminuting step in a first comminuting device and subsequently a further comminuting step in a further comminuting device; wherein the plurality of the fine non- surface-crosslinked water-absorbent polymer particles is fed into the further comminuting device in process step (vi). A preferred further comminuting device is a mincer, such as a meat grinder.

In an embodiment of the invention in process step (viii) the water-absorbent polymer particles are subjected to a further sizing step, thereby obtaining the plurality of the fine non- surface-crosslinked water- absorbent polymer particles. Preferably, the further sizing step is performed by a further sizing device. A preferred further sizing device is an appropriate sieve, such as for example a tumbler sieve, a vibrating sieve and a sieve comprising a rectangular tilted screen.

In an embodiment of the invention prior to adding the plurality of the fine non- surface-crosslinked water-absorbent polymer particles to the polymer gel or the polymer gel particles or both in process step (vi) the plurality of the fine non-surface-crosslinked water- absorbent polymer particles is contacted with water in an amount of 65 to 150 wt.-% , preferably from 70 to 140 wt.-%. more preferably from 75 to 130 wt.-%, more preferably from 80 to 120 wt.-%, more preferably from 85 to 1 10 wt.-%, most preferably from 90 to 100 wt.-%, based on the weight of the plurality of the fine non-surface-crosslinked water-absorbent polymer particles in a fines mixing device. In an embodiment of the invention the fines mixing device comprises a tluidized bed of the fine non-surface-crosslinked water-absorbent polymer particles or the fine surface- crosslinked water- absorbent polymer particles or both. Therein, a preferred fluidized bed is mechanically generated by an action of mixing tools. Preferably, mixing tools of the fines mixing device rotate close to a wall of a mixing drum of the fines mixing device, thereby lifting fines from a product bed into an open mixing area and scooping material from the wall of the mixing drum counteracting a centrifugal force. A preferred fines mixing device comprises a plurality of mixing blades or chopping tools or preferably both. Another preferred fines mixing device comprises a horizontally aligned rotating mixing shaft. Another preferred fines mixing device comprises a horizontal mixing drum. A particularly preferred fines mixing device is a Continuous Ploughshare ® Mixer, preferably of type KM 600 DW, by Gebriider Lo- dige Maschinenbau GmbH, Paderborn, Germany.

In an embodiment of the invention a weight ratio of the plurality of the fine surface- crosslinked water-absorbent polymer particles to the plurality of the fine non-surface- crosslinked water-absorbent polymer particles being added to the polymer gel or the polymer gel particles or both in process step (vi) is in the range of from 0.3 : 0.7 to 0.7 : 0.3, preferably from 0.4 : 0.6 to 0.6 : 0.4, more preferably from 0.45 : 0.55 to 0.55 to 0.44, most preferably from 0.48 : 0.52 to 0.52 : 0.48. Therein, the weight ratio is based on the fines prior to an optional contacting with water.

In an embodiment of the invention in process step (ix) the crosslinking composition further comprises a reducing agent or a poly alkylene glycol or both. A preferred poly al- kylene glycol is a poly ethylene glycol. A preferred poly ethylene glycol has a molecular weight in the range of from 50 to 1000, preferably from 150 to 750, more preferably from 200 to 500, most preferably from 350 to 450.

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

In an embodiment of the invention in process step (ix) the contacting is performed in a first mixing device, wherein the first mixing device comprises a rotating shaft and a plurality of mixing tools, wherein the mixing tools are connected to the rotating shaft, wherein in process step (ix) the rotating shaft rotates at a speed in the range of from 500 to 1200 rpm, preferably from 550 to 1200, more preferably from 600 to 1200, more preferably from 650 to 1200, more preferably from 700 to 1200, more preferably from 750 to 1 150, more preferably from 800 to 1 100, more preferably from 850 to 1050, most preferably from 900 to 1000. In an embodiment of the invention in process step (ιχ) the contacting is pertormed in a first mixing device, wherein the first mixing device comprises a plurality of mixing tools, wherein the mixing tools are rods or paddles or both. A preferred plurality of mixing tools comprises rods and paddles.

In an embodiment of the invention in process step (ix) the contacting is performed in a first mixing device, wherein the first mixing device comprises an annular layer of the at least part of the water-absorbent polymer particles. A preferred annular layer is shaped by a centrifugal force or a wall of a mixing chamber or preferably both.

In an embodiment of the invention in process step (ix) the contacting is a mixing by a plurality of contacts of the at least part of the water-absorbent polymer particles to a rotating component, wherein the mixing is performed for a duration in the range of from 0.1 to 60 seconds, preferably from 0.1 to 55 seconds, more preferably from 0.1 to 40 seconds, more preferably from 0.1 to 30 seconds, more preferably from 0.1 to 20 seconds, most preferably from 0.1 to 10 seconds. Therein, the contact of the water-absorbent polymer particles to the rotating component is preferably made a plurality of times during the duration of the mixing.

In an embodiment of the invention the polymer gel being discharged in process step (vi) 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 (vi) 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 (vi) 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 (v) is performed in presence of a blowing agent. The blowing agent may be added in one selected from the group consisting of step (i), step (ii), step (iii), step (iv), and step (v), 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 (v) is initiated. Particularly preferably, the blowing agent is added to the monomer solution after or simultaneously to adding the initiator or a component of an initiator system. Preferably the blowing agent is added to the monomer solution in an amount in the range of from 500 to 4000 ppm by weight, preferably from 1000 to 3500 ppm by weight, more preferably from 1500 to 3200 ppm by weight, most preferably from 2000 to 3000 ppm by weight, based on the total weight of the monomer solution.

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

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

A preferred nitrite is ammonium nitrite. A preferred peroxide is hydrogen peroxide. A preferred aromatic azo compound is one selected from the group consisting of a triazene, ar- ylazosulfones, arylazotriarylmethanes, a hydrazo compound, a diazoether, and diazoamino- benzene, or a combination of at least two thereof. A preferred hydrazine is phenylhydrazine. A preferred azide is a carbonyl azide or a sulfonyl azide or both. A preferred Ν,Ν'- Dinitro- soamide is N,N'-dimethyl-N,N'-dinitrosoterephthalamide.

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

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

b) a further container, designed to take at least one crosslinker (a3); c) a further mixing device, wherein the further mixing device is

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

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

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

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

e) a first comminuting device, wherein the first comminuting device

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

ii) is designed to comminute the polymer gel;

f) a further comminuting device, wherein the further comminuting device i) is located down-stream to the first comminuting device, ii) is designed to comminute the polymer gel to which a plurality of fine surface-crosslinked water-absorbent polymer particles has been added, thereby obtaining polymer gel particles;

g) a belt dryer, wherein the belt dryer is

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

h) 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;

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

i) located down-stream to the grinding device,

ii) designed to contact at least a part of the water- absorbent polymer particles with a crosslinking composition, comprising a further crosslinker, thereby obtaining surface-crosslinked water-absorbent polymer particles;

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

i) located down-stream to the first mixing device, ii) designed to size the surface-crosslinked water-absorbent polymer particles, thereby obtaining the plurality of the fine surface- crosslinked water-absorbent polymer particles.

Therein, the further mixing device may be identical to the polymerization reactor. Moreover, the polymerization reactor may be identical to the first comminuting device. Hence, the further mixing device, the polymerization reactor, and the first comminuting device may be identical. Preferred components or devices or both of the device according to the invention are designed according to the process according to the invention. A preferred first mixing device is the first mixing device according to the process according to the invention. A preferred further mixing device is the further mixing device according to the process according to the invention. A preferred first comminuting device is the first comminuting device according to the process according to the invention. A preferred further comminuting device is the further comminuting device according to the process according to the invention. A preferred first sizing device is the first sizing device according to the process according to the invention, n. A preferred crosslinking composition is the crosslinking composition according to the process according to the invention. In an embodiment of the invention the device further comprises a further sizing device, wherein the further sizing device is

a) located down-stream to the grinding device,

b) designed to size the water-absorbent polymer particles, thereby obtaining a plurality of fine non-surface-crosslinked water- absorbent polymer particles;

wherein the further comminuting device is further designed to comminute the polymer gel to which the plurality of the fine surface-crosslinked water-absorbent polymer particles and the plurality of the fine non-surface-crosslinked water-absorbent polymer particles have been added, thereby obtaining polymer gel particles. A preferred further sizing device is the further sizing device according to the process according to the invention.

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

A contribution to the solution of at least one of the above objects is provided by a surface-crosslinked water-absorbent polymer particle, obtainable by the process according to the invention. A 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 Si0 2 in an amount in the range of from 500 to 3,000 ppm by weight, preferably from 1 ,000 to 2,000 ppm by weight;

each based on the weight of the plurality of surface-crosslinked water-absorbent polymer particles. 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 surface-crosslinked water-absorbent polymer particle according to the invention.

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

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

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

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

The retention capacity of water-absorbent polymer particles is measured according to a standard test method for superabsorbent materials defined by the ED ANA. Said test method is described in ED AN A, Harmonized Test Methods Nonwovens and Related Industries, 2012 Edition as "Fluid Retention Capacity in Saline, After Centrifugation" under the method number WSP 241.2.R3 (12).

[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- aery late monomer solution is degased with nitrogen.

B) Polymerization of the monomer solution

1 wt.-part of the monomer solution prepared in step A) is mixed with 0.001 wt.-parts of trimethylol propane triacrylate as crosslinker, 0.001 wt.-parts of sodium peroxodisulfate as first initiator component, 0.000034 wt.-parts of 2,2-dimethoxy-l,2-diphenylethan-l-one (Ci- ba ® Irgacure ® 651 by Ciba Specialty Chemicals Inc., Basel, Switzerland) as a second initiator component, up to 0.1 wt.-parts of acrylic acid particles (with a particle size of less than 150 μηι) 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 ot a conveyer belt reactor and the polymerization is initiated by UV radiation. The conveyor belt has a length of at least 20 m and a width of 0.8 m. The conveyor belt is formed as a trough to keep the solution on the belt prior to and while being polymerized. The dimensions of the conveyor belt and the conveying speed of the conveyer belt are selected in a way that a poly-acrylic acid gel is formed at a downstream end of the belt. At the end of this step a water-absorbent polymer gel is achieved. The polymer gel has a water content of about 52 wt.-%, based on the total weight of the polymer gel.

C) Comminuting and drying of the polymer gel

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

cl) The polymer gel is cut into a plurality of gel strands by a crusher as shown in figure 12. The gel strips have a length in the range from 10 to 20 cm and a width in the range of from 30 to 50 mm, then

c2) a mincer according to figure 13 a) and 13 b) is used to shred the strands into gel particles having a width in the range from 12 to 14 mm.

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

D) Milling and sizing

The dried polymer gel is ground in three steps. First the dried polymer gel is fed through a Herbold Granulator HGM 60/145 (HERBOLD Meckesheim GmbH) 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 μιη. 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 tne invention are nave a panicle size of more than 850 μιη. The obtained water-absorbent polymer particles are named precursor I.

Therein, water-absorbent polymer particles having particle diameters of less than 150 μηι are separated and referred to as fine non-surface-crosslinked water absorbent polymer particles. Said fine non-surface-crosslinked water absorbent polymer particles are used in the comparative examples and examples in table 1 below if fine non-surface-crosslinked water- absorbent polymer particles are recycled. E) Silicon dioxide treatment

In a treatment step the precursor I is mixed in a disc mixer with about 0.01 wt.-part (+- 10 %) of silicon dioxide (Si0 2 ), based on the total weight of the precursor I plus Si0 2 . The silicon dioxide is used in form of Sipernat ® 22 obtainable from Evonik Industries AG, Essen, Germany. When mixing the precursor I with the Si0 2 , the precursor still has a temperature 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.-% Na 2 S0 3 , 40 wt.-% poly ethylene glycol with a molecular weight of 400 g/mol, each based on the total amount of the crosslinker. The ingredients of the crosslinker are mixed in a line static mixer. The crosslinker is mixed in a ringlayer mixer CoriMix ® CM 350 (Ge- bruder Lodige Mascheninenbau GmbH, Paderborn, Germany) with precursor II. The mixture is heated to a temperature in the range of from 130 to 160 °C. The mixture is then dried in a paddle dryer Andritz Gouda Paddle Dryer, preferably of type GPWD12W120, by Andritz AG, Graz, Austria for 45 minutes at a temperature in the range of from 130 to 160°C. Surface- cross-linked absorbent polymer particles are obtained.

In a cooling device in the form of a fluid bed, the temperature of the surface-cross- linked absorbent polymer particles is decreased to below 60 °C, obtaining cooled surface- cross-linked absorbent polymer particles referred as to precursor III.

G) Post treatment

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

The fine surface-crosslinked water absorbent polymer particles obtained here are used in the comparative examples and examples in table 1 below if fine surface-crosslinked water- absorbent polymer particles are recycled.

The following scale is used to compare the results of measuring the parameters given in table 1 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 : Impact of fines recycling on process parameters and performance of surface- crosslinked water-absorbent polymer particles produced thereby.

In the comparative example 1 no fines are recycled. In the comparative example 2 fine non-surface-crosslinked water absorbent polymer particles are recycled into the monomer solution on the polymerization belt. As said recycled fines occupy some space one the belt, the reactor capacity for the reactants is reduced with respect to the comparative example 1. However, recycling fines increases the overall yield of the production process with respect to a produced amount of surface-crosslinked water-absorbent polymer particles in the desired particle size range of from 150 to 850 μιη. Surprisingly, recycling lines into me monomer solution increases the amount of fine non-surface-crosslinked water absorbent polymer particles produced in the process. In the comparative example 3 in addition to the fine non- surface-crosslinked water absorbent polymer particles also fine surface-crosslinked water ab- 5 sorbent polymer particles are recycled into the monomer solution. The reactor capacity is reduced even more, the overall yield is increased even more, and even more fine non-surface- crosslinked water absorbent polymer particles are produced. Recycling all the fine non- surface-crosslinked water absorbent polymer particles again and again may lead to a "fines catastrophe" and hence to a drastic decrease of the overall yield of the process. So an int o creased production of fine non-surface-rcrosslinked water absorbent polymer particles is undesirable.

Instead, in the example 1 according to the invention fine surface-crosslinked water absorbent polymer particles are recycled into the polymer gel. Prior to adding said fines to the gel, the fine surface-crosslinked water absorbent polymer particles are mixed with water in an

15 amount of 95 wt.-% based on the fine surface-crosslinked water absorbent polymer particles in a Continuous Ploughshare ® Mixer of type KM 600 DW by Gebriider Lodige Maschinenbau GmbH, Paderborn, Germany. The obtained wetted fine surface-crosslinked water absorbent polymer particles are recycled into the further comminuting device (mincer) of step C) c2) above. Here, the reactor capacity is not reduced, the overall process yield is increased due to 0 the recycling, and surprisingly the amount of fine non-surface-crosslinked water absorbent polymer particles produced is decreased. Additionally, recycling fine surface-crosslinked water absorbent polymer particles into the polymer gel in example 2 increases the overall yield even more. Therein, the fine non-surface-crosslinked water absorbent polymer particles are wetted with water prior to adding to the polymer gel as described above for the fine surface- 5 crosslinked water absorbent polymer particles. All the other parameters are constant with respect to example 1. In all the examples and comparative examples the retention capacity of the surface-crosslinked water-asborbent polymer particles produced is approximately constant. In conclusion, the examples 1 and 2 according to the invention show a superior combination of: a reduced amount of fine non-surface-crosslinked water absorbent polymer particles pro- 0 duced, a high reactor capacity, and an increased overall yield of the process.

Figure 1 shows a flow chart diagram depicting the steps 101 to 109 of a process 100 for the preparation of surface-crosslinked water-absorbent polymer particles 610 according to the invention. In a first step 101 an aqueous monomer solution comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups ( l) and at least one crosslinker (a3) is provided. Preferabl y , -A

aqueous solution of partially neutralized acrylic acid, further comprising crosslinkers. In a second step 102 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 third step 103 the oxygen content of the aqueous monomer solution is decreased by bubbling nitrogen into the aqueous monomer solution. In a fourth step 104 the monomer solution is charged onto a belt of a polymerization belt reactor as a polymerization reactor 904. The belt is an endless conveyor belt. In a fifth step 105 the aqueous monomer solution is polymerized to a polymer gel 601. In a sixth step -106 the polymer gel 601 is discharged from the belt. Subsequently, the polymer gel 601 is comminuted, whereby polymer gel particles 604 are obtained. Further in the sixth step 106 a plurality of fine surface-crosslinked water- absorbent polymer particles 61 1 is added to the polymer gel 601. In a seventh step 107 the polymer gel particles 604 are charged onto a belt of a belt dryer 605 and subsequently dried at a temperature of about 120 to 150°C. The dried polymer gel particles 606 are discharged from the belt dryer and subsequently in an eighth step 108 ground to obtain water-absorbent polymer particles 608. The ground water-absorbent polymer particles 608 are contacted with a crosslinking composition 612, comprising a further crosslinker, thereby obtaining surface- crosslinked water- absorbent polymer particles 610 in a nineth step 109.

Figure 2 shows a flow chart diagram depicting the steps 101 to 109 of another process 100 for the preparation of surface-crosslinked water-absorbent polymer particles 610 according to the invention. The process 100 shown in figure 2 is the same as the process 100 in figure 1 , wherein the second process step 102 and the third process step 103 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 , 102, 104 to 109 of another process 100 for the preparation of surface-crosslinked water-absorbent polymer particles 610 according to the invention. The process 100 shown in figure 3 is the same as the process 100 in figure 1 , wherein the third step 103 is not part of the process 100 according to figure 3.

Figure 4 shows a flow chart diagram depicting the steps 101 to 109 of another process 100 for the preparation of surface-crosslinked water-absorbent polymer particles 610 according to the invention. The process 100 shown in figure 4 is the same as the process 100 in figure 1 , wherein the process 100 of figure 4 is a continuous process, wherein the sequence of the process steps 101 to 109 is repeated as cycles of the continuous process 100. Figure 5 shows a flow chart diagram depicting tne steps ι υι to ι ι υ or anotner process 100 for the preparation of surface-crosslinked water- absorbent polymer particles 610 according to the invention. The process 100 shown in figure 5 is the same as the process 100 in figure 4, wherein the process 100 of figure 5 further comprises as part of the sequence a tenth step 1 10 of subjecting the surface-crosslinked water- absorbent polymer particles 610 to a first sizing step; thereby obtaining the plurality of the fine surface-crosslinked water-absorbent polymer particles 61 1 which is added to the polymer gel 601 in the sixth step 106.

Figure 6 shows a flow chart diagram of a sequence of process steps (vi) 106 to (ix) 109 of a process 100 according to the invention. The polymer gel 601 is provided by polymerization of monomers in a monomer solution in a polymerization reactor 904 which is a kneading reactor. The kneading reactor is also a first comminuting device 602. Subsequently, the polymer gel 601 is discharged from the kneading reactor and fed into a further comminuting device 603, which is a mincer. In the further comminuting device 603 a plurality of fine surface-crosslinked water-absorbent polymer particles 61 1 is added to the polymer gel 601 and to the polymer gel particles 604 obtained by comminuting the polymer gel 601. Subsequently, the polymer gel particles 604 are introduced into a belt dryer 605 and dried, thereby obtaining dried polymer gel particles 606. The dried polymer gel particles 606 are discharged from the belt dryer 605 and fed into a grinding device 607 which is a Herbold Granulator (type SML 60/145-SX7-2 by Herbold Meckenheim GmbH, Meckenheim, Germany). By grinding the dried polymer gel particles 606, water-absorbent polymer particles 608 are obtained. The water-absorbent polymer particles 608 are contacted with a crosslinking composition 612, comprising a further crosslinker, in a first mixing device 609, thereby obtaining surface- crosslinked water-absorbent polymer particles 610.

Figure 7 shows a flow chart diagram of another sequence of process steps (vi) 106 to (x) 1 10 of a process 100 according to the invention. The process 100 is a continuous process, wherein a sequence of process steps (i) 101 to (x) 1 10 is repeated as cycles of the continuous process 100. The polymer gel 601 obtained on a conveyor belt as a polymerization reactor 904 is fed into a first comminuting device 602 which comminutes the polymer gel 601 by contact to a plurality of counter rotating toothed wheels. Subsequently, the polymer gel 601 is fed into a further comminuting device 603, which is a mincer. In the further comminuting device 603 a plurality of fine surface-crosslinked water-absorbent polymer particles 61 1 is added to the polymer gel 601 and to the polymer gel particles 604 obtained by comminuting the polymer gel 601. Subsequently, the polymer gel particles 604 are introduced into a belt dryer 605 and dried, thereby obtaining dried polymer gel particles 606. The dried polymer gel particles 606 are discharged from the belt dryer 605 and ted into a grinding device bU / which is a Bauermeister Roll Crusher (type SWR 350.1 x 1800, 3-stage crusher by Bauermeister Zerkleinerungstechnik GmbH, Norderstedt, Germany). By grinding the dried polymer gel particles 606, water-absorbent polymer particles 608 are obtained. The water-absorbent poly- mer particles 608 are contacted with a crosslinking composition 612, comprising a further crosslinker, in a first mixing device 609, thereby obtaining surface-crosslinked water- absorbent polymer particles 610. The first mixing device 609 is high-performance ringlayer mixer CoriMix® CM 350 by Gebriider Lodige Maschinenbau GmbH, Paderborn, Germany. The surface-crosslinked water- absorbent polymer particles 610 are subjected to a first sizing step in a first sizing device 701 which is a sieve. Thereby, the plurality of fine surface- crosslinked water-absorbent polymer particles 61 1 is obtained.

Figure 8 shows a flow chart diagram of another sequence of process steps (vi) 106 to (x) 1 10 of a process 100 according to the invention. The process 100 is a continuous process, wherein a sequence of process steps (i) 101 to (x) 1 10 is repeated as cycles of the continuous process 100. The polymer gel 601 obtained on a conveyor belt as a polymerization reactor 904 is fed into a first comminuting device 602 which comminutes the polymer gel 601 by contact to a plurality of counter rotating toothed wheels. Subsequently, the polymer gel 601 is fed into a further comminuting device 603, which is a mincer. In the further comminuting device 603 a plurality of fine surface-crosslinked water-absorbent polymer particles 61 1 and a plurality of fine non-surface-crosslinked water-absorbent polymer particles 802 are added to the polymer gel 601 and to the polymer gel particles 604 obtained by comminuting the polymer gel 601. Subsequently, the polymer gel particles 604 are introduced into a belt dryer 605 and dried, thereby obtaining dried polymer gel particles 606. The dried polymer gel particles 606 are discharged from the belt dryer 605 and fed into a grinding device 607 which is a Bau- ermeister Roll Crusher (type SWR 350.1 x 1800, 3-stage crusher by Bauermeister Zerkleinerungstechnik GmbH, Norderstedt, Germany). By grinding the dried polymer gel particles 606, water-absorbent polymer particles 608 are obtained. The water- absorbent polymer particles 608 are subjected to a further sizing step in a further sizing device 801 which is an appropriate vibrating sieve. Thereby, the plurality of the fine non-surface-crosslinked water- absorbent polymer particles 802 is obtained. The water- absorbent polymer particles 608, which have not been separated as part of the plurality of the fine non-surface-crosslinked water-absorbent polymer particles 802, are contacted with a crosslinking composition 612, comprising a further crosslinker, in a first mixing device 609, thereby obtaining surface- crosslinked water-absorbent polymer particles 610. The first mixing device 609 is high- performance ringlayer mixer CoriMix® CM 350 by Georuuer j_,ouige iviascnmenoau urnon, Paderborn, Germany. The surface-crosslinked water-absorbent polymer particles 610 are subjected to a first sizing step in a first sizing device 701 which is a sieve. Thereby, the plurality of fine surface-crosslinked water-absorbent polymer particles 61 1 is obtained. Prior to adding the plurality of fine surface-crosslinked water-absorbent polymer particles 61 1 and the plurality of fine non-surface-crosslinked water-absorbent polymer particles 802 to the polymer gel 601 and to the polymer gel particles 604 said plurality of fine surface-crosslinked water- absorbent polymer particles 61 1 and said plurality of fine non-surface-crosslinked water- absorbent polymer particles 802 are mixed with water in a 1 : 1 weight ratio in a Continuous Ploughshare ® Mixer of type KM 600 DW by Gebriider Lodige Maschinenbau GmbH, Paderborn, Germany.

Figure 9 shows a block diagram of a device 900 for the preparation of surface- crosslinked water-absorbent polymer particles 610 according to the invention. The arrows show a direction of a process stream 905 of the preparation of the surface-crosslinked water- absorbent polymer particles 610. The device 900 comprises a first container 901 , a further container 902, downstream a further mixing device 903, downstream a polymerization belt reactor as a polymerization reactor 904, downstream a first comminuting device 602, downstream a further comminuting device 603, downstream a belt dryer 605, downstream a grinding device 607, downstream a first mixing device 609, and downstream a first sizing device 701 , each according to the invention.

Figure 10 shows a block diagram of another device 900 for the preparation of surface- crosslinked water-absorbent polymer particles 610 according to the invention. The arrows show a direction of a process stream 905 of the preparation of the surface-crosslinked water- absorbent polymer particles 610. The device 900 comprises a first container 901 , a further container 902, downstream a further mixing device 903, downstream a polymerization belt reactor as a polymerization reactor 904, downstream a first comminuting device 602, downstream a further comminuting device 603, downstream a belt dryer 605, downstream a grinding device 607, downstream a further sizing device 801 , downstream a first mixing device 609, and downstream a first sizing device 701, each according to the invention.

Figure 1 1a) shows a scheme of a longitudinal cross section of a first mixing device

609 according to the invention. The first mixing device 609 comprises an inlet 1 101, a mixing chamber 1 102 which is limited by a mixing chamber wall 1 103, and an outlet 1 104. The ground water-absorbent polymer particles 608 are fed via the inlet 1 101 into the mixing chamber 1 102. Therein, a rotating shaft 1 107 with mixing tools 1 108 (not shown in figure 11a)) rotate at a speed in the range of from 500 to 120u rpm. uuc iu a ucmruugai lurce me polymer particles distribute over the mixing chamber wall 1 103, thereby forming an annular layer 1 105 of the ground water-absorbent polymer particles 608. A cross section at the axial position 1 106 of the mixing chamber 1 102 is shown in figure l ib). The first mixing device 609 is a high-performance ringlayer mixer CoriMix® CM 350 by Gebruder Lodige Maschi- nenbau GmbH, Paderborn, Germany.

Figure l ib) shows a scheme of a transversal cross section of the first mixing device 609 in figure 11a). The transversal cross section is taken at the axial position 1 106 is figure 1 la). Figure 1 lb) additionally shows the rotating shaft 1 107 and one of a plurality of mixing tools 1 108. The mixing tool 1 108 is a paddle.

Figure 12 shows a scheme of a first comminuting device 602 according to the invention. The first comminuting device 602, a crusher, comprises a plurality of toothed wheels 1201. A first portion of the toothed wheels 1202 rotates around a first axis of rotation 1201 and a further portion of the toothed wheels 1202 rotates around a further axis of rotation 1201. The toothed wheels 1202 of the first portion rotate in counter direction and towards the toothed wheels 1202 of the further portion. A polymer gel 601 being fed between the rotating toothed wheels 1202 is comminuting by the first comminuting device 602 and polymer gel particles 604 are obtained.

Figure 13a) shows a scheme of a further comminuting device 603 according to the in- vention in an external view. The further comminuting device 603 is a mincer ("meat grinder") comprising a static hole plate 1301 , a rotating screw 1302, and a feed unit 1303 for feeding polymer gel particles 604 into the mincer.

Figure 13b) shows a scheme of inner parts the further comminuting device 603 of figure 13a) in an exploded view. The further comminuting device 603 comprises a screw 1302 which rotates together with a rotating hole plate 1304. Thereby, the screw 1302 conveys the polymer gel particles 604 towards the static hole plate 1301 and through the holes of the static hole plate 1301. As the rotating hole plate 1304 rotates with respect to the static hole plate 1301 circular cutting edges 1305 of holes of the rotating hole plate 1304 comminute the polymer gel particles 604 to obtain polymer gel particles (not shown) having a particle diameter which is less.