[Invention Title]

HYDROGEL COMMINUTING DEVICE COMPRISING ROTATING STEEL COMPONENT IN THE PRODUCTION OF WATER-ABSORBENT POLYMER PARTICLES

[Technical Field]

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

[Background Art]

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

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

The polymer gel, also called hydrogel, obtained after the polymerization is usually 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 cross- linking reaction.

The acid-group-carrying monomers can be polymerized in the presence of the crosslinkers in a batch process or in a continuous process. Both in continuous and in batchwise polymerization, partially neutralized acrylic acid is typically used as the monomer. Suitable neutraliza- tion 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 A 1 and WO 2007/028747 Al .

[Disclosure] [Technical Problem] Various comminuting devices for comminuting polymer gels are known in the prior art. Known comminuting devices of the prior art are designed to cut the polymer gel, to shred the polymer gel, or to mince the polymer gel. All these comminuting devices have a lifetime over which they can effectively comminute polymer gels. After a certain amount of polymer gel has been comminuted by the comminuting device of the prior art, the comminuting device has suffered tribological damage, for example by an interaction of parts of the comminuting device with polymer gels to be comminuted or by an interaction of at least two parts of the comminuting device with each other, or corrosion. In consequence, the comminuting device can no longer be operated safely or the quality of comminution by the comminuting device has been lowered due to the tribological or corrosion damage. Such a lower quality of com- minuting can lead to increased drying times of the polymer gel. The tribological or corrosion damage which limits the lifetime of the comminuting device occurs usually at certain components, such as rotating components, of the comminuting device. These components effectively limit the lifetime of the comminuting device. Hence, said components have to be exchanged after a certain lifetime. Such a maintenance operation is expensive and time consuming. Dur- ing maintenance the production of water-absorbent polymers is at least limited to a lower level of productivity. If the maintenance operation is too expensive or no suitable spare parts are available, the comminuting device has to be exchanged completely. [Technical Solution]

Generally, it is an object of the present invention to at least partly overcome a disadvantage arising form the prior art in the context of the production of water-absorbent polymer particles. A further object is to provide a process for the production of water-absorbent polymers, being characterized by a comminuting device with a higher durability. A further object is to provide a process for the production of water-absorbent polymers, being characterized by a comminuting device with an increased operational lifetime. A further object is to provide a process for the production of water-absorbent polymers, being characterized by a comminuting device which needs less maintenance. A further object is to provide a process for the production of water-absorbent polymers, which is less often interrupted or restricted due to maintenance work. A further object is to provide a process for the production of water-absorbent polymers, which is characterized by a decreased drying time of the polymer gel. A further object is to provide a process for the production of water-absorbent polymers, which is characterized by a decreased drying time of the polymer gel, in a dryer such as a belt dryer, or an increased operational lifetime of a comminuting device, or both. A further object is to provide a process for the production of water-absorbent polymers, being characterized by a comminuting device which shows less tribological damage or less corrosion or both due to operation. A further object is to provide a process for the production of water-absorbent polymers, being charac- terized by a comminuting device having a rotating component which comminutes a polymer gel and which has a balanced combination of good mechanical properties, such has tensile strength, and good corrosion resistance. It is a further object of the present invention to provide a device for producing water-absorbent polymer particles by a process having at least one of the above advantages. A further object is to provide water-absorbent polymer particles which have been produced by a less expensive process. It is a further object of the present invention to provide a composite material comprising a water-absorbent polymer particle produced by a process having at least one of the above advantages, wherein the composite material shows no reduction of quality. A contribution to the solution of at least one of the above objects is given by the independent claims. The dependent claims provide preferred embodiments of the present invention which also serve solving at least one of the above mentioned objects.

[Advantageous Effects] A composite material comprising a water-absorbent polymer particle produced by a process according to the invention shows no reduction of quality.

[ Description of Drawings ]

Fig. 1 is a flow chart diagram depicting the steps of a process according to the

invention;

Fig. 2 is a flow chart diagram depicting the steps of another process according to

the invention;

Fig. 3 is a flow chart diagram depicting the steps of another process according to

the invention;

Fig. 4 is a scheme of a comminuting device according to the invention;

Fig. 5a) is a scheme of another comminuting device according to the invention in

an external view;

Fig. 5b) is a scheme of inner parts the comminuting device of figure 5a) in an

exploded view; and

Fig. 6 is a block diagram of a device for the preparation of water-absorbent polymer particles according to the invention.

List of references

100 process according to the invention

101 step (i)

102 step (ii)

103 step (iii)

104 step (iv)

105 step (v)

106 step (vi)

107 step (vii)

108 step (viii)

109 step (ix)

1 10 step (x)

1 1 1 step (xi)

400 comminuting device

401 axis of rotation

402 toothed wheel 403 polymer gel particle

501 static hole plate

502 screw

503 feed unit

504 rotating hole plate

505 circular cutting edge

600 device for the preparation of water-absorbent polymer particles

601 first container

602 further container

603 mixing device

604 polymerization reactor

605 belt dryer

606 grinding device

607 sizing device

608 process stream

[Best Mode]

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

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

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

(iii) adding a polymerization initiator or a at least one component of a polymerization initiator system that comprises two or more components to the aqueous monomer solution;

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

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

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

(vii) discharging the polymer gel out of the polymerization reactor and prior or after the discharging comminuting the polymer gel by a comminuting device, thereby obtaining polymer gel particles; (viii) drying the polymer gel particles;

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

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

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

wherein in process step (vii) the comminuting device comprises a rotating component;

wherein the rotating component comprises, preferably is made of, a component steel;

wherein the component steel comprises as alloying elements

a) 0.1 wt.-% or less, preferably 0.07 wt.-% or less, more preferably 0.05 wt.-% or

less, of C,

b) 0.15 wt.-% or less, preferably 0.1 wt.-% or less, more preferably 0.07 wt.-% or

less, most preferably 0.06 wt.-% or less, of Si,

c) 2 wt.-% or less, preferably 1.7 wt.-% or less, more preferably 1.5 wt.-% or less, most preferably 1.0 wt.-% or less, of Mn,

d) Ni in the range of from 3 to 5 wt.-%, preferably from 3.2 to 4.7 wt.-%. more preferably from 3.5 to 4.5 wt.-%,

e) 0.1 wt.-% or less, preferably 0.07 wt.-% or less, more preferably 0.04 wt.-% or

less, most preferably 0.035 wt.-% or less, of P,

f) 0.05 wt.-% or less, preferably 0.03 wt.-% or less, more preferably 0.015 wt- %

or less, of S,

g) Cr in the range of from 8 to 18 wt.-%, preferably from 10 to 16 wt.-%, more preferably from 12 to 14 wt.-%, most preferably from 12.5 to 14 wt.-%, h) Mo in the range of from 0.1 to 1.0 wt.-%, preferably from 0.2 to 0.8 wt.-%, more preferably from 0.3 to 0.7 wt.-%, most preferably from 0.4 to 0.7 wt.-%, i) 0.01 wt.-% or more, preferably 0.015 wt.-% or more, more preferably 0.02 wt.-% or more, of N,

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

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

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

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

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

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

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

Preferred crosslinkers (oc3) 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 reac- tion or a ring-opening reaction (crosslinker class III), or polyvalent metal cations (cross-linker class IV). Thus with the compounds of crosslinker class I a crosslinking of the polymer is achieved by radical polymerization of the ethylenically unsaturated groups of the crosslinker molecules with the monoethylenically unsaturated monomers (al) or (a2), while with the compounds of crosslinker class II and the polyvalent metal cations of crosslinker class IV a crosslinking of the polymer is achieved respectively via condensation reaction of the functional groups (crosslinker class II) or via electrostatic interaction of the polyvalent metal cation (crosslinker class IV) with the functional groups of the monomer (al) or (a2). With compounds of cross-linker class III a cross-linking of the polymers is achieved correspondingly by radical polymerization of the ethylenically unsaturated groups as well as by conden- sation 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 (cc3) 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, 1ST -methylene bisacrylamide is even more preferred, and as compounds of crosslinker class IV, Al ₂ (S0 ₄ ) ₃ and its hydrates are particularly preferred.

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

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

The aqueous monomer solution may further comprise water-soluble polymers (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 (a4), preferably synthetic, such as polyvinyl alcohol, can not only serve as a graft base for the monomers to be polymerized. It is also conceivable for these water-soluble polymers to be mixed with the polymer gel or the already dried, water-absorbent polymer.

The aqueous monomer solution can furthermore also comprise auxiliary substances (a5), these auxiliary substances including, in particular, complexing agents, such as, for example, EDTA.

The relative amount of monomers (al) and (a2) and of crosslinking agents (oc3) and water- soluble polymers (a4) and auxiliary substances (a5) in the aqueous monomer solution is pref- erably 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 particularly preferably to the extent of 0.01 to 2.5 wt.-% on the crosslinking agents (a3), to the extent of 0 to 30 wt.-%, preferably to the extent of 0 to 5 wt.-% and particularly preferably to the extent of 0.1 to 5 wt.-% on the water-soluble polymers (a4), to the extent of 0 to 20 wt.-%, preferably to the extent of 0 to 10 wt.-% and particularly preferably to the extent of 0.1 to 8 wt.-% on the auxiliary substances (a5), and to the extent of 0.5 to 25 wt.-%, preferably to the extent of 1 to 10 wt.-% and particularly preferably to the extent of 3 to 7 wt.-% on water (a6) the sum of the amounts by weight (al) to (a6) being 100 wt.-%.

Optimum values for the concentration in particular of the monomers, crosslinking agents and water-soluble polymers in the monomer solution can be determined by simple preliminary experiments or from the prior art, in particular from the publications US 4,286,082, DE 27 06 135 Al , US 4,076,663, DE 35 03 458 Al , DE 40 20 780 C I , DE 42 44 548 A1 , _. DE 43 33 056 Al and DE 44 18 818 Al . In process step (ii) fine particles of a water-absorbent polymer may optionally be added to the aqueous monomer solution. Independent of optional step (ii) fine water-absorbent polymer particles may be added to the aqueous monomer solution at one selected from the group consisting of after step (iii), after step (iv), and before step (v), or a combination of at least two thereof.

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

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

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

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

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

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

In process step (iii) of the process according to the present invention a polymerization initiator 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 polymerization 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 polymerization initiators are preferred: azo compounds such as azo- bis-isobutyronitrol, azo-bis-dimethylvaleronitril, azo-bis-ami-dinopropane dihydrochloride, 2,2'-azobis-(N,N-dimethylene)isobutyramidine di-hydrochloride, 2-

(carbamoylazo)isobutyronitrile and 4,4'-azobis-(4-cyano-valeric acid). The aforementioned compounds are used in conventional amounts, preferably in a range from 0.01 to 5 mol-%, more preferably 0.1 to 2 mol-%, respectively based on the amount of the monomers to be polymerized.

Redox catalysts comprise two or more components, usually one or more of the peroxo compounds listed above, and at least one reducing component, preferably ascorbic acid, glucose, sorbose, mannose, ammonium or alkali metal hydrogen sulfite, sulfate, thiosulfate, hyposulfite or sulfide, metal salts such as iron II ions or silver ions or sodium hydroxymethyl sulfoxylate. Preferably ascorbic acid or sodium pyrosulfite is used as reducing component of the redox catalyst. 1 ^χ 10 ^"5 to 1 mol-% of the reducing component of the redox catalyst and 1 x 10 ^"5 to 5 mol-% of the oxidizing component of the redox catalyst are used, in each case referred to the amount of monomers used in the polymerization. Instead of the oxidising component of the redox catalyst, or as a complement thereto, one or more, preferably water- soluble azo compounds may be used. The polymerization is preferably initiated by action of energetic radiation, so-called photo- initiators are generally used as initiator. These can comprise for example so-called a-splitters, H-abstracting systems or also azides. Examples of such initiators are benzophenone derivatives such as Michlers ketone, phenanthrene derivatives, fluorine derivatives, anthraquinone derivatives, thioxanthone derivatives, cumarin derivatives, benzoinether and derivatives thereof, azo compounds such as the above-mentioned radical formers, substituted hexaaryl- bisimidazoles or acylphosphine oxides. Examples of azides are: 2-(N,N-dimethylamino)ethyl- 4-azidocinnamate, 2-(N,N-dimethylamino)ethyl-4-azidonaphthylketone, 2-( ,N-di- methylamino)ethyl-4-azidobenzoate, 5-azido-l -naphthyl-2'-(N,N-dimethylami- no)ethylsulfone, N-(4-sulfonylazidophenyl)maleinimide, N-acetyl-4-sulfonyl-azidoaniline, 4- sulfonylazidoaniline, 4-azidoaniline, 4-azidophenacyl bromide, p-azidobenzoic acid, 2,6- bis(p-azidobenzylidene)cyclohexanone and 2,6-bis(p-azidobenzylidene)-4- methylcyclohexanone. A further group of photo-initiators are di-alkoxy ketales such as 2,2- dimethoxy-l,2-diphenylethan-l-one. The photo-initiators, when used, are generally employed in quantities from 0.0001 to 5 wt.-% based on the monomers to be polymerized.

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

iiia. a peroxodisulfate; and

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

wherein the initiator comprises the peroxodisulfate and the organic initiator molecule in a molar ratio in the range of from 20: 1 to 50: 1. In one aspect of this embodiment it is preferred that the concentration of the initiator component iiia. is in the range from 0.05 to 2 wt.-%, based on the amount of monomers to be polymerized. In another aspect of this embodiment it is preferred that the organic initiator molecule is selected from the group consisting of 2,2- dimethoxy- 1 ,2-diphenylethan- 1 -one, 2,2-azobis-(2-amidinopropane)dihydrochloride, 2,2- azobis-(cyano valeric acid) or a combination of at least two thereof. In a further aspect of this embodiment it is preferred that the peroxodisulfate is of the general formula M ₂ S ₂ C with M being selected from the group consisting of NH ₄ , Li, Na, Ka or at least two thereof. The above described components are in particular suitable for UV initiation of the polymerization in step (vi) of the process of the present invention. Employing this composition further yields low residual monomer and reduced yellowing in the water-absorbent polymer particle, obtainable by the process according to the present invention. In this context it should also be noted that step (iii), adding the polymerization initiator, may be realized before step (iv), simultaneously to step (iv), or overlapping in time with step (iv), i.e. when the oxygen content of the aqueous monomer solution is decreased. If a polymerization initiator system is used that comprises two or more components, one or more of the components of such a polymerization initiator system may, for example, be added before process step (iv), whereas the remaining component or the remaining components which are necessary to complete the activity of the polymerisation initiator system, are added after process step (iv), perhaps even after process step (v). Independent of optional step (iv), decreasing the oxygen content of the aqueous monomer solution may also be performed before process step (iii) according to the invention.

In process step (iv) of the process according to the present invention the oxygen content of the aqueous monomer solution is optionally decreased. Independent of optional step (iv), decreas- ing the oxygen content of the aqueous monomer solution may also be performed before, during or after process step (ii) according to the invention. Preferably, the oxygen content of the aqueous monomer solution is decreased after the fine particles have been added in process step (ii). Whenever the oxygen content of the aqueous monomer solution is decreased, this may be realized by bringing the aqueous monomer solution into contact with an inert gas, such as nitrogen. The phase of the inert gas being in contact with the aqueous monomer solution is free of oxygen and is thus characterized by a very low oxygen partial pressure. As a consequence oxygen converts from the aqueous monomer solution into the phase of the inert gas until the oxygen partial pressures in the phase of the inert gas and the aqueous monomer solution are equal. Bringing the aqueous monomer phase into contact with a phase of an inert gas can be accomplished, for example, by introducing bubbles of the inert gas into the monomer solution in co-current, countercurrent or intermediate angles of entry. Good mixing can be achieved, for example, with nozzles, static or dynamic mixers or bubble columns. The oxygen content of the monomer solution before the polymerization is preferably lowered to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, based on the monomer solution. In process step (v) of the process according to the present invention the aqueous monomer solution is charged into a polymerization reactor, preferably onto a conveyor belt, especially preferred at an upstream position of the conveyor belt and in process step (vi) the monomers in the aqueous monomer solution are polymerized in the polymerization reactor, thereby obtaining a polymer gel. If polymerization is performed on a conveyor belt as the polymerization reactor, a polymer gel sheet is obtained in a downstream portion of the conveyor belt, which, before drying, is 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 (vii) 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 device may be the polymerization reactor or a part of the polymerization reactor, or a separate device, or both. Hence, comminuting the polymer gel may be performed before, during, or after discharging the polymer gel out of the polymerization reactor. A preferred polymerization reactor which is the comminuting device is a kneading reactor. If the comminuting is performed in the polymerization reactor, the polymer gel particles obtained are preferably further comminuted after discharging out of the polymerization reactor. If the polymerization reactor is a conveyor belt, the comminuting is preferably performed after discharging the polymer gel as a polymer gel sheet from the conveyor belt in a comminuting device, wherein the comminuting device is a separate device. Preferably, the polymer gel sheet is discharged from the conveyor belt as a continuous sheet that is of a soft semi-solid consistency and is then passed on for further processing such as comminuting.

Comminution of the polymer gel is preferably performed in at least three steps, wherein the comminuting device according to the invention may be applied in any of the three steps: - in a first step, a cutting unit, preferably a knife, for example a knife as disclosed in WO- A-96/36464, is used for cutting the polymer gel into flat gel strips, preferably with a length within the range of from 5 to 500 mm, preferably from 10 to 300 mm and particularly preferably from 100 to 200 mm, a height within the range of from 1 to 30 mm, preferably from 5 to 25 mm and particularly preferably from 10 to 20 mm as well as a width within the range of from 1 to 500 mm, preferably from 5 to 250 mm and particularly preferably from 10 to 200 mm; in a second step, a shredding unit, preferably a breaker, is used for shredding the gel strips into gel pieces, preferably with a length within the range of 3 to 100 mm, prefera- bly from 5 to 50 mm, a height within the range from 1 to 25 mm, preferably from 3 to

20 mm as well as a width within the range from 1 to 100 mm, preferably from 3 to 20 mm and in a third step a "wolf (grinding) unit, preferably a mincer, preferably having a screw and a hole plate, whereby the screw conveys against the hole plate is used in order to grind and crush gel pieces into polymer gel particles which are preferably smaller than the gel pieces.

The comminuting device according to the invention may comprise one selected from the group consisting of the cutting unit, the shredding unit, and the wolf unit, or a combination of at least two thereof. An optimal surface-volume ratio is achieved by comminuting, which has an advantageous effect on the drying behaviour in process step (viii). 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 kernels. Another preferred comminution of the polymer gel is performed in at least two steps, wherein the comminuting device according to the invention may be applied in any of the two steps:

- in a first step the polymer gel is crushed by a plurality of rotating discs, preferably rotating toothed wheels. Thereby a plurality of polymer gel strands is produced. in a second step a "wolf (grinding) unit, preferably a mincer, preferably having a screw and a hole plate, whereby the screw conveys against the hole plate is used in order to grind and crush the polymer gel strands into polymer gel particles which are preferably smaller than the polymer gel strands. Therein, holes of the hole plate pref- erably comprise planar cutting edges.

Therein, the rotating component according to the invention is particularly preferable a rotating disc. A preferred rotating disc is a toothed wheel. In process step (viii) 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 polymer gel particles. Ro- tary 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 par- ticularly 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 pref- erably 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, multistage or multistory system. The present invention is preferably practiced using a belt dryer having at least one belt. One-belt dryers are very particularly preferred. To ensure optimum performance of the belt-drying operation, the drying properties of the water-absorbent 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 of the belt dryer by means of a swivel belt. The feed height, i.e. the vertical distance between the swivel belt and the belt of the belt dryer, is preferably not less than 10 cm, more preferably not less than 20 cm and most preferably not less than 30 cm and preferably up to 200 cm, more preferably up to 120 cm and most preferably up to 40 cm. The thickness on the belt dryer of the polymer gel particles to be dried is preferably not less than 2 cm, more preferably not less than 5 cm and most preferably not less than 8 cm and preferably not more than 20 cm, more preferably not more than 15 cm and most preferably not more than 12 cm. The belt speed of the belt dryer is preferably not less than 0.005 m/s, more preferably not less than 0.01 m/s and most pref- erably not less than 0.015 m/s and preferably up to 0.05 m/s, more preferably up to 0.03 m/s and most preferably up to 0.025 m/s.

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

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

In process step (xi) of the process according to the present invention the surface of the ground and sized water-absorbent polymer particles is optionally treated. As measures to treat the surface of water-absorbent polymer particles any measure can be used the person skilled in the art considers as appropriate for such a purpose. Examples of surface treatments include, for example, surface crosslinking, the treatment of the surface with water-soluble salts, such as aluminium sulfate or aluminium lactate, the treatment of the surface with inorganic particles, such as silicon dioxide, and the like. Preferably, the components used to treat the surface of the polymer particles (cross-linker, water soluble salts) are added in the form of aqueous solutions to the water-absorbent polymer particles. After the particles have been mixed with the aqueous solutions, they are heated to a temperature in the range of from 150 to 230°C, preferably 160 to 200°C in order to promote the surface-crosslinking reaction. In an embodiment of the invention the component steel comprises no further alloy element except of Fe in an amount of more than 0.05 wt.-%, preferably of more than 0.025 wt.-%, more preferably of more than 0.01 wt.-%, based on the total weight of the component steel.

In an embodiment of the invention the rotating component is one selected from the group consisting of a disc, a shaft, a screw, and a knife, or a combination of at least two thereof. A preferred rotating component is a disc. A preferred disc is a toothed wheel.

In an embodiment of the invention the comminuting device comprises at least one, preferably at least two, more preferably at least three, more preferably at least 5, more preferably at least 10, even more preferably at least 15, even more preferably at least 20, most preferably at least 22, further rotating components, wherein the further rotating components comprise the component steel. Preferably, each further rotating component comprises the component steel.

In an embodiment of the invention the further rotating component is one selected from the group consisting of a disc, a shaft, a screw, and a knife, or a combination of at least two thereof. Preferably, each further rotating component is one selected from the group consisting of a disc, a shaft, a screw, and a knife, or a combination of at least two thereof. A preferred further rotating component is 'a disc. Preferably, each further rotating component is a disc. A preferred disc is a toothed wheel.

In an embodiment of the invention the comminuting device further comprises at least two rotating shafts. Preferably, each of the at least two shafts is connected to at least one, preferably at least two, more preferably at least three, more preferably at least four, more preferably at least 5, more preferably at least 10, most preferably at least 1 1 , rotating components or fur- ther rotating components. Preferably, the at least two rotating shafts rotate in different, preferably in opposite directions of rotation. In an embodiment of the invention the component steel is characterized by at least one, preferably two or more, of the following criteria:

a) a tensile strength R _m in the range of from 500 to 1500 MPa, preferably from 550 to 1450 MPa, more preferably from 600 to 1400 MPa, more preferably from 650 to 1350 MPa, more preferably from 700 to 1300 MPa, more preferably from 750 to 1250 MPa, more preferably from 800 to 1200 MPa, more preferably from 850 to 1 150 MPa, most preferably from 880 to 1 120 MPa;

b) a 0.2 % proof strength R _{p 0 2} in the range of from 450 to 1000 MPa, preferably from 500 to 950 MPa, more preferably from 550 to 900 MPa, more preferably from 600 to 900 MPa, more preferably from 650 to 900 MPa, more preferably from 700 to 900 MPa, more preferably from 750 to 850 MPa, more preferably from 770 to 830 MPa, most preferably from 790 to 810 MPa;

c) a minimum elongation at fracture in the range of from 1 to 40 %, preferably from 2 to 35 %, more preferably from 3 to 30 %, more preferably from 4 to 25 %, more preferably from 505 to 23 %, more preferably from 7 to 21 %, most preferably from 9 to 19 %, based on the length of the component steel before elongation;

d) a Brinell hardness in the range of from 200 to 500 HB, preferably from 250 to 450 HB, more preferably from 270 to 400 HB, more preferably from 280 to 370 HB, more preferably from 290 to 350 HB, most preferably from 310 to 330 HB; e) an impact energy at 20°C in the range of from 20 to 100 J, preferably from 30 to 90 J, more preferably from 40 to 80 J, more preferably from 45 to 75 J; f) an impact energy at -20°C in the range of from 10 to 70 J, preferably from 20 to 50 J, more preferably from 30 to 50 J, more preferably from 32 to 47 J, more preferably from 35 to 45 J, most preferably from 38 to 42 J.

g) a density in the range of from 5 to 10 g/cm ³ , preferably from 6 to 9 g/cm ³ , more preferably from 7 to 8 g/cm ³ , most preferably from 7.5 to 7.9 g/cm ³ ; h) an electrical resistivity in the range of from 0.3 to 1.0 Ω mmVm, preferably from 0.4 to 0.8 Ω mm ² /m, more preferably from 0.55 to 0.65 Ω mmVm;

i) a thermal conductivity in the range of from 10 to 40 W/(m K) , preferably from 20 to 30 W/(m K), more preferably from 22 to 27 W/(m K); and

j) a specific heat capacity in the range of from 350 to 500 J/(kg K) , preferably from 380 to 470 J/(kg K), more preferably from 420 to 440 J/(kg K). A preferred component steel meets all of the above criteria a) to j). Another preferred component steel meets the above criteria a) to f). Another preferred component steel meets the above criteria a) to c). Another preferred component steel meets the above criteria g) to j). A particularly preferred component steel conforms number 1.4313 in EN 10088-3.

In an embodiment of the invention in process step (vii) an antisticking agent is added to the comminuting device. A preferred antisticking agent is a liquid, preferably an emulsion. Preferably the antisticking agent is added by spraying or dripping or both.

In an embodiment of the invention the antisticking agent comprises one selected from the group consisting of a solvent, a polyalkylsiloxane, and a surfactant, or a combination of at least two thereof. A preferred solvent is water. A preferred polyalkylsiloxane is polydimethyl- siloxane.

In an embodiment of the invention the surfactant is a compound according to the general formula R-(-0-CH ₂ -CH ₂ -) _n -OX, wherein R is selected from C ₄ to C ₂₀ , n is an integer in the range of from 10 to 10,000, preferably from 50 to 5,000, more preferably from 100 to 500, and X is H or M, wherein M is a metal ion. A particularly preferred compound according to said general formula is polyethylene glycol trimethylnonyl ether.

In an embodiment of the invention the antisticking agent comprises

a) the solvent in an amount in the range of from 95 to 99 wt.-%, preferably from 95.5 to 98.5 wt.-%, more preferably from 96.5 to 97.5 wt.-%,

b) the polyalkylsiloxane in an amount in the range of from 0.5 to 5 wt.-%, preferably from 1 to 4 wt.-%, more preferably from 2.4 to 3.2 wt.-%, and c) the surfactant in an amount in the range of from 0.01 to 1 wt.-%, preferably from 0.05 to 0.7 wt.-%, more preferably from 0.1 to 0.3 wt.-%, each based on the total weight of the antisticking agent and the amounts in wt.-% adding up to a total of 100 wt.-%.

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

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

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

A blowing agent is a substance which is capable of producing a cellular structure or pores or both via a foaming process during polymerization of the monomers. The foaming process is preferably endothermic. A preferred endothermic foaming process is started by heat from an exothermic polymerisation or crosslinking or both reaction. A preferred blowing agent is a physical blowing agent or a chemical blowing agent or both. A preferred physical blowing agent is one selected from the group consisting of a CFC, a HCFC, a hydrocarbon, and C0 ₂ , or a combination of at least two thereof. A preferred C0 ₂ is liquid C0 ₂ . A preferred hydrocar- bon is one selected from the group consisting of pentane, isopentane, and cyclopentane, or a combination of at least two thereof. A preferred chemical blowing agent is one selected from the group consisting of a carbonate blowing agent, a nitrite, a peroxide, calcined soda, an oxalic acid derivative, an aromatic azo compound, a hydrazine, an azide, a Ν,Ν'- Dinitrosoamide, and an organic blowing agent, or a combination of at least two thereof. A very particularly preferred blowing agent is a carbonate blowing agent. Carbonate blowing agents which may be used according to the invention are disclosed in US 5, 1 18, 719 A, and are incorporated herein by reference. A preferred carbonate blowing agent is a carbonate con- taining salt, or a bicarbonate containing salt, or both. Another preferred carbonate blowing agent comprises one selected from the group consisting of 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 MgCC^, 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 ( H ₄ ) ₂ C0 ₃ . The MgCC>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 , ( H ₄ ) ₂ C0 ₃ , MgC0 ₃ , CaCC-3, NaHC0 ₃ , KHC0 ₃ , NH ₄ HC0 ₃ , Mg(HC0 ₃ ) ₂ , Ca(HC0 ₃ ) ₂ , ZnC0 ₃ , and BaCC>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 pre- ferred aromatic azo compound is one selected from the group consisting of a triazene, aryla- zosulfones, arylazotriarylmethanes, a hydrazo compound, a diazoether, and diazoaminoben- zene, 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'- Dinitroso- amide is N,N'-dimethyl-N,N'-dinitrosoterephthalamide.

A contribution to solving at least one of the above objects is provided by a device for the preparation of water-absorbent polymer particles in a process stream, comprising a) a first container, designed to take an aqueous monomer solution, comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al);

b) a further container, designed to take at least one crosslinker (cc3);

c) a mixing device, wherein the mixing device is

i) located down-stream to the first container and the further container, ii) designed to mix the monomer solution and the at least one crosslinker (<x3);

d) a polymerization reactor, wherein the polymerization reactor is

i) located down-stream to the first container and the further container, ii) designed to comprise the aqueous monomer solution and the at least one crosslinker (a3) during polymerizing the monomers in the aqueous monomer solution, thereby obtaining a polymer gel;

e) a comminuting device, wherein the comminuting device

i) is located down-stream to the first container and the further container, ii) is designed to comminute the polymer gel, thereby obtaining polymer gel particles,

iii) comprises a rotating component;

f) a belt dryer, wherein the belt dryer is

i) located down-stream to the comminuting device,

ii) designed to dry the polymer gel particles,

g) a grinding device, wherein the grinding device is

i) located down-stream to the belt dryer,

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

h) a sizing device, wherein the sizing device is

i) located down-stream to the grinding device,

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

wherein the rotating component comprises a component steel; wherein the component steel comprises as alloying elements

A. 0.1 wt.-% or less, preferably 0.07 wt.-% or less, more preferably 0.05 wt.-% or less, of C,

B. 0.15 wt.-% or less, preferably 0.1 wt.-% or less, more preferably 0.07 wt.-% or less, most preferably 0.06 wt.-% or less, of Si, C. 2 wt.-% or less, preferably 1.7 wt.-% or less, more preferably 1.5 wt.-% or less, most preferably 1.0 wt.-% or less, of Mn,

D. Ni in the range of from 3 to 5 wt.-%, preferably from 3.2 to 4.7 wt.-%. more preferably from 3.5 to 4.5 wt.-%,

E. 0.1 wt.-% or less, preferably 0.07 wt.-% or less, more preferably 0.04 wt.-% or less, most preferably 0.035 wt.-% or less, of P,

F. 0.05 wt.-% or less, preferably 0.03 wt.-% or less, more preferably 0.015 w - % or less, of S,

G. Cr in the range of from 8 to 18 wt.-%, preferably from 10 to 16 wt.-%, more preferably from 12 to 14 wt.-%, most preferably from 12.5 to 14 wt.-%,

H. Mo in the range of from 0.1 to 1.0 wt.-%, preferably from 0.2 to 0.8 wt.-%, more preferably from 0.3 to 0.7 wt.-%, most preferably from 0.4 to 0.7 wt.-%,

I. 0.01 wt.-% or more, preferably 0.015 wt.-% or more, more preferably 0.02 wt.-% or more, of N,

each based on the total weight of the component steel. Therein, the mixing device may be identical to the polymerization reactor. Moreover, the polymerization reactor may be identical to the comminuting device. Hence, the mixing device, the polymerization reactor, and the comminuting device may be identical. A preferred component steel is a component steel according to the process according to the invention. A preferred comminuting device is a comminuting device according to the process according to the invention. Preferred components or devices or both of the device according to the invention are designed 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 water-absorbent polymer particles in the device according to the invention. Preferably, the process comprises the process steps (i) to (xi) according to the invention.

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

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

c) a Si0 ₂ in an amount in the range of from 500 to 3,000 ppm by weight, pref- erably 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 plurality of water-absorbent polymer particles, comprising a polyalkylsiloxane, or a compound according to a general formula R-(-0-CH ₂ -CH ₂ -) _n -OX, or both; wherein in the general formula R is C ₄ to C ₂ o, n is an integer in the range of from 10 to 10,000, preferably from 50 to 5,000, more preferably from 100 to 500, and X is H or M; wherein M is a metal ion. A particularly preferred compound according to said general formula is polyethylene glycol trimethylnonyl ether. A preferred polyalkylsiloxane is polydimethylsiloxane.

In an embodiment of the invention the plurality of water-absorbent polymer particles comprises

a) the polyalkylsiloxane in an amount in the range of from 0.01 to 1 wt.-%, preferably from 0.02 to 0.9 wt.-%, more preferably from 0.03 to 0.8 wt.-%, more prefer- ably from 0.04 to 0.7 wt.-%, more preferably from 0.05 to 0.6 wt.-%, more preferably from 0.05 to 0.5 wt.-%, more preferably from 0.05 to 0.4 wt.-%, more preferably from 0.05 to 0.3 wt.-%, more preferably from 0.05 to 0.2 wt.-%, more preferably from 0.06 to 0.15 wt.-%, most preferably from 0.07 to 0.1 1 wt.-%, or b) the compound according to the general formula R-(-0-CH ₂ -CH ₂ -) _n -OX in an amount in the range of from 0.001 to 0.1 wt.-%, preferably from 0.002 to 0.09 wt.-%, more preferably from 0.003 to 0.08 wt.-%, more preferably from 0.002 to 0.07 wt.-%, more preferably from 0.002 to 0.06 wt.-%, more preferably from 0.002 to 0.05 wt.-%, more preferably from 0.002 to 0.04 wt.-%, more preferably from 0.002 to 0.03 wt.-%, more preferably from 0.002 to 0.02 wt.-%, more pref- erably from 0.002 to 0.01 wt.-%, more preferably from 0.003 to 0.01 wt.-%, more preferably from 0.004 to 0.01 wt.-%, most preferably from 0.005 to 0.009 wt.-%, or

c) both,

each based on the total weight of the plurality of water-absorbent polymer particles.

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

In an embodiment of the invention the composite material according to the invention comprises one selected from the group consisting of a foam, a shaped article, a fibre, a foil, a film, a cable, a sealing material, a liquid-absorbing hygiene article, a carrier for plant and fungal growth-regulating agents, a packaging material, a soil additive, and a building material, or a combination of at least two thereof. A preferred cable is a blue water cable. A preferred liquid-absorbing hygiene article is one selected from the group consisting of a diaper, a tampon, and a sanitary towel, or a combination of at least ^' two thereof. A preferred diaper is a baby's diaper or a diaper for incontinent adults or both. A contribution to the solution of at least one of the above objects is provided by a process for the production of a composite material, wherein the water-absorbent polymer particle according to the invention or the plurality of water-absorbent polymer particles according to the invention, and a substrate, and optionally an auxiliary substance are brought into contact with one another.

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

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

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

The water content after drying is determined according to the Karl Fischer method. tensile strength, proof strength, elongation at fracture

Tensile tests are performed according to DIN EN 10002-1. From these tensile tests the tensile strength R _m , the 0.2 % proof strength R _{p 0 2} and the minimum elongation at ^' fracture are determined. The 0.2 % proof strength R _{p 0 2} gives the tensile force per area of the component steel which has to be applied to a sample of the component steel in order to elongate the sample by 0.2 % along the direction of the tensile force. hardness according to Brinell

The hardness according to Brinell is determined according to DIN EN IDO 6506-1. impact energy

The impact energy is determined according to DIN EN ISO 148 at 20°C and at -20°C. corrosion resistance

The corrosion resistance of rotating components of comminuting devices is tested according to DIN EN ISO 9227. Therein, a test duration of 720 hours is chosen. After the test duration the samples are rinsed with water and checked first visually by eye and subsequently by an optical microscope. If no sign of corrosion is observed the sample is subjected to another test duration of 720 hours and checked again afterwards. Out of two components, the one which shows no signs of corrosion for a longer total test duration or less corrosion for the same test duration is assessed to have the better corrosion resistance. surface spalling resistance

The component made of the steel to be studied is used in a comminuting device in a process as described below under examples. The comminuting device is applied for 100 operational days, wherein each day 20 t of surface-crosslinked water-absorbent polymer particles are produced. Subsequently, the component is checked first visually by eye and subsequently by an optical microscope, i.e. for cracks, micro cracks, fissure, flaking, chipping and spalling. If no sign of surface spalling is observed the component is used in the comminuting device for another 100 operational days and checked again afterwards. Out of two components, the one which shows no signs of surface spalling for a longer operational time or less surface spalling for the same operational time is assessed to have the better surface spalling resistance. abrasion resistance

The component made of the steel to be studied is used in a comminuting device in a process as described below under examples. The comminuting device is applied for 100 operational days, wherein each day 20 t of surface-crosslinked water-absorbent polymer particles are produced. Subsequently, the component is checked first visually by eye and subsequently by an optical microscope for a loss of material due to abrasion at its surface. Out of two components, the one which shows no signs of abrasion for a longer operational time or less abrasion for the same operational time is assessed to have the better abrasion resistance. density

The density of the steel is measured by measuring the mass of a component made of the steel using a weighing scale suited to the mass of the component and measuring the volume of the component by means of the volume of water displaced by introducing the component into a basin of water. The ratio of said mass to said volume is the density. chemical composition of steel

The chemical analysis of the steels, providing the amounts of the alloying elements, is per- formed according to the following standard methods. Therefor, several samples of the same steel are applied.

C - DIN EN 10036: 1990-04,

Si - DIN EN 24829-1 : 1992-10 (Si-content above 0.05 wt.-%) or DIN EN 24829-2: 1992-10 (Si-content in the range of from 0.01 to 0.05 wt.-%) for the respective range of Si-content, Mn - DIN EN 10071 :2013-01,

Ni - DIN EN 10136: 1990-04,

P - DIN EN 10315:2006-10,

S - DIN EN 24935: 1992-07,

Cr - DIN EN 10188: 1990-04,

Mo - ISO 4941 : 1994- 12, and

N - DIN EN 10179: 1990-04

[Mode for Invention] Examples

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

A) Preparation of a partially neutralized acrylic acid monomer solution

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

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

B) Polymerization of the monomer solution

1 wt.-part of the monomer solution prepared in step A) is mixed with 0.001 wt.-parts of trimethylol propane triacrylate as crosslinker, 0.001 wt.-parts of sodium peroxodisulfate as first initiator component, 0.000034 wt.-parts of dimethyl ketal (Ciba ^® Irgacure ^® 651 by Ciba Specialty Chemicals Inc., Basel, Switzerland) as a second initiator component, up to 0.1 wt.- parts of acrylic acid particles (with a particle size of less than 150 μιτι) in a container to achieve a mixed solution. If according to table 2 below a blowing agent is added, 0.1 wt.-part, based on the total amount of the mixed solution, of sodium carbonate are added to the mixed solution. In the comparative examples 1 to 4 no blowing agent is applied.

A sufficient amount of the mixed solution is subjected to further treatment in order to obtain a polymer gel and further downstream water-absorbent polymer particles and further down- stream surface-crosslinked water-absorbent polymer particles as well as further downstream a water-absorbent product which is post treated. Details of the further treatment are given below. Subsequently, the mixed solution is placed on the belt of a conveyer belt reactor and the polymerization is initiated by UV radiation. The conveyor belt has a length of at least 20 m. The conveyor belt is formed as a trough to keep the solution on the belt prior and during polymerization. The dimensions of the conveyor belt and the conveying speed of the conveyer belt are selected in a. way that a poly-acrylic acid gel is formed at a downstream end of the belt. At the end of this step a water-absorbent polymer gel is achieved. The 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:

- The rubbery poly-acrylic acid gel is cut into semi-endless spaghetti-like gel strips by a crusher as shown in figure 4. Therein, in the examples and the comparative examples the toothed wheels of the crusher are made of the steel given in table 1 respectively.

- Then a mincer according to figure 5a) and 5b) is used to shred the strips into gel pieces in the range from 5 to 10 mm,

wherein the respected steels for the toothed wheels of the crusher and the screw of the mincer used in the examples are given in table 1 below. The same applies to the comparison examples.

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

D) Milling and sizing

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

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

E) Silicon dioxide treatment

In a treatment step the precursor I is mixed in a disc mixer with about 0.01 wt.-part (+- 10 %) of silicon dioxide (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, Ger- many. 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 +- 10 °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 sur- face 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 (Gebriider Lodige Mascheninenbau GmbH, Paderborn, Germany) with precursor II. The mixture is heated to a temperature in the range of from 130 to 160 °C. The mixture is then dried in a paddle dryer Andritz Gouda Paddle Dryer, preferably of type 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 μιη. At least 50 wt.-% of the surface-crosslined absorbent polymer particles have a particles size in the range of from 300 to 600 μιη. 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 μπϊ. Post treated crosslinked water- absorbent polymer particles are obtained.

The following scale is used to compare the results of measuring the parameters given in tables 1 and 2 for the examples and the comparative examples. In the order given in the following the measurement results are getting better from left to right:— , -, 0, +, ++, +++.

The following steels are used in the examples and the comparative examples as given in table l and 2.

In the examples 1 to 3 according to the invention steel 1.4313(QT900) is used. 1.4313 refers to the steel of steel number 1.4313 listed in the standard DIN EN 10088-1 :2005 and properties of which are given in the standards DIN EN 10088-2:2005 and/or DIN EN 10088-3 :2005. Therein, the standard according to the desired shape of the steel product is to be chosen. The steel 1.4313(QT900) has been heat treated according to QT900 (quenched and tempered) in DIN 17022-1 : 1994-10 part 1. The steel is obtainable from Stahlhandel Groditz GmbH, Groditz Germany and Deutsche Edelstahlwerke GmbH, Witten, Germany. The steel comprises according to chemical analysis as alloying elements: 0.05 wt.-% or less of C, 0.06 wt- % or less of Si, 1.0 wt.-% or less of Mn, Ni in the range of from 3.5 to 4.5 wt.-%, 0.035 wt.-% or less of P, 0.015 wt.-% or less of S, Cr in the range of from 12.5 to 14 wt.-%, Mo in the range of from 0.4 to 0.7 wt.-%, and 0.02 wt.-% or more of N, each based on the total weight of the said steel.

In the comparative examples 1 to 4 the steels 1.4003(A), 1.4005(A), 1.4828 and 1.2002 are used. Therein, 1.4003(A), 1.4005(A) and 1.4828 refer to the steels of the respective steel numbers listed in the standard DIN EN 10088-1 :2005. The properties of 1.4003(A) are given in the standard DIN EN 10088-2:2005 and/or DIN EN 10088-3:2005. Therein, the standard according to the desired shape of the steel product is to be chosen. The properties of 1.4005(A) are given in the standard DIN EN 10088-3:2005. Both, 1.4003(A) and 1.4005(A), have been heat treated according to A (annealed) in DIN 17022-1 : 1994-10 part 1. The properties of 1.4828 are given in the standard DIN EN 10095: 1999. The properties of 1.2002 are given in the standard DIN EN 10132-4:2000. All the above steels are obtainable from Deutsche Edelstahlwerke GmbH, Witten, Germany.

Table 1 : Steels used for the toothed wheels of the crusher applied for comminuting the polymer gel, as well as the resulting corrosion resistance, abrasion resistance and surface spalling of the toothed wheels.

Table 1 gives the steels from which the toothed wheels of the crusher used to comminute the polymer gel obtained on the conveyor belt are made of for the comparative examples 1 to 4 and the example 1. Therein, comparative example 3 shows the best corrosion resistance of the toothed wheels of the crusher. Example 1 shows a corrosion resistance which is still better than that of the comparative examples 1 , 2 and 4. Moreover, example 1 shows the best abrasion resistance and surface spalling resistance. The drying time in table 1 , meaning the time required to dry the comminuted polymer gel as given above to a water content of 5 wt.-% based on the dried polymer gel, has been measured after operating the crusher for an operating time after which the toothed wheels in the comparative example 1 showed at least one of abrasion, spalling, and corrosion. As can be seen, said drying time is the shortest in example 1. Hence, example 1 shows the most balanced combination of high resistance against corrosion, abrasion, surface spalling and drying time. steel of crusher blowing agent antisticking drying time per kg agent polymer gel particles example 1 1.4313 (QT900) none none +

example 2 1.4313 (QT900) sodium carbonate none ++

example 3 1.4313 (QT900) sodium carbonate diluted EG-601 +++

Table 2: Time required for drying the polymer gel particles depending on the use of a blowing agent and the addition of an antisticking agent into the crusher during comminuting.

Table 2 gives the steel from which the toothed wheels of the crusher used to comminute the polymer gel obtained on the conveyor belt are made of for examples 1 to 3. In each of these examples steel 1.4313(QT900) according to the invention is used. Therein, example 1 corresponds to example 1 in table 1. Therein, no blowing agent is used. In example 2 sodium carbonate is used as the blowing agent. This results in a decreased time required for drying a kg of the comminuted polymer gel to the desired water content. Moreover, example 3 shows that adding an antisticking agent during comminuting the polymer gel further decreases said drying time. The antisticking agent used in example 3 is obtained from a polydimethyl siloxane emulsion (EG-601 by Eugene Industry, 166, Nongso-ri, Juchon-myon, Gimhae-si, Gyengnam, Korea) by diluting the emulsion with water to a water content of 97 wt.-% based on the weight of the diluted emulsion. The antisticking agent is sprayed into the crusher while the polymer gel is comminuted in the crusher.

Figure 1 shows a flow chart diagram depicting the steps 101 to 1 1 1 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. In a first step 101 an aqueous monomer solution comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al) and at least one crosslinker (a3) is provided. Preferably, the aqueous monomer solution is an aqueous solution of partially neutralized acrylic acid, further comprising crosslinkers. In a second step 102 fine particles of a water-absorbent polymer may be added to the aqueous monomer solution. In a third step 103 a polymerization initiator or at least one component of a polymerization initiator system that comprises two or more components is added to the aqueous monomer solution. In a fourth step 104 the oxygen content of the aqueous monomer solution is decreased by bubbling nitrogen into the aqueous monomer solution. In a fifth step 105 the monomer solution is charged onto a belt of a polymerization belt reactor as a polymerization reactor 604. The belt is an endless conveyor belt. In a sixth step 106 the aqueous monomer solution is pol- ymerized to a polymer gel. in a seventh step 107 the polymer gel is discharged from the belt. Subsequently, the polymer gel is comminuted, whereby polymer gel particles 403 are obtained. In an eighth step 108 the polymer gel particles 403 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 are discharged from the belt dryer 605 and subsequently in a ninth step 109 grinded to obtain water-absorbent polymer particles. In a tenth step 1 10 the water-absorbent polymer particles are sized to obtain water-absorbent polymer particles having a well defined particle size distribution. In an eleventh step 11 1 the surface of the water-absorbent polymer particles is treated in terms of a surface crosslinking.

Figure 2 shows a flow chart diagram depicting the steps 101 to 11 1 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. The process 100 shown in figure 2 is the same as the process 100 in figure 1 , wherein the third process step 103 and the fourth process step 104 overlap in time. While the polymerization initiator is added to the aqueous monomer solution, nitrogen is bubbled into the aqueous monomer solution in order to decrease its oxygen content.

Figure 3 shows a flow chart diagram depicting the steps 101 , 103, 105 to 110 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. The process 100 shown in figure 3 is the same as the process 100 in figure 1 , wherein the second step 102, the fourth step 104, and the eleventh step 1 1 1 are not part of the process 100 according to figure 3.

Figure 4 shows a scheme of a comminuting device 400 according to the invention. The comminuting device 400 comprises a plurality of toothed wheels 402. A first portion of the toothed wheels 402 rotates around a first axis of rotation 401 and a further portion of the toothed wheels 402 rotates around a further axis of rotation 401. The toothed wheels 402 of the first portion rotate in counter direction and towards the toothed wheels 402 of the further portion. A polymer gel being fed between the rotating toothed wheels 402 is comminuting by the comminuting device 400 and polymer gel particles 403 are obtained. The toothed wheels 402 are rotating components according to the invention. Hence, the toothed wheels are made of a component steel comprising as alloying elements 0.05 wt.-% of C, 0.6 wt.-% of Si, 1.0 wt.-% of Mn, 4.0 wt.-% of Ni, 0.035 wt.-% of P, 0.015 wt.-% of S, 13.0 wt.-% of Cr, 0.6 wt.- % of Mo, 0.02 wt.-% of N, and Fe up to the remainder completing the sum to 100 wt.-%, each based on the total weight of the component steel.

Figure 5a) shows a scheme of another comminuting device 400 according to the invention in an external view. The comminuting device 400 is a mincer ("meat grinder") comprising a static hole plate 501 , a rotating screw 502, and a feed unit 503 for feeding a polymer gel into the mincer.

Figure 5b) shows a scheme of inner parts the comminuting device 400 of figure 5a) in an exploded view. The comminuting device 400 comprises a screw 502 which rotates together with a rotating hole plate 504. Thereby, the screw 502 conveys the polymer gel towards the static hole plate 501 and through the holes of the static hole plate 501. As the rotating hole plate 504 rotates with respect to the static hole plate 501 circular cutting edges 505 of holes of the rotating hole plate 504 comminute the polymer gel to obtain polymer gel particles 403 (not shown). The screw 502 is a rotating components according to the invention. Hence, the screw 502 is made of a component steel comprising as alloying elements 0.05 wt.-% of C, 0.6 wt.-% of Si, 1.0 wt.-% of Mn, 4.0 wt.-% of Ni, 0.035 wt.-% of P, 0.015 wt.-% of S, 13.0 wt.-% of Cr, 0.6 wt.-% of Mo, 0.02 wt.-% of N, and Fe up to the remainder completing the sum to 100 wt.-%, each based on the total weight of the component steel.

Figure 6 shows a block diagram of a device 600 for the preparation of water-absorbent poly- mer particles according to the invention. The arrows show a direction of a process stream

608 of the preparation of water-absorbent polymer particles. The device 600 comprises a first container 601 , a further container 602, downstream a mixing device 603, downstream a polymerization reactor 604 which is a polymerization belt reactor comprising an endless conveyor belt, downstream a comminuting device 400, downstream a belt dryer 605, downstream a grinding device 606, and downstream a sizing

device 607, each according to the invention.