WO2015163514A1

WO2015163514A1 - Monomer preparation with over-neutralization for production of water-absorbent polymer particles

Info

Publication number: WO2015163514A1
Application number: PCT/KR2014/003672
Authority: WO
Inventors: Jeong Beom Park
Original assignee: Songwon Industrial Co., Ltd.
Priority date: 2014-04-25
Filing date: 2014-04-25
Publication date: 2015-10-29
Also published as: EA201691537A1; EA031397B1; KR20160149238A; CN106232630B; KR102223611B1; CN106232630A

Abstract

The invention generally relates to a process for the preparation of water-absorbent polymer particles, comprising the process steps of (i) preparing an aqueous monomer solution comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (α1) and at least one crosslinker (α3); (ii) optionally adding fine particles of a water-absorbent polymer to the aqueous monomer solution; (iii) adding a polymerization initiator or at least one component of a polymerization initiator system to the aqueous monomer solution; (iv) optionally decreasing the oxygen content of the aqueous monomer solution; (v) charging the aqueous monomer solution into a polymerization reactor; (vi) polymerizing the monomers, thereby obtaining a polymer gel; (vii) discharging the polymer gel out of the polymerization reactor and optionally comminuting the polymer gel; (viii) drying the optionally comminuted polymer gel; (ix) grinding the dried polymer gel thereby obtaining water-absorbent polymer particles; (x) sizing the grinded water-absorbent polymer particles; and (xi) optionally treating the surfaces of the grinded and sized water-absorbent polymer particles; wherein in process step (i) preparing the aqueous monomer solution comprises: (i1) providing a first portion of acrylic acid; and (i2) contacting the first portion of acrylic acid with a hydroxide in a first contacting step, thereby obtaining a first acrylate phase; and (i3) contacting the first acrylate phase with a further portion of acrylic acid in a second contacting step, thereby obtaining the monomer solution.

Description

[DESCRIPTION]

[Invention Title]

MONOMER PREPARATION WITH OVER-NEUTRALIZATION FOR PRODUCTION OF

WATER-ABSORBENT POLYMER PARTICLES

[Technical Field]

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

[Background Art]

Superabsorbers, also known as super absorbing polymers (SAP), 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 fonnation of hydro gels, and to retain such fluids under a certain pressure. By virtue of those characteristic properties, such polymers are chiefly used for incorporation into sanitary articles, such as, for example, baby's nappies/diapers, incontinence products or sanitary towels.

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

The polymer gel, also called hydrogel, obtained after the polymerization is usually comminuted, dried and classified in order to obtain a particulate superabsorber with a well defined particle size distribution. In a further process step these superabsorbent particles are often surface crosslinked in order to improve the absorption behavior. For this purpose the particles are mixed with an aqueous solution containing a surface crosslinking agent and op- tionally further additives and the thus obtained mixture is heat treated in order to promote the crosslinking reaction.

The acid-group-carrying monomers can be polymerized in the presence of the cross- linkers in a batch process or in a continuous process. Both in continuous and in batchwise polymerization, partially neutralized acrylic acid is typically used as the monomer. Suitable neutralization processes are described, for example, in EP 0 372 706 A2, EP 0 574 260 Al , WO 2003/051415 Al, EP 1 470 905 Al, WO 2007/028751 Al , WO 2007/028746 Al and WO 2007/028747 Al .

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

[Disclosure] [Technical Problem]

A major disadvantage of the processes for the preparation of water-absorbent polymer particles is a high residual monomer content, in particular when used in diapers. Residual monomers staying in the water-absorbent polymer particles can cause skin irritations when diapers are frequently used.

[Technical Solution]

Generally, it is an object of the present invention to at least partly overcome a disadvantage arising form the prior art in the context of the production of water-absorbent polymer particles.

It is an object of the present invention to provide a process for the production of water- absorbent polymer particles, which can be used in diapers with a reduced tendency to cause skin irritations. It is a further object to provide water-absorbent polymer particles with low residual monomer content. Moreover, it is an object to provide a device suitable for producing water-absorbent polymer particles with a low residual monomer content.

It is a further object of the present invention to provide a water-absorbent polymer par- ticle produced by a process having at least one of the above advantages, wherein the water- absorbent polymer particle shows no reduction in quality. It is a further object of the present invention to provide a composite material comprising a water-absorbent polymer particle pro- duced by a process having at least one of the above advantages, wherein the composite material shows no reduction in quality. It is a further object of the present invention to provide a device for producing water-absorbent polymer particles by a process having at least one of the above advantages.

A contribution to the solution of at least one of the above objects is given by the independent claims. The dependent claims provide preferred embodiments of the present invention which also serve solving at least one of the above mentioned objects.

A process for the preparation of water-absorbent polymer particles includes the process steps of

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

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

(iii) adding a polymerization initiator or 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 (400);

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

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

(viii) drying the optionally comminuted polymer gel;

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

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

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

wherein in process step (i) preparing the aqueous monomer solution further comprises as steps:

(11) providing a first portion of acrylic acid; and

(12) contacting the first portion of acrylic acid with a hydroxide in a first contacting step, thereby obtaining a first acrylate phase, wherein the first acrylate phase has a pH of 10 or more,

wherein the first acrylate phase has a degree of neutralization of at least

0.95; and

(i3) contacting the first acrylate phase with a further portion of acrylic acid in a second contacting step, thereby obtaining the monomer solution, wherein the further portion of acrylic acid has a water content which is less than a water content of the first acrylate phase,

wherein the monomer solution has a pH which is less than the pH of the first acrylate phase,

wherein the monomer solution has a degree of neutralization in the range from 0.5 to 0.9;

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

A device for the preparation of water-absorbent polymer particles in a process stream includes:

a) a first container (501), designed to take acrylic acid;

b) a second container(502), designed to take a metal hydroxide;

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

i) located down-stream to the second container (),

ii) designed to cool the metal hydroxide;

d) a first mixing device (504), wherein the first mixing device (504) is

i) located down-stream to the first container (501) and the cooling device (503),

ii) equipped with a cooler and a propelled mixing means;

e) a third container (505), wherein the third container (505) is

i) located down-stream to the first mixing device (504),

ii) designed to take an aqueous monomer solution, comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups ( l);

f) a fourth container (506), designed to take at least one crosslinker (a3);

g) a second mixing device (507), wherein the second mixing device (507) is

i) located down-stream to the third container (505) and the fourth container (506), ii) designed to mix the monomer solution and the at least one crosslinker ( 3);

h) a polymerization belt reactor (400), wherein the polymerization belt reactor (400) i) is located down-stream to the second mixing device (507), ii) is designed to comprise the aqueous monomer solution and the at least one crosslinker (a3) during polymerizing the monomers in the aqueous monomer solution, thereby obtaining a polymer gel;

j) a comminuting device (508), wherein the comminuting device (508) is

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

k) a belt dryer (509), wherein the belt dryer (509) is

i) located down-stream to the comminuting device (508),

ii) designed to dry the polymer gel;

1) a grinding device (510), wherein the grinding device ( 10) is

i) located down-stream to the belt dryer (509),

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

m) a sizing device (51 1), wherein the sizing device (51 1) is

i) located down-stream to the grinding device (510),

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

[Advantageous Effects]

Water-absorbent polymer particles which can be used in diapers with a reduced tendency to cause skin irritations and with low residual monomer content and a composite material including the water-absorbent polymer are provided. Further, a device suitable for producing water-absorbent polymer particles with a low residual monomer content is provided.

[Description of Drawings]

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

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

Figure 3 is a flow chart diagram depicting the steps of another process according to the invention; Figure 4 is a scheme of a basic setup of a polymerization belt reactor according to the invention; and

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

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)

112 step (il)

1 13 step (i2)

1 14 step (i3)

400 polymerization reactor; polymerization belt reactor

401 belt

402 guide roller

403 direction of movement of the polymer gel on the belt

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

405 longitudinal directions

406 transversal directions

500 device for the preparation of water-absorbent polymer particles

501 first container

502 second container

503 cooling device

504 first mixing device

505 third container

506 fourth container 507 second mixing device

508 comminuting device

509 belt dryer

510 grinding device

511 sizing device

512 process stream

[Best Mode]

A contribution to the solution of at least one of these objects is made by a process (100) for the preparation of water-absorbent polymer particles, comprising the process steps of

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

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

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

(viii) drying the optionally comminuted polymer gel;

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

wherein in process step (i) preparing the aqueous monomer solution further comprises as steps: (11) providing a first portion of acrylic acid; and

(12) contacting the first portion of acrylic acid with a hydroxide in a first contacting step, thereby obtaining a first acrylate phase,

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

wherein the first acrylate phase has a degree of neutralization of at least 0.95, preferably at least 0.98, more preferably at least 0.99 ; and

(13) contacting the first acrylate phase with a further portion of acrylic acid in a second contacting step, thereby obtaining the monomer solution,

wherein the further portion of acrylic acid has a water content which is less than a wa- ter content of the first acrylate phase,

wherein the monomer solution has a degree of neutralization in the range from 0.5 to 0.9, preferably in the range from 0.6 to 0.85, and more preferably in the range from 0.65 to 0.8;

wherein the content of acrylate and acrylic acid in the aqueous monomer solution is in the range from 30 to 50 wt.-%, preferably in the range from 35 to 45 wt.-%, each based on the total weight of the aqueous monomer solution.

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 degree of neutralization as used through out this text is the ratio of mole hydroxide to mole acrylic acid.

The process according to the present invention is preferably a continuous process in which the aqueous monomer solution is continuously provided and is continuously fed into the polymerization reactor. Preferably the polymerization reactor is a polymerization belt reactor. Preferably, the aqueous monomer solution is added continuously and is thus continuously fed onto the belt of the polymerization belt reactor. The polymer gel obtained is continuously discharged out of the polymerization reactor and is continuously optionally comminuted, dried, grinded and sized in the subsequent process steps. This continuous process may, however, be interrupted in order to, for example,

substitute certain parts of the process equipment, like the belt material of the conveyor belt if a conveyor belt is used as the polymerization reactor,

clean certain parts of the process equipment, especially for the purpose of removing polymer deposits in tanks or pipes, or start a new process when water-absorbent polymer particles with other absorption characteristics have to be prepared.

Water-absorbent polymer particles which are preferred according to the invention are particles that have an average particle size in accordance with WSP 220.2 (test method of „Word Strategic Partners" ED ANA and INDA) in the range of from 10 to 3,000 μηι, preferably 20 to 2,000 μιη and particularly preferably 150 to 850 μιη. 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 μνα to be at least 30 wt.-%, particularly preferably at least 40 wt.-% and most preferably at least 50 wt.-%, based on the total weight of the water-absorbent poly- mer particles.

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

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

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

The neutralization of the monomers bearing carboxylic acid groups is preferably established by addition of sodium hydroxide to at least a part of the aqueous monomer solution at the beginning of step (i), in step (i2). The acrylic acid comprises optionally a polymerization inhibitor. The polymerization inhibitor is preferably selected from the group consisting of mono methyl ether hydroquinone (MEHQ) or hydroquinone (HQ) or both.

The acrylic acid preferably comprises the polymerization inhibitor, e.g. mono methyl ether hydroquinone (MEHQ) or hydroquinone (HQ) preferably in an amount from 100 to 2000 ppm, or preferably in an amount from 120 to 1000 ppm, or preferably in an amount from 150 to 500 ppm, based on the total weight of the first portion of acrylic acid. Preferably, the acrylic acid comprises mono methyl ether hydroquinone (MEHQ).

In step (i2) of the process according to the invention, the first portion of acrylic acid is contacted with the hydroxide, wherein a pH of 10 or more, preferably of more than 10.5, or preferably more than 10.8, is obtained. In some cases a maximum pH of 12 is accomplished. The first portion of acrylic acid preferably comprises the hydroxide in a weight ratio to the acrylic acid in the range from 0.1 : 1 to 1.5: 1 , or preferably in the range from 0.2: 1 to 1.3: 1 , or preferably in the range from 0.3: 1 to 1 : 1. The addition of hydroxide to the acrylic acid results in a conversion of at least a part of the acrylic acid to acrylate. According to the invention in the first acrylate phase at least 95 mol-% of the acrylic acid in an acrylate.

The first acrylate phase is contacted in step (i3) with a further portion of acrylic acid to obtain the monomer solution. The contacting in step (i3) can be performed with any method the person skilled in the art would choose for a contacting of two aqueous phases. The further portion of acrylic acid has a smaller water content than the first acrylate phase. The water con- tent of the further portion of acrylic acid is smaller than the water content of the first portion of acrylic acid. The first portion of acrylic acid has preferably a water content in the range from 30 to 80 wt.-%, or in the range from 35 to 75 wt.-%, or in the range from 40 to 70 wt.-%, each based on the total weight of the first portion of acrylic acid. The further portion of acrylic acid has preferably a water content in the range from 0 to 10 wt.-%, or preferably in the range from 0 to 8 wt.-%, or preferably in the range from 0 to 6 wt.-%, each based on the total weight of the further portion of acrylic acid.

The monomer solution received in step (i3) of the process according to the invention has a pH smaller than the pH of the first acrylate phase. Preferably, the pH of the monomer solution differs from the pH of the first acrylate phase by 0.5 to 6 pH units, or preferably by 1 to 5 pH units, or preferably by 1.5 to 4.5 pH units. It is preferred according to the invention that 60 to 80 mol-%, preferably 65 to 78 mol-%, or preferably 67 to 76 mol-% of the acrylic acid in the monomer solution is an acrylate.

In step (i) the preparation of the aqueous monomer solution preferably comprises several further steps:

a. providing the first portion acrylic acid comprising acrylate and optionally MEHQ or

HQ and/or;

β. providing acrylic acid monomer and/or;

χ. providing a further monoethylenically unsaturated monomers (ot2);

The steps β to χ can be provided in any order and in any combination with step a. In one preferred embodiment only step a is provided. In a further preferred embodiment steps a and β are provided. In yet a further preferred embodiment steps a and χ are provided. Also the order can be varied in step (i). In a preferred embodiment first the crosslinker (a3) is provided first and then step a and optionally one of steps β and/or χ can be provided. In a further preferred embodiment step a alone or in a combination with one of steps β or χ is firstly provided and afterwards the crosslinker (a3) is added.

The acrylate and acrylic acid content of the aqueous monomer solution is less than 55 wt.-%, preferably less than 50 wt.-%, or preferably less than 45 wt.-%, related to the total weight of the aqueous monomer solution. It is furthermore preferred that the acrylate and acrylic acid content of the aqueous monomer solution is not below 30 wt.-%.

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

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

Preferred crosslinkers (a3) are all those compounds which are_. cited in DE 102 23 060 Al as crosslinkers (a3) of the crosslinker classes I, II, III and IV, whereby

as compounds of crosslinker class I, N, N^'-methylene bisacrylamide, polyethylenegly- col di(meth)acrylates, triallylmethylammonium chloride, tetraallylammonium chloride and allylnonaethyleneglycol acrylate produced with 9 mol ethylene oxide per mol acrylic acid are particularly preferred, wherein N, TNT -methylene bisacrylamide is even more preferred, and as compounds of crosslinker class IV, Al₂ (S0₄)3 and its hydrates are particularly preferred.

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

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

The aqueous monomer solution may further comprise water-soluble polymers (a4).

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

The aqueous monomer solution can furthermore also comprise auxiliary substances (<x5), these auxiliary substances including, in particular, complexing agents, such as, for example, EDTA.

The relative amount of monomers (al) and (a2) and of crosslinking agents (a3) and water-soluble polymers (a4) and auxiliary substances (a5) in the aqueous monomer solution is preferably chosen such that the water-absorbent polymer structure obtained after drying the optionally comminuted polymer gel is based

to the extent of from 20 to 99.999 wt.-%, preferably to the extent of 55 to 98.99 wt.-% and particularly preferably to the extent of 70 to 98.79 wt.-% on monomers ( l),

to the extent of from 0 to 80 wt.-%, preferably to the extent of 0 to 44.99 wt.-% and particularly preferably to the extent of 0.1 to 44.89 wt.-% on the monomers (a2),

to the extent of from 0 to 5 wt.-%, preferably to the extent of 0.001 to 3 wt.-% and particularly preferably to the extent of 0.01 to 2.5 wt.-% on the crosslinking agents (a3),

- to the extent of from 0 to 30 wt.-%, preferably to the extent of 0 to

5 wt.-% and particularly preferably to the extent of 0.1 to 5 wt.-% on the water-soluble polymers (a4),

to the extent of from 0 to 20 wt.-%, preferably to the extent of 0 to 10 wt.-% and particularly preferably to the extent of 0.1 to 8 wt.-% on the auxiliary substances (a5), and

to the extent of from 0.5 to 25 wt.-%, preferably to the extent of 1 to 10 wt.-% and particularly preferably to the extent of 3 to 7 wt.-% on water (a6)

the sum of the amounts by weight (al) to (a6) being 100 wt.-%.

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

Water-absorbent fine particles are preferably water-absorbent polymer particles the composition of which corresponds to the composition of the above described water-absorbent polymer particles, wherein it is preferred that at least 90 wt.-% of the water-absorbent fine particles, preferably at least 95 wt.-% of the water-absorbent fine particles and most preferred at least 99 wt.-% of the water-absorbent fine particles based on the total weight of the water- absorbent fine particles have a particle size of less than 200 μιτι, 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 realised in a so called "Rotor Stator Mixer" which comprises in its mixing area a preferably cylindrically shaped, non-rotating stator, in the centre of which a likewise preferably cylindrically shaped rotor is rotating. The walls of the rotor as well as the walls of the stator are usually provided with notches, for example notches in the form of slots, through which the mixture of fine particles and aqueous monomer solution can be sucked through and thus can be subjected to high shear forces.

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

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

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

In process step (iii) of the process according to the present invention a polymerization initiator or at least one component of a polymerization initiator system that comprises two or more components is added to the aqueous monomer solution.

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

As thermal initiators may be used all compounds known to the person skilled in the art that decompose under the effect of an increased temperature to form radicals. Particularly preferred are thermal polymerisation initiators with a half life of less than 10 seconds, more preferably less than 5 seconds at less than 180°C, more preferably at less than 140°C. Peroxides, hydroperoxides, hydrogen peroxide, persulfates and azo compounds are particularly preferred thermal polymerization initiators. In some cases it is advantageous to use mixtures of various thermal polymerization initiators. Among such mixtures, those consisting of hydrogen peroxide and sodium or potassium peroxodisulfate are preferred, which may be used in any desired quantitative ratio. Suitable organic peroxides are preferably acetylacetone peroxide, methyl ethyl ketone peroxide, benzoyl peroxide, lauroyl peroxide, acetyl peroxide, capryl peroxide, isopropyl peroxidicarbonate,2-ethylhexyle peroxidicarbonate, tert. -butyl hydroperoxide, cumene hydroperoxide, and peroxides of tert- amyl perpivalate, tert.-butyl perpivalate, tert.-butyl perneohexonate, tert.-butyl isobutyrate, tert.-butyl per-2-ethylhexenoate, tert.-butyl perisononanoate, tert.-butyl permaleate, tert.-butyl perbenzoate, tert.-butyl-3,5,5-trimethylhexanoate and amyl perneodecanoate. Furthermore, the following thermal polymerisation initiators are preferred: azo compounds such as azo-bis- isobutyronitril, azo-bis-dimethylvaleronitril, azo-bis-ami-dinopropane dihydrochloride, 2,2'- azobis-(N,N-dhnethylene)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 oxidizing component of the redox catalyst, or as a complement thereto, one or more, preferably water- soluble azo compounds may be used.

The polymerization is preferably initiated by action of energetic radiation, so-called photo-initiators are generally used as initiator. These can comprise for example so-called a- splitters, H-abstracting systems or also azides. Examples of such initiators are benzophenone derivatives such as Michlers ketone, phenanthrene derivatives, fluorine derivatives, anthra- quinone derivatives, thioxanthone derivatives, cumarin derivatives, benzoinether and derivatives thereof, azo compounds such as the above-mentioned radical formers, substituted hex- aarylbisimidazoles or acylphosphine oxides. Examples of azides are: 2-(N,N- dimethylamino)ethyl-4-azidocinnamate, 2-(N,N-dimethylamino)ethyl-4-azidonaphthylketone, 2-(N,N-di-methylamino)ethyl-4-azidobenzoate, 5-azido-l-naphthyl-2'-(N,N-dimethylami- no)ethylsulfone, N-(4-sulfonylazidophenyl)maleinimide, N-acetyl-4-sulfonyl-azidoaniline, 4- sulfonylazidoaniline, 4-azidoaniline, 4-azidophenacyl bromide, p-azidobenzoic acid, 2,6- bis(p-azidobenzylidene)cyclohexanone and 2,6-bis(p-azidobenzylidene)-4- methylcyclohexanone. A further group of photo-initiators are di-alkoxy ketales such as 2,2- dimethoxy-l,2-diphenylethan-l-one. The photo-initiators, when used, are generally employed in quantities from 0.0001 to 5 wt.-% based on the monomers to be polymerized.

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

iiia. a peroxodisulfate; and

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

wherein the initiator comprises the peroxodisulfate and the organic initiator molecule in a molar ratio in the range of from 20:1 to 50:1. In one aspect of this embodiment it is preferred that the concentration of the initiator component iiia. is in the range from 0.05 to 2 wt.-%, based on the amount of monomers to be polymerized. In another aspect of this embodiment it is preferred that the organic initiator molecule is selected from the group consisting of 2,2-dimethoxy-l,2-diphenylethan-l-one, 2,2-azobis-(2- amidinopropane)dihydrochloride, 2,2-azobis-(cyano valeric acid) or a combination of at least two thereof. In a further aspect of this embodiment it is preferred that the peroxodisulfate is of the general formula M₂S₂0₈, with M being selected from the group consisting of NH₄, Li, Na, Ka or at least two thereof. The above described components are in particular suitable for UV initiation of the polymerization in step (vi) of the process of the present invention. Employing this composition further yields low residual monomer and reduced yellowing in the water- absorbent polymer particle, obtainable by the process according to the present invention.

In this context it should also be noted that step (iii), adding the polymerization initiator, may be realized before step (iv), simultaneously to step (iv), or overlapping in time with step (iv), i.e. when the oxygen content of the aqueous monomer solution is decreased. If a polymerization initiator system is used that comprises two or more components, like the preferred initiator system that comprises hydrogen peroxide, sodium peroxodisulfate and ascorbic acid and that is active only if all the components have been added, 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). In process step (iv) of the process according to the present invention the oxygen content of the aqueous monomer solution is optionally decreased. Optional step (iv), decreasing the oxygen content of the aqueous monomer solution, may be performed before, during or after process step (ii) Preferably, the oxygen content of the aqueous monomer solution is de- creased after the fine particles have been added in process step (ii).

Decreasing of the oxygen content in the aqueous monomer solution may be realized by bringing the aqueous monomer solution into contact with an inert gas, such as nitrogen. The phase of the inert gas being in contact with the aqueous monomer solution is free of oxygen and is thus characterized by a very low oxygen partial pressure. As a consequence oxygen converts from the aqueous monomer solution into the phase of the inert gas until the oxygen partial pressures in the phase of the inert gas and the aqueous monomer solution are equal. Bringing the aqueous monomer phase into contact with a phase of an inert gas can be accomplished, for example, by introducing bubbles of the inert gas into the monomer solution in concurrent, countercurrent or intermediate angles of entry. Good mixing can be achieved, for example, with nozzles, static or dynamic mixers or bubble columns. The oxygen content of the monomer solution before the polymerization is preferably lowered to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, based on the total amount of 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 polymerization belt as the polymerization reactor, a polymer gel strand is obtained in a downstream portion of the conveyor belt, which, before drying, is preferably comminuted in order to obtain gel particles.

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

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

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

In a preferred embodiment of the invention the monomer solution is subjected to radical polymerization in order to obtain a precursor polymer. The radical polymerization can be initiated by any means the person skilled in the art would choose. Preferably, the initiation is chosen from the group consisting of heat, radiation or a combination of these. The initiation by radiation is preferred. The radiation is preferably provided by UV radiation. Preferably, the precursor polymer is subjected to a further cross-linking step. The further cross-linking step can be provided by any step the person skilled in the art would choose for a cross-linking process. The further cross-linking step can comprise the following steps: al . adding monomers to the polymer precursor;

a2. adding a crosslinker (o 3) to the polymer precursor;

a3. adding an initiator to the polymer precursor;

a4. initiating the polymerization.

The steps al . to a4. can be provided solely or in any combination with each other and in any sequence.

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

Comminution of the polymer gel or gel strand is preferably performed in at least three steps:

in a first step, a cutting unit, preferably a knife, for example a knife as disclosed in WO- A-96/36464, is used for cutting the polymer gel into flat gel strips, preferably with a length within the range of from 5 to 500 mm, preferably from 10 to 300 mm and particularly preferably from 100 to 200 mm, a height within the range of from 1 to 30 mm, preferably from 5 to 25 mm and particularly preferably from 10 to 20 mm as well as a width within the range of from 1 to 500 mm, preferably from 5 to 250 mm and particularly preferably from 10 to 200 mm;

in a second step, a shredding unit, preferably a breaker, is used for shredding the gel strips into gel pieces, preferably with a length within the range of 3 to 100 mm, preferably from 5 to 50 mm, a height within the range from 1 to 25 mm, preferably from 3 to 20 mm as well as a width within the range from 1 to 100 mm, preferably from 3 to 20 mm and

- in a third step a "wolf (grinding) unit, preferably a mincer, preferably having a screw and a hole plate, whereby the screw conveys against the hole plate is used in order to grind and crush gel pieces into polymer gel particles which are preferably smaller than the gel pieces.

An optimal surface-volume ratio is achieved hereby, which has an advantageous effect on the drying behaviour in process step (viii). A gel which has been comminuted in this way is particularly suited to belt drying. The three-step comminution offers a better "airability" because of the air channels located between the granulate kernels.

In process step (viii) of the process according to the present invention the polymer gel is dried. During this drying step (viii) water is preferably removed from the polymer gel in the range from 120 to 240 kg minute, preferably in the range from 130 to 230 kg/minute, or preferably in the range from 140 to 220 kg minute, or preferably in the range from 150 to 210 kg/minute, or preferably in the range from 160 to 200 kg/minute, based on a polymer gel portion in the range from 0.5 to 5 t, or preferably in the range from 0.8 to 4.5 t, or preferably in the range from 1 to 4 t. Preferably, the reduction of water from 40 to 60 wt.-%, based on the total weight of the polymer gel from 3 to 7 wt.-%, based on the total weight of the dried polymer is achieved in a time period in the range from 1 to 60 minutes, or preferably in the range from 2 to 50 minutes, or preferably in the range from 3 to 40 minutes. The obtained dry polymer preferably has a water content in the range from 3 to 7 wt.-%, or preferably in the range from 3.5 to 6.5 wt.-%, or preferably in the range 4 to 6 wt.-%, based on the total weight of the dried polymer. The content of water is measured by the Karl-Fischer titration method.

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

The size and design of the dryers depend on the product to be processed, the manufacturing capacity and the drying duty. A belt dryer can be embodied as a single-belt, multi-belt, multi-stage or multistory system. The present invention is preferably practiced using a belt dryer having at least one belt. One-belt dryers are very particularly preferred. To ensure optimum performance of the belt-drying operation, the drying properties of the water-absorbent polymers are individually determined as a function of the processing parameters chosen. The hole size and mesh size of the belt is conformed to the product. Similarly, certain surface en- hancements, such as electropolishing or Teflonizing, are possible.

The polymer gel to be dried is preferably applied to the belt of the belt dryer by means of a swivel belt. The feed height, i.e. the vertical distance between the swivel belt and the belt of the belt dryer, is preferably not less than 10 cm, more preferably not less than 20 cm and most preferably not less than 30 cm and preferably up to 200 cm, more preferably up to 120 cm and most preferably up to 40 cm. The thickness on the belt dryer of the polymer gel to be dried is preferably not less than 2 cm, more preferably not less than 5 cm and most preferably not less than 8 cm and preferably not more than 20 cm, more preferably not more than 15 cm and most preferably not more than 12 cm. The belt speed of the belt dryer is preferably not less than 0.005 m/s, more preferably not less than 0.01 m/s and most preferably not less than 0.015 m/s and preferably up to 0.05 m/s, more preferably up to 0.03 m/s and most preferably up to 0.025 m/s.

Furthermore, it is preferable according to the invention that the polymer gel is dried to a water content in the range of from 0.5 to 25 wt.-%, preferably from 1 to 10 wt.-% and particularly preferably from 3 to 7 wt.-%, based on the dried polymer gel. In process step (ix) of the process according to the present invention the dried polymer gel is ground thereby obtaining water-absorbent polymer particles.

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

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

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 parti- cles, such as silicon dioxide, and the like. Preferably, the components used to treat the surface of the polymer particles (cross-linker, water soluble salts) are added in the form of aqueous solutions to the water-absorbent polymer particles. After the particles have been mixed with the aqueous solutions, they are heated to a temperature in the range from 150 to 230°C, preferably 160 to 200°C in order to promote the surface-crosslinking reaction. A preferable fur- ther treatment of the water-absorbent polymer particles or the surface-crosslinked water- absorbent particles is the mixing of the particles in one, two or more fractions with zeolites. The zeolites preferably comprise one of the elements selected from the group consisting of Ag, Na, Zn or a mixture of at least two thereof. Preferably, the zeolites are mixed with the water- absorbent particles in a range from 0.05 to 5 wt.-%, or preferably in the range from 0.1 to 13 wt.-%, or preferably in the range from 0.2 to 10 wt.-%, based on the total weight of the water-absorbent polymer particles. By this kind of treatment post-treated water-absorbent polymer particles are achieved.

In a preferred embodiment of the invention the degree of neutralization of the acrylate phase in step (i2) is more than 1 , preferably more than 1.05, more preferably more than 1.1 , most preferably more than 1 ,15. Preferably, the degree of neutralization of the acrylate phase in step (i2) is not more than 1.2.

In a preferred embodiment of the invention the temperature of the first portion of acrylic acid and the temperature of the hydroxide differ by 2 K or more, preferably by 5 K or more, more preferably by 10 K or more, more preferably by 15 K or more, more preferably by 20 K or more, most preferably by 25 K or more. Preferably the hydroxide has a temperature in the range from 0 to 20°C, more preferably from 0 to 15°C, more preferably from 0 to 10°C, most preferably from 0 to 5°C.

In a preferred embodiment of the invention the further portion of acrylic acid compris- es less than 5 wt.-% of water, preferably less than 3 wt.-% of water, more preferably less than 2 wt.-% of water, based on the total weight of the further portion of acrylic acid.

In a preferred embodiment of the invention the first portion of acrylic acid comprises 50 wt.-% or more of water, preferably 60 wt.-% or more of water, more preferably 60 to 70 wt.-% of water, more preferably 62 to 68 wt.-% of water, more preferably 62 to 66 wt.-% of water, most preferably 63 to 64 wt.-% of water, based on the total weight of the first portion of acrylic acid.

In a preferred embodiment of the invention the pH of the monomer solution is in the range from 8 to 12, preferably from 8 to 1 1 , more preferably from 8 to 10, most preferably from 8 to 9.

In a preferred embodiment of the invention the hydroxide is one selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide or a mixture of at least two thereof.

In a preferred embodiment of the invention the oxygen gas content of the monomer solution is in the range from 3 to 15 wt.-ppm, preferably from 5 to 12 wt.-ppm, more prefera- bly from 6 to 10 wt.-ppm, based on the weight of the monomer solution. Preferably, the monomer solution is saturated with oxygen gas.

In a preferred embodiment of the invention the monomer solution is contacted with active carbon in a further contacting step. Preferably, the further contacting step is performed prior to step (i2). Preferably, the further contacting step is performed for a duration in the range from 60 to 240 minutes, preferably from 80 to 220 minutes, more preferably from 100 to 200 minutes, most preferably from 120 to 180 minutes. Preferably, the amount of the active carbon used in the further contacting step is in the range from 0.05 to 10 wt.-%, preferably from 0.1 to 5 wt.-%, more preferably from 0.2 to 1 wt.-%, based on the total weight of the monomer solution. Preferably, the active carbon is a powder comprising granules. Preferably, the diameter of the granules are less than 1.0 mm, preferably less than 0.7 mm, more preferably than 0.5 mm.

In a preferred embodiment of the invention at least the first contacting step or the second contacting step or both are completed in a time period in the range from 10 to 300 minutes, preferably from 20 to 250 minutes, more preferably from 30 to 200 minutes.

In a preferred embodiment of the invention at least the first contacting step or the second contacting step or both, preferably the sum of both, are perforaied at a temperature of the aqueous monomer solution of less than 40 °C preferably less than 30 °C, more preferably less than 25 °C.

In a preferred embodiment of the invention in step (v) the polymerization reactor is a polymerization belt reactor. A preferred polymerization belt reactor comprises a conveyor belt, preferably an endless conveyor belt.

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

In a preferred 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 from 10 to 200 mm, preferably from 10 to 100 mm, more preferably from 15 to 75 mm, most preferably from 15 to 50 mm.

In a preferred embodiment of the 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 immediately 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 cells 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 exo- thermic polymerisation or crosslinking or both reaction. A preferred blowing agent is a physical blowing agent or a chemical blowing agent or both. A preferred physical blowing agent is one selected from the group consisting of a CFC, a HCFC, a hydrocarbon, and C0₂, or a combination of at least two thereof. A preferred C0₂ is liquid C0₂. A preferred hydrocarbon is one selected from the group consisting of pentane, isopentane, and cyclopentane, or a com- bination of at least two thereof. A preferred chemical blowing agent is one selected from the group consisting of a carbonate blowing agent, a nitrite, a peroxide, calcined soda, an oxalic acid derivative, an aromatic azo compound, a hydrazine, an azide, a Ν,Ν'- Dinitrosoamide, and an organic blowing agent, or a combination of at least two thereof.

A very particularly preferred blowing agent is a carbonate blowing agent. Carbonate blowing agents which may be used according to the invention are disclosed in US 5, 1 18, 719 A, and are incorporated herein by reference. A preferred carbonate blowing agent is a carbonate containing salt, or a bicarbonate containing salt, or both. Another preferred carbonate blowing agent comprises one selected from the group consisting of C0₂ as a gas, C0₂ as a solid, ethylene carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, magnesium carbonate, or magnesium hydroxic carbonate, calcium carbonate, barium carbonate, a bicarbonate, a hydrate of these, other cations, and naturally occurring carbonates, or a combination of at least two thereof. A preferred naturally occurring carbonate is dolomite. The above mentioned carbonate blowing agents release C0₂ when being heated while dissolved or dispersed in the monomer solution. A particularly preferred carbonate blowing agent is MgC0₃, which may also be represented by the formula (MgC0₃)₄ Mg(OH)₂-5-H₂0. Another preferred carbonate blowing agent is agent is (NH )₂C0₃. The MgC0₃ and (NH₄)₂C0₃ may also be used in mixtures. Preferred carbonate blowing agents are carbonate salts of multivalent cations, such as Mg, Ca, Zn, and the like. Examples of such carbonate blowing agents are Na₂CQ₃, ₂C0₃, (NH₄)₂C0₃, MgC0₃, CaC0₃, NaHC0₃, KHC0₃, NH₄HC0₃, Mg(HC0₃)₂, Ca(HC0₃)₂, ZnC0₃, and BaC0₃. Although certain of the multivalent transition metal cations may be used, some of them, such as ferric cation, can cause color staining and may be subject to reduction oxidation reactions or hydrolysis equilibria in water. This may lead to difficulties in quality control of the final polymeric product. Also, other multivalent cations, such as Ni, Ba, Cd, Hg would be unacceptable because of potential toxic or skin sensitizing effects.

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

A contribution to the solution of at least one of the above objects is provided by a device for the preparation of water-absorbent polymer particles in a process stream, comprising a) a first container, designed to take acrylic acid;

b) a second container, designed to take a metal hydroxide;

c) a cooling device, wherein the cooling device is

i) located down-stream to the second container,

ii) designed to cool the metal hydroxide;

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

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

ii) equipped with a cooler and a propelled mixing means;

e) a third container, wherein the third container is

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

ii) designed to take an aqueous monomer solution, comprising at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups ( l); f) a fourth container, designed to take at least one crosslinker (a3);

g) a second mixing device, wherein the second mixing device is

i) located down-stream to the third container and the fourth container,

ii) designed to mix the monomer solution and the at least one crosslinker (<x3); h) a polymerization belt reactor, wherein the polymerization belt reactor i) is located down-stream to the second mixing device, ii) is designed to comprise the aqueous monomer solution and the at least one crosslinker (a3) during polymerizing the monomers in the aqueous monomer solution, thereby obtaining a polymer gel;

j) a comminuting device, wherein the comminuting device is

i) located down-stream to the polymerization belt reactor,

ii) designed to comminute the polymer gel;

k) a belt dryer, wherein the belt dryer is

i) located down-stream to the comminuting device,

ii) designed to dry the polymer gel;

1) a grinding device, wherein the grinding device is

i) located down-stream to the belt dryer,

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

m) a sizing device, wherein the sizing device is

i) located down-stream to the grinding device,

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

Preferred components or devices or both of the device for the preparation of water- absorbent polymer particles according to the invention are designed according to the process according to the invention.

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

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

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

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

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

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

The belt can be made of various materials, although these preferably have to meet the requirements of good tensile strength and flexibility, good fatigue strength under repeating bending stresses, good deformability and chemical resistance to the individual reaction components under the conditions of the polymerization. These demands are usually not met by a single material. Therefore, a multi-layer material is commonly used as belt for a process 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.

The polymerization belt reactor 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 may be upwardly extending supporting elements, the lon- gitudinal axes of which intersect at a point that is beneath the upper run, and which shape the conveyor belt that is supported by them so that it becomes suitably trough-shaped.

Another means of providing a trough-like configuration is arranging weir elements on an approximately flat belt. Therein, at least one weir element is arranged in longitudinal direction at each lateral edge of the belt. At least two further weir elements are arranged in trans- versal direction on different longitudinal positions of the belt, wherein the trough longitudinally extends between those two positions. The height of the weir elements determines the deepness of the trough and hence the amount of reaction components which can be fed into the trough. The longitudinal weir elements have to be designed to be longitudinally bent with the belt at turning sections of the belt. Therefor, the longitudinal weir elements may be made of a flexible material or they may consist of a plurality of shorter longitudinal weir elements which are arranged close to each other.

A trough-like configuration of the polymerization belt can also be achieved by a combination of transversally oriented weir elements and shaping the belt in the transversal direction by supporting elements, or by a combination of longitudinally oriented weir elements and shaping the belt in the longitudinal direction by supporting elements.

Test Methods

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

The water content is determined according to the Karl Fischer method. residual monomer content The residual monomer content is measured according to a standard test method for su- perabsorbent materials defined by the EDANA. Said test method is described in EDANA, Harmonized Test Methods Nonwovens and Related Industries, 2012 Edition as "Residual Monomers" under the method number WSP 210.2.R3 (12).

[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 degree of neutralization of 0.7 is achieved. In order to obtain this monomer solution an aqueous acrylic acid solution with a water content of 63 wt.-% and a temperature of 25°C is provided. An aqueous NaOH solution with an NaOH content of 30 wt.-% is cooled to a temperature given in table 1 and added to the aqueous acrylic acid solution in a mixer to obtain a degree of neutralization given in table 1 without exceeding 35°C during neutralization. Five minutes after neutralization, glacial acrylic acid is added until a degree of neutralization of 0.7 is obtained without exceeding 35°C during neutralization.

Optionally the sodium-acrylate monomer solution is degased with nitrogen. B) Polymerization of the monomer solution

1 wt.-part of the monomer solution prepared in step A) is mixed with 0.001 wt.-parts of trimethylol propane triacrylate as crosslinker, 0.001 wt.-parts of sodium peroxodisulfate as first initiator component, 0.000034 wt.-parts of 2,2-dimethoxy-l ,2-diphenylethan-l -one (Ci- ba^® Irgacure^® 651 by Ciba Specialty Chemicals Inc., Basel, Switzerland) as a second initiator component, up to 0.1 wt.-parts of acrylic acid particles (with a particle size of less than 150 μηι) in a container to achieve a mixed solution.

A sufficient amount of the mixed solution is subjected to further treatment in order to obtain a polymer gel and further downstream water-absorbent polymer particles and further downstream surface-crosslinked water-absorbent polymer particles as well as further downstream a water-absorbent product which is post treated. Details of the further treatment are given below.

Subsequently, the mixed solution is placed on the belt of a conveyer belt reactor, as shown in figure 4, and the polymerization is initiated by UV radiation. The conveyor belt has a length of at least 20 m. The conveyor belt is formed as a trough to keep the solution on the belt prior and during polymerization. The dimensions of the conveyor belt and the conveying speed of the conveyer belt are selected in a way that a poly-acrylic acid gel is formed at a downstream end of the belt. At the end of this step a water-absorbent polymer gel is achieved. The polymer gel has a water content of about 52 wt.-%, based on the total weight of the polymer gel.

C) Comminuting and drying of the polymer gel

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

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

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

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

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

D) Milling and sizing

The dried polymer gel is ground in three steps. First the dried polymer gel is fed through a Herbold Granulator HGM 60/145 (HERBOLD Meckesheim GmbH) 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 pol- ymer gel parts are then milled in a roller mill of Bauermeister Type 350.1 x 1800 (3-stage crusher) (Bauermeister Zerkleinerungstechnik GmbH) to obtain water-absorbent polymer particles having a particle size of less than 1 mm.

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

E) Silicon dioxide treatment

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

F) Surface cros slinking

In a further step 1 wt.-part of the precursor II is mixed with 0.003 wt.-part (+-10 %) of a surface crosslinker, based on the total weight of the mixture of precursor II and crosslinker. The surface crosslinker is composed of 19 wt.-% water, 40 wt.-% ethylene glycol diglycidyl ether, 1 wt.-% Na₂S0₃, 40 wt.-% poly ethylene glycol with a molecular weight of 400 g/mol, each based on the total amount of the crosslinker. The ingredients of the crosslinker are mixed in a line static mixer. The crosslinker is mixed in a ringlayer mixer CoriMix^® CM 350 (Ge- briider L5dige 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 μιτι. Less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples according to the invention are smaller than 150 μηι, less than 5 wt.-% of the surface-crosslinked absorbent polymer particles of the examples according to the invention are have a particle size of more than 850 μηι. Post treated crosslinked water-absorbent polymer particles are obtained.

NaOH.

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

It is apparent from the above table that the residual monomer content of the surface- crosslinked water-absorbent polymer particles can be significantly decreased by over- neutralization with an aqueous NaOH solution having a low temperature.

Figure 1 shows a flow chart diagram depicting the steps (101 to 1 11) provided after steps 1 12 and 1 14 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. The first step (il) 1 12 comprises: providing a first portion of acryl- ic acid comprising mono methyl ether hydroquinone (MEHQ) or hydroquinone (HQ) or both. The further step (i2) 113 comprises: contacting the first portion of acrylic acid with sodium hydroxide (NaOH) in a first contacting step, wherein a pH of 10 or more is obtained resulting in a first aqueous sodium-acrylate comprising phase. It is preferred that the first aqueous so- dium-acrylate comprising phase has a degree of neutralization of 1 or more. In step (i3) 1 14 the first acrylate phase with a further portion of acrylic acid in a second contacting step, thereby obtaining the monomer solution. In step (i) 101 the aqueous monomer solution comprising the at least one partially neutralized, monoethylenically unsaturated monomer bearing carboxylic acid groups (al). The monomer solution is prepared in steps (il) 1 12, step (i2) 1 14, step (i3) 1 16. Preferably, the aqueous monomer solution is an aqueous solution of partially neutralized acrylic acid, further comprising crosslinkers. Preferably, the partially neutralized acrylic acid is provided as a solution in the form of a first portion of acrylic acid comprising MEHQ neutralized with sodium hydroxide NaOH to a first acrylate phase. After neutralization, in the first acrylate phase about 96 mol-% of the acrylic acid is an acrylate. The aqueous monomer solution can also comprise a further monomer like acrylic acid or acrylamide. The monomer solution and at least one crosslinker (a3) is provided in step (i) 101.

In a second step 102 fine particles of a water-absorbent polymer may be added to the aqueous monomer solution. Preferably, these particles comprise poly acrylic acid particles. In a third step 103 a polymerization initiator or at least one component of a polymerization initi- ator system that comprises two or more components is added to the aqueous monomer solution. In a fourth step 104 the oxygen content of the aqueous monomer solution is decreased by bubbling nitrogen into the aqueous monomer solution. In a fifth step 105 the monomer solution is charged onto a belt 401 of a polymerization belt reactor 400. The belt 401 is a continuous conveyor belt. In a sixth step 106 the aqueous monomer solution is polymerized to a pol- ymer gel. In a seventh step 107 the polymer gel is discharged from the belt 401. Subsequently, the polymer gel is comminuted, whereby polymer gel particles are obtained. In an eighth step 108 the polymer gel particles are charged onto a belt of a belt dryer and subsequently dried at a temperature of about 120 to 150°C. The dried polymer gel particles are discharged from the belt dryer and subsequently in a ninth step 109 grinded to obtain water-absorbent polymer particles. In a tenth step 110 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 undergoes surface crosslinking treatment. Figure 2 shows a flow chart diagram depicting the steps 101 to 1 11 of a process 100 for the preparation of water-absorbent polymer particles according to the invention. The process 100 shown in figure 2 is the same as the process 100 in figure 1, wherein the third process step 103 and the fourth process step 104 overlap in time. While the polymerization initia- tor is added to the aqueous monomer solution, nitrogen is bubbled into the aqueous monomer solution in order to decrease its oxygen content.

Figure 3 shows a flow chart diagram depicting the steps 101, 103, and 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 basic setup of a polymerization belt reactor 400 according to the invention. The polymerization belt reactor 400 comprises a belt 401. The belt 401 is an endless conveyor belt. The belt 401 passes over two guide rollers 402 such that an upper run of the belt 401 moves downstream. The downstream movement of the upper run of the belt 401 determines the direction of movement 403 of the polymer gel (not shown here) on the belt 401, indicated by an arrow. Another arrow indicates a counter direction 404 to a direction of movement of the polymer gel on the belt. Another arrow indicates both longitudinal directions 405 of the belt 401 and yet another arrow the transversal directions 406 of the belt. The polymerization belt reactor 400 may comprise further components which are not shown in the figure, such as support elements, a supply and metering system, irradiating means, cooling and heating devices, and a removal system.

Figure 5 shows a block diagram of a device 500 for the preparation of water-absorbent polymer particles according to the invention. The arrows show a direction of a process stream 512 of the preparation of water-absorbent polymer particles. The device 500 comprises a first container 501, a second container 502, downstream to the second container 502 a cooling device 503, downstream to the cooling device 503 and the first container 501 a first mixing device 504, downstream to the first mixing device 504 a third container 505, a fourth container 506, downstream to the third container 505 and the fourth container 506 a second mixing de- vice 507, downstream a polymerization belt reactor 400, downstream a comminuting device 508, downstream a belt dryer 509, downstream a grinding device 510, and downstream a sizing device 51 1, each according to the invention.

Claims

[CLAIMS]

[Claim 1 ]

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

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

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

(viii) drying the optionally comminuted polymer gel;

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

(11) providing a first portion of acrylic acid; and

wherein the first acrylate phase has a pH of 10 or more, wherein the first acrylate phase has a degree of neutralization of at least

0.95; and (i3) contacting the first acrylate phase with a further portion of acrylic acid in a second contacting step, thereby obtaining the monomer solution, wherein the further portion of acrylic acid has a water content which is less than a water content of the first acrylate phase,

[Claim 2]

The process (100) according to claim 1, wherein the degree of neutralization of the acrylate phase in step (i2) is more than 1.

[Claim 3]

The process (100) according to claim 1 or 2, wherein the temperature of the first portion of acrylic acid and the temperature of the hydroxide differ by 2 or more.

[Claim 4]

The process (100) according to any of the preceding claims, wherein the further portion of acrylic acid comprises less than 5 wt.-% of water, based on the total weight of the further portion of acrylic acid.

[Claim 5]

The process (100) according to any of the preceding claims, wherein the first portion of acrylic acid comprises 50 wt.-% or more of water, based on the total weight of the first portion of acrylic acid.

[Claim 6]

The process (100) according to any of the preceding claims, wherein the pH of the monomer solution is in the range from 8 to 12.

[Claim 7] The process (100) according to any of the preceding claims, wherein the hydroxide is one selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide or a mixture of at least two thereof.

[Claim 8]

The process (100) according to any of the preceding claims, wherein the oxygen gas content of the monomer solution is in the range from 3 to 15 wt.-ppm, based on the weight of the monomer solution.

[Claim 9]

The process (100) according to any of the preceding claims, wherein the monomer solution is contacted with active carbon in a further contacting step.

[Claim 10]

The process (100) according to any of the preceding claims, wherein at least the first contacting step or the second contacting step or both are completed in a time period in the range from 10 to 300 minutes.

[Claim 1 1 ]

The process (100) according to any of the preceding claims, wherein at least the first contacting step or the second contacting step or both are performed at a temperature of the aqueous monomer solution of less than 40 °C.

[Claim 12]

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

[Claim 13]

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

[Claim 14]

The process (100) according to any of the preceding claims, wherein the polymer gel being discharged in process step (vii) is a polymer gel sheet;

wherein the polymer gel sheet is characterized by a thickness in the range from 10 to 200 mm. [Claim 15]

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

[Claim 16]

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

[Claim 17]

A device (500) for the preparation of water-absorbent polymer particles in a process stream (512), comprising

a) a first container (501), designed to take acrylic acid;

b) a second container(502), designed to take a metal hydroxide;

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

i) located down-stream to the second container (),

ii) designed to cool the metal hydroxide;

d) a first mixing device (504), wherein the first mixing device (504) is

ii) equipped with a cooler and a propelled mixing means;

e) a third container (505), wherein the third container (505) is

i) located down-stream to the first mixing device (504),

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

f) a fourth container (506), designed to take at least one crosslinker (a3);

g) a second mixing device (507), wherein the second mixing device (507) is

i) located down-stream to the third container (505) and the fourth container (506),

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

h) a polymerization belt reactor (400), wherein the polymerization belt reactor (400) i) is located down-stream to the second mixing device (507),

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

j) a comminuting device (508), wherein the comminuting device (508) is

k) a belt dryer (509), wherein the belt dryer (509) is

i) located down-stream to the comminuting device (508),

ii) designed to dry the polymer gel;

1) a grinding device (510), wherein the grinding device (510) is

i) located down-stream to the belt dryer (509),

m) a sizing device (511), wherein the sizing device (511) is

i) located down-stream to the grinding device (510),

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

[Claim 18]

A process for the preparation of water-absorbent polymer particles in the device (500) according to claim 17.

[Claim 19]

A water-absorbent polymer particle, obtainable by the process according to any of claims 1 to 16, or 18.

[Claim 20]

A composite material comprising a water-absorbent polymer particle according to claim 19.

[Claim 21 ]

The composite material according to claim 20, comprising one selected from the group consisting of a foam, a shaped article, a fibre, a foil, a film, a cable, a sealing material, a liquid-absorbing hygiene article, a carrier for plant and fungal growth- regulating agents, a packaging material, a soil additive, and a building material, or a combination of at least two thereof. [Claim 22]

A process for the production of a composite material, wherein a water-absorbent polymer particle according to claim 19 and a substrate and optionally an auxiliary substance are brought into contact with one another.

[Claim 23]

A composite material obtainable by a process according to claim 22.

[Claim 24]

A use of the water-absorbent polymer particle according to claim 19 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.