Water-absorbing polymer structure with improved color stability

1. A process for the production of water-absorbing polymer structures, comprising the process steps:

i) providing an aqueous monomer solution containing

a polymerizable, monoethylenically unsaturated monomer (α1) carrying acid groups or a salt thereof,

optionally a monoethylenically unsaturated monomer (α2) which can be polymerized with the monomer (α1), and

optionally a crosslinking agent (α3),

ii) radical polymerizing the aqueous monomer solution to give a polymer gel,

iii) optionally comminutating the polymer gel,

iv) drying of the optionally comminuted polymer gel to give water-absorbing polymer structures,

v) optionally grinding and sieving of the water-absorbing polymer structures and

vi) surface post-crosslinking of the optionally ground and sieved water-absorbing polymer structures,

wherein

a reducing agent comprising a sulphonate, a salt of a sulphonate or a mixture of a sulphonate and a salt of a sulphonate is added to

I) the aqueous monomer solution before or while carrying out process step ii),

II) the polymer gel after carrying out process step ii),

III) the optionally comminuted polymer gel after carrying out process step iii),

IV) the water-absorbing polymer structure after carrying out process step iv),

V) the water-absorbing polymer structure after carrying out process step v), or

VI) the water-absorbing polymer structure after carrying out process step vi).

2. The process according to claim 1 , wherein the reducing agent is added after carrying out process step iv) or after carrying out process step v) but before carrying out process step vi).

3. The process according to claim 1 , wherein the reducing agent comprises less than 10 wt. % of sulphur compounds which differ from sulphonates, based on the total weight of the reducing agent.

4. The process according to claim 3 , wherein the reducing agent comprises less than 10 wt. % of sulphites, bisulphites, sulphinates or salts of these sulphur compounds, based on the total weight of the reducing agent.

5. The process according to claim 1 , wherein the sulphonate or the sulphonate salt has the structure I

in which X is a hydrogen atom, an ammonium cation or an alkali metal cation and R is

a saturated or unsaturated, aliphatic or aromatic C 1 -C 20 -hydrocarbon radical,

a radical of the structure YOOC—R 1 —, in which Y is a hydrogen atom, an ammonium cation or a monovalent metal cation and R 1 is a C 1 -C 7 -alkylene radical,

a radical of the structure ZOOC—R 2 —, in which Z is a hydrogen atom, an ammonium cation or a monovalent metal cation and R 2 is a C 1 -C 7 -alkylene radical, which carries a hydroxyl group on at least one of the carbon atoms,

a radical of the structure R′R″N—R 3 —, in which R 3 is a C 1 -C 9 -alkylene radical and the radicals R′ and R″ can be a hydrogen atom, a C 1 -C 5 -alkyl radical or a radical of the structure R″′—CO—, in which R′″ is a saturated or unsaturated C 1 -C 5 -hydrocarbon, or

a radical of the structure R 4 —[OCH 2 CH 2 ] n —O—R 5 - or a radical of the structure R 4 —[OCH 2 CHCH 3 ] n —O—R 5 —, in which R 4 is a C 1 -C 10 -alkyl radical or a C 1 -C 9 -acyl radical, n is an integer in a range of from 2 to 50 and R 5 is a C 1 -C 9 -alkylene radical.

6. The process according to claim 1 , wherein the sulphonate or the sulphonate salt is chosen from the group consisting of vinylsulphonic acid, allylsulphonic acid, vinyltoluenesulphonic acid, styrenesulphonic acid, sulphoethyl acrylate, sulphoethyl methacrylates, sulphopropyl acrylates, sulphopropyl methacrylates, 2-vinyl-4-ethylbenzenesulphonic acid, 2-allylbenzenesulphonic acid, 1-phenylethylenesulphonic acid, 2-hydroxy-3-methacryloxypropylsulphonic acid, 2-acrylamide-2-methylpropanesulphonic acid, para-toluenesulphonic acid, 2-hydroxy-2-sulphonatoacetic acid, salts of the abovementioned compounds or a mixture of at least two of these.

7. The process according to claim 1 , wherein the sulphonate salt comprises disodium salt of 2-hydroxy-2-sulphonatoacetic acid.

8. The process according to claim 1 , wherein the reducing agent is employed in an amount in a range of from about 0.001 to about 10 wt. %, based on the solids content of the water-absorbing polymer structure.

9. The process according to claim 1 , wherein the reducing agent is added in the form of a fluid comprising the reducing agent and a solvent.

10. The process according to claim 9 , wherein the solvent is water.

11. A water-absorbing polymer structure obtainable by a process according to claim 1 .

12. A water-absorbing polymer structure comprising about 10 to about 100,000 ppm, based on the solids content of the water-absorbing polymer structure, of a non-polymerized sulphonate, a non-polymerized salt of a sulphonate or a mixture of a non-polymerized sulphonate and a non-polymerized salt of a sulphonate, and less than 1,000 ppm, based on the solids content of the water-absorbing polymer structure, of non-polymerized sulphites, non-polymerized bisulfites, non-polymerized sulphinates or non-polymerized salts of these compounds.

13. A water-absorbing polymer structure, the surface of which has been brought into contact with about 0.001 to about 10 wt. %, based on the solids content of the water-absorbing polymer structure, of a reducing agent comprising a sulphonate, a salt of a sulphonate or a mixture of a sulphonate and a salt of a sulphonate.

14. The water-absorbing polymer structure according to claim 13 , wherein the reducing agent comprises less than 10 wt. %, based on the total weight of the reducing agent, of sulphur compounds which differ from sulphonates.

15. The water-absorbing polymer structure according to claim 13 , wherein the reducing agent comprises less than 10 wt. %, based on the total weight of the reducing agent, of sulphites, bisulphites, sulphinates or salts of these sulphur compounds.

16. The water-absorbing polymer structure according to claim 12 , wherein the sulphonate or the salt of a sulphonate is a sulphonate or a salt of a sulphonate

has the structure I

in which X is a hydrogen atom, an ammonium cation or an alkali metal cation and R is

a saturated or unsaturated, aliphatic or aromatic C 1 -C 20 -hydrocarbon radical,

a radical of the structure YOOC—R 1 —, in which Y is a hydrogen atom, an ammonium cation or a monovalent metal cation and R 1 is a C 1 -C 7 -alkylene radical,

a radical of the structure ZOOC—R 2 —, in which Z is a hydrogen atom, an ammonium cation or a monovalent metal cation and R 2 is a C 1 -C 7 -alkylene radical, which carries a hydroxyl group on at least one of the carbon atoms,

a radical of the structure R′R″N-R 3 —, in which R 3 is a C 1 -C 9 -alkylene radical and the radicals R′ and R″ can be a hydrogen atom, a C 1 -C 5 —alkyl radical or a radical of the structure R′″—CO—, in which R′″ is a saturated or unsaturated C 1 -C 5 -hydrocarbon, or

a radical of the structure R 4 —[OCH 2 CH 2 ] n —O—R 5 - or a radical of the structure R 4 —[OCH 2 CHCH 3 ] n —O—R 5 —, in which R 4 is a C 1 -C 10 -alkyl radical or a C 1 -C 9 -acyl radical, n is an integer in a range of from 2 to 50 and R 5 is a C 1 -C 9 -alkylene radical.

17. The water-absorbing polymer structure according to claim 11 having at least one of the following properties:

((β1) a whiteness index, determined in accordance with the test method described herein, of at least about 7.5 after storage of the water-absorbing polymer structure for 20 days at 60° C. and at 75% relative atmospheric humidity;

((β2) an absorption, determined in accordance with ERT 442.2-02, under a pressure of about 50 g/cm 2 (0.7 psi) of at least about 18 g/g;

((β 3 ) an SFC value (SFC=“saline flow conductivity”), determined in accordance with the test method described herein, at a TB value of <26 g/g, determined in accordance with the test method described herein, of at least about 80×10 −7 cm 3 ·s·g −1 ,

(β4) an SFC value, determined in accordance with the test method described herein, at a TB value in the range of >26 to <27 g/g, determined in accordance with the test method described herein, of at least about 70×10 −7 cm 3 ·s·g −1 ,

(β5) an SFC value, determined in accordance with the test method described herein, at a TB value in the range of >27 to <28 g/g, determined in accordance with the test method described herein, of at least about 60× 10 −7 cm 3 ·s·g −1 ,

(β6) an SFC value, determined in accordance with the test method described herein, at a TB value in the range of >28 to <29 g/g, determined in accordance with the test method described herein, of at least about 45 × 10 7 cm 3 ·s·g −1 ,

(β7) an SFC value, determined in accordance with the test method described herein, at a TB value in the range of >29 to <30 g/g, determined in accordance with the test method described herein, of at least about 30× 10 −7 cm 3 ·s·g −1 ,

(β8) an SFC value, determined in accordance with the test method described herein, at a TB value in the range of >30 to <31 g/g, determined in accordance with the test method described herein, of at least about 20×10 −7 cm 3 ·s·g −1 ,

(β9) an SFC value, determined in accordance with the test method described herein, at a TB value in the range of >31 g/g, determined in accordance with the test method described herein, of at least about 10× 10 −7 cm 3 ·s·g −1 .

18. The water-absorbing polymer structure according to claim 11 , comprising about 10 to about 100,000 ppm, based on the solids content of the water-absorbing polymer structure, of a non-polymerized sulphonate, a non-polymerized salt of a sulphonate or a mixture of a non-polymerized sulphonate and a non-polymerized salt of a sulphonate, and less than about 1,000 ppm, based on the solids content of the water-absorbing polymer structure, of non-polymerized sulphites, non-polymerized bisulfites, non-polymerized sulphinates or non-polymerized salts of these compounds.

19. A composite comprising a water-absorbing polymer structure according to claim 11 and a substrate.

20. A process for the production of a composite, wherein a water-absorbing polymer structure according to claim 11 and a substrate and optionally an auxiliary substance are brought into contact with one another.

21. A composite obtainable by a process according to claim 20 .

22. An article selected from foams, shaped articles, fibers, foils, films, cables, sealing materials, liquid-absorbing hygiene articles, carriers for plant and fungal growth-regulating agents, packaging materials, soil additives or building materials comprising the water-absorbing polymer structure according to claim 11 .

23. A use of the water-absorbing polymer structure according to claim 11 , the use comprising providing the water-absorbing polymer structure in foams, shaped articles, fibers, foils, films, cables, sealing materials, liquid-absorbing hygiene articles, carriers for plant and fungal growth-regulating agents, packaging materials, soil additives, for controlled release of active compounds, or in building materials.

24. The process according to claim 1 , wherein the sulphonate salt comprises 2-hydroxy-2-sulfonatoacetic acid.

25. The water-absorbing polymer structure according to claim 12 , wherein the sulphonate salt comprises 2-hydroxy-2-sulfonatoacetic acid.

26. The water-absorbing polymer structure according to claim 13 , wherein the sulphonate salt comprises 2-hydroxy-2-sulfonatoacetic acid.