Water-absorbing polysaccharide and method for producing the same

1. A process for producing a particulate water-absorbent polycarboxypolysaccharide wherein said water-absorbent polycarboxypolysaccharide is chemically modified with an average degree of carboxyl substitution within the range from 0.4 to about 1.5 and has a (α1) an absorption under load value at a pressure of 0.9 psi within a range from 10 to 22 g/g with a centrifuge retention capacity value within a range from 15 to 20 g/g; or (α2) an absorption under load value at a pressure of 0.9 psi within a range from 6 to 20 g/g with a centrifuge retention capacity value within a range from 20 to 25 g/g, the process comprising the steps of:

a) preparing an aqueous solution having a pH of from about 8 to about 12 by mixing a polycarboxypolysaccharide with a crosslinking agent consisting essentially of polyphosphoric acid in an amount within a range from 0.001 to 20 wt. %, based upon the weight of the polycarboxypolysaccharide, in the presence of water to form said water-absorbent polycarboxypolysaccharide, wherein the mixing step occurs in a kneader wherein the kneader has at least two kneading shafts; and

b) drying the water-absorbent polycarboxypolysaccharide gel on a belt drier at a temperature of from about 120° C. to about 130° C. for a time period of from about 50 minutes to about 150 minutes.

2. The process according to claim 1 , wherein the at least two kneader shafts have a contour which at least partially reaches into each other.

3. The process according to claim 1 , wherein the at least two kneader shafts form a conveying channel running at least partially axial to one of the knead shafts.

4. The process according to claim 1 , wherein the carboxyl groups of the uncrosslinked polycarboxypolysaccharide are neutralized to at least about 50 mol %.

5. The process according to claim 1 , wherein the crosslinking or the drying occurs at a temperature above about 70° C.

6. The process according to claim 1 , wherein the mixing step of the polycarboxypolysaccharide with the crosslinking agent occurs in the absence of an organic solvent.

7. The process according to claim 1 , wherein the water-absorbent polycarboxypolysaccharide comprises a salt content of less than about 20 wt. %, based upon the total weight of the polycarboxypolysaccharide.

8. The process according to claim 1 , wherein the polyphosphate comprises as crosslinking agent the composition M n+2 I [P n O 3n+1 ] or M n I [H 2 P n O 3n+1 ], wherein M I is a monovalent metal and n has a value of at least 2.

9. The process according to claim 1 , wherein the polyphosphoric acid as crosslinking agent has the composition H n+2 P n O 3n+1 or (HPO 3 ) n , in which n has a value of at least 2.

10. The process according to claim 1 , wherein the polycarboxypolysaccharide gel is comminuted before the drying and/or wherein the dried, crosslinking polysaccharide is milled, so that particulate, crosslinked polycarboxypolysaccharides are obtained.

11. The process according to claim 10 , wherein the particulate, crosslinked polycarboxypolysaccharide is post-crosslinked in the outer part of the particle with a post-crosslinking agent.

12. The process according to claim 11 , wherein the post-crosslinking agent is used in the form of about 0.01 to about 80 wt. % aqueous solution.

13. The process according to claim 11 , wherein the post-crosslinking agent is a polyphosphate or polyphosphoric acid.

14. The process according to claim 11 , wherein the post-crosslinking of the crosslinked polycarboxypolysaccharides with the post-crosslinking agent occurs in the presence of inorganic particles.

15. A particulate, water-absorbent polycarboxypolysaccharide obtainable by a process for producing the particulate water-absorbent polycarboxypolysaccharide wherein said water-absorbent polycarboxypolysaccharide is chemically modified with an average degree of carboxyl substitution within the range from 0.4 to about 1.5 and has a (α1) an absorption under load value at a pressure of 0.9 psi within a range from 10 to 22 g/g with a centrifuge retention capacity value within a range from 15 to 20 g/g; or (α2) an absorption under load value at a pressure of 0.9 psi within a range from 6 to 20 g/g with a centrifuge retention capacity value within a range from 20 to 25 g/g, the process comprising the process steps of:

a) preparing an aqueous solution having a pH of from about 8 to about 12 by mixing a polycarboxypolysaccharide with a crosslinking agent consisting essentially of polyphosphoric acid in an amount within a range from 0.001 to 20 wt. %, based upon the weight of the polycarboxypolysaccharide, in the presence of water to form the water-absorbent polycarboxypolysaccharide gel wherein the mixing step occurs in a kneader wherein the kneader has at least two kneading shafts; and

b) drying the water-absorbent polycarboxypolysaccharide gel on a belt drier at a temperature of from about 120° C. to about 130° C. for a time period of from about 50 minutes to about 150 minutes.

16. The particulate, water-absorbent polycarboxypolysaccharide according to claim 15 , wherein the polysaccharide is an at least partially neutralized polycarboxypolysaccharide.

17. The particulate, water-absorbent polycarboxypolysaccharide according to claim 15 , wherein the polycarboxypolysaccharide is present in particulate form with a particle diameter within a range from 150 to 850 μm.

18. The particulate, water-absorbent polycarboxypolysaccharide according to claim 15 , wherein the particulate, water-absorbent polycarboxypolysaccharide has a Gel Bed Permeability of from about 20×10 −9 cm 2 to 500×10 −9 cm 2 .

19. A composite comprising a water-absorbent, at least partially neutralized polycarboxypolysaccharide according to claim 15 and a substrate.