Water-absorbing polymer structure with improved absorption properties

1. A water-absorbing polymer structure, which is based at least to 50 wt. % on carboxylate groups containing monomers wherein the water-absorbing polymer structure has been dried and comminuted into dried water-absorbing polymer structure particles wherein at least 50 wt % of the dried water-absorbing polymer structure particles have a particle size of from 300 μm to 600 μm wherein the dried water-absorbing polymer structure particles have an inner region and an outer region surrounding the inner region wherein the surface of the water-absorbing polymer structure particles has been brought into contact with a combination of an organic post-crosslinker, a metal salt of a bivalent or trivalent metal cation and zinc oxide, wherein at least 50 wt. % of the zinc oxide have a particle size in a range of 10 to 5,000 nm and wherein the metal salt is a water-soluble metal salt wherein the metal salt has a water-solubility at a temperature of 25° C. of at least 100 g in 1 liter of distilled water; and the contacted water-absorbing polymer structure particles have been heated to a temperature of from 150° C. to 250° C. whereby the outer region of the water-absorbing polymer structure particles is more strongly crosslinked than the inner region.

2. The water-absorbing polymer structure according to claim 1 wherein the polymer structure has an R/L-ratio of at least 2.00 g/g %.

3. The water-absorbing polymer structure according to claim 1 , wherein the metal salt is a sulfate.

4. A water-absorbing polymer structure according to claim 1 wherein the organic post-crosslinker comprises ethylene carbonate.

5. A process for treatment of the surface of water-absorbing polymer structures, comprising the process steps:

i) providing an untreated water-absorbing polymer structure which is based to at least 50 wt. % on carboxylate groups containing monomers;

ii) drying the untreated water-absorbing polymer structure to form dried water-absorbent polymer structures;

iii) comminuting the dried water-absorbent polymer structures into dried water-absorbing polymer structure particles wherein at least 50 wt % of the dried water-absorbing polymer structure particles have a particle size of from 300 μm to 600 μm wherein the dried water-absorbing polymer structure particles have an inner region and an outer region surrounding the inner region

iv) bringing into contact the surface of the dried water-absorbing polymer structure particles with a combination of an organic post-crosslinker, a metal salt of a bivalent or trivalent metal cation and zinc oxide at a temperature from 150 to 250° C., wherein at least 50 wt. % of the zinc oxide has a particle size from 10 to 5,000 nm, and wherein the metal salt is a water soluble metal salt having a water-solubility at a temperature of 25° C. of at least 100 g in 1 liter of distilled water;

whereby the outer region of the dried water-absorbing polymer structure particles is more strongly crosslinked than the inner region.

6. The process according to claim 5 wherein the metal salt and the zinc oxide in the form of a fluid F 1 comprising a solvent, the metal salt, and the zinc oxide are brought into contact with the water-absorbing polymer structure.

7. The process according to claim 6 , wherein the fluid F 1 additionally comprises an organic post-crosslinker which is capable of reacting in a condensation reaction with functional groups of the water-absorbing polymer structure.

8. The process according to claim 5 wherein the metal salt in an amount from 0.001 to 5 wt. % and the zinc oxide in an amount from 0.001 to 5 wt. %, respectively based on the weight of the untreated water-absorbing polymer structure, are brought into contact with the untreated water-absorbing polymer structure.

9. The process according to claim 5 wherein the metal salt is a salt of aluminum.

10. A composite comprising a water-absorbing polymer structure according to claim 1 and a substrate.