Process for producing water-absorbent polymer particles by polymerizing droplets of a monomer solution

1. Water-absorbent polymer particles prepared according to by polymerizing droplets of

a) at least one ethylenically unsaturated monomer which bears an acid group and optionally is at least partly neutralized,

b) at least one crosslinker,

c 1 ) from 0.01 to less than 0.3% by weight, based on monomer a), of at least one persulfate,

c 2 ) at least 0.05% by weight, based on monomer a), of at least one azo initiator,

d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a),

e) optionally one or more water-soluble polymer, and

f) water,

in a surrounding heated gas phase in a reaction zone and thermal posttreatment in a fluidized bed, wherein a temperature of a gas leaving the reaction zone is less than 150° C., a temperature of the water-absorbent polymer particles during the thermal posttreatment is less than 100° C., and a residence time of the water-absorbent polymer particles in the fluidized bed is from 60 to 300 minutes.

2. Polymer particles according to claim 1 , wherein the heated gas phase is flowing cocurrent to the droplets through the reaction zone.

3. Polymer particles according to claim 1 , wherein the monomer solution comprises from 0.05 to 0.15% by weight, based on monomer a), of the at least one persulfate.

4. Polymer particles process according to claim 1 , wherein the monomer solution comprises from 0.2 to 0.5% by weight, based on monomer a), of the at least one azo initiator.

5. Polymer particles according to claim 1 , wherein the temperature of the gas leaving the reaction zone is from 110 to 120° C.

6. Polymer particles according to claim 1 , wherein the temperature of the gas entering the reaction zone is from 160 to 200° C.

7. Polymer particles according to claim 1 , wherein a gas velocity inside the reaction zone is from 0.1 to 2.5 m/s.

8. Polymer particles according to claim 1 , wherein the temperature of the water-absorbent polymer particles during the thermal posttreatment is from 60 to 80° C.

9. Polymer particles according to claim 1 , wherein the residence time of the water-absorbent polymer particles in the fluidized bed is from 120 to 240 minutes.

10. Polymer particles according to claim 1 , wherein a gas velocity inside the fluidized bed is from 0.3 to 2.5 m/s.

11. Polymer particles according to claim 1 , wherein the gas entering the fluidized bed comprises from 0.02 to 0.15 kg steam per kg of dry gas.

12. Polymer particles according to claim 1 , wherein the ethylenically unsaturated monomer which bears an acid group is an ethylenically unsaturated carboxylic acid.

13. Polymer particles according to claim 1 , wherein the ethylenically unsaturated monomer which bears an acid group is acrylic acid.

14. A fluid-absorbent article, comprising

(A) an upper liquid-pervious layer,

(B) a lower liquid-impervious layer and

(C) a fluid-absorbent core between the layer (A) and the layer (B), comprising from 5 to 90% by weight fibrous material and from 10 to 95% by weight water-absorbent polymer particles according to claim 1 ,

(D) an optional acquisition-distribution layer between (A) and (C), comprising from 80 to 100% by weight fibrous material and from 0 to 20% by weight water-absorbent polymer particles according to claim 1 ,

(E) an optional tissue layer disposed immediately above and/or below (C); and

(F) other optional components.

15. A fluid-absorbent article, comprising water-absorbent polymer particles according to claim 1 and less than 15% by weight fibrous material and/or adhesives in the absorbent core.

16. Polymer particles according to claim 1 , wherein the monomer solution comprises from 0.1 to 0.15% by weight, based on monomer a) of the persulfate.

17. Water-absorbent polymer particles having a bulk density of at least 0.65 g/cm 3 , a centrifuge retention capacity of at least 35 g/g, and a HC 60 value of at least 80.

18. Polymer particles according to claim 17 , wherein the polymer particles have a bulk density of at least 0.7 g/cm 3 .

19. Polymer particles according to claim 17 , wherein the polymer particles have a centrifuge retention capacity of at least 40 g/g.

20. Polymer particles according to claim 17 , wherein the polymer particles have a HC 60 value of at least 90.

21. A fluid-absorbent article, comprising

(A) an upper liquid-pervious layer,

(B) a lower liquid-impervious layer and

(C) a fluid-absorbent core between the layer (A) and the layer (B), comprising from 5 to 90% by weight fibrous material and from 10 to 95% by weight water-absorbent polymer particles according to claim 17 ,

(D) an optional acquisition-distribution layer between (A) and (C), comprising from 80 to 100% by weight fibrous material and from 0 to 20% by weight water-absorbent polymer particles according to claim 17 ,

(E) an optional tissue layer disposed immediately above and/or below (C); and

(F) other optional components.