METHOD FOR PRODUCING WATER-ABSORBENT POLYMER PARTICLES WITH A HIGHER PERMEABILITY BY POLYMERISING DROPLETS OF A MONOMER SOLUTION

Abstract

A process for preparing water-absorbing polymer beads with high permeability by polymerizing droplets of a monomer solution, comprising monomers bearing acid groups, in a gas phase surrounding the droplets, wherein the monomer solution comprises polyvalent cations and the polymer beads have a mean diameter of at least 150 m.

Claims

1. A process for preparing water-absorbing polymer beads comprising polymerizing droplets of a monomer solution comprising a) at least one ethylenically unsaturated monomer bearing acid groups, b) at least one crosslinker, c) at least one initiator, and d) water, in a gas phase surrounding the droplets, wherein the monomer solution comprises polyvalent cations and the polymer beads have a mean diameter of at least 150 m.

2. The process according to claim 1, wherein the monomer solution comprises from 0.001 to 0.25% by weight of polyvalent cations, based on monomer a).

3. The process according to claim 1, wherein the polyvalent cation is at least trivalent.

4. The process according to claim 1, wherein monomer a) is acrylic acid to an extent of at least 50 mol %.

5. The process according to claim 1, wherein the polymer beads have a mean diameter of at least 200 m.

6. The process according to claim 1, wherein at least 90% by weight of the polymer beads have a diameter of from 100 to 800 m.

7. The process according to claim 1, wherein a carrier gas flows through a reaction chamber.

8. The process according to claim 7, wherein the carrier gas leaving the reaction chamber is recycled at least partly after one pass.

9. The process according to claim 7, wherein an oxygen content of the carrier gas is from 0.001 to 0.15% by volume.

10. The process according to claim 1, wherein the polymer beads are dried and/or postcrosslinked in at least one further process step.

11.-22. (canceled)

23. The process according to claim 1, wherein the acid groups of the ethylenically unsaturated monomer are at least party neutralized.

24. The process according to claim 16, wherein the ethylenically unsaturated monomer is acrylic acid to an extent of at least 50 mol %.

25. The process according to claim 1 wherein the water-absorbing polymer beads have a mean sphericity of at least 0.84, a content of hydrophobic solvents of less than 0.005%, by weight, and a permeability of at least 510.sup.7 cm.sup.3 s/g.

26. The process according to claim 1 wherein the content of the hydrophobic solvents in the water-absorbing beads is less than 0.005%, by weight.

27. The process according to claim 1 wherein the water-absorbing beads are essentially free of the hydrophobic solvents.

28. The process according to claim 1 wherein the water-absorbing beads are free of the hydrophobic solvents.

29. The process according to claim 1, wherein the water-absorbing beads have a permeability of at least 1510.sup.7 cm.sup.3 s/g.

30. The process according to claim 1 wherein the water-absorbing beads have a mean sphericity of at least 0.86.

31. The process according to claim 1 wherein the water-absorbing beads have a mean sphericity of at least 0.88.

32. The process according to claim 1 wherein the water-absorbing beads have a mean sphericity of at least 0.9.

Description

EXAMPLES

[0136] 14.6 kg of sodium acrylate (37.5% by weight solution in water) and 1.4 kg of acrylic acid were mixed with 28.0 g of 15-tuply ethoxylated trimethylolpropane triacrylate and 0 to 55.9 g of aluminum sulfate. The solution was dropletized into a heated dropletization tower filled with nitrogen atmosphere (180 C., height 12 m, width 2 m, gas velocity 0.1 m/s in cocurrent). The metering rate was 16 kg/h. The dropletizer plate had 37 bores of 170 m. The diameter of the dropletizer plate was 65 mm. The initiator was metered into the monomer solution via a Venturi mixer just upstream of the dropletizer. The initiator used was a 15% by weight solution of 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride in water. The metering rate of the initiator solution was 0.224 kg/h. The gas exit temperature from the dropletization tower was 130 C. The mean particle diameter of the resulting polymer beads was 270 m.

[0137] The resulting water-absorbing polymer beads were then analyzed. The results are summarized in Table 1:

TABLE-US-00001 TABLE 1 Influence of the polyvalent cation Amount of aluminum Aluminum sulfate CRC SFC Example sulfate content*.sup.) [g/g] [10.sup.7 cm.sup.3s/g] 1 0.0 g none 39.2 4 2 28.0 g 0.50% by wt. 41.2 7 3 41.9 g 0.75% by wt. 40.4 12 4 55.9 g 1.00% by wt. 39.3 19 *.sup.)based on acrylic acid