Method for producing water-absorbent polymer particles with a higher permeability by polymerising droplets of a monomer solution
A process for preparing water-absorbing polymer beads with high permeability by polymerizing droplets of a monomer solution in a gas phase surrounding the droplets, wherein the monomer solution comprises at least 0.5% by weight of a crosslinker, based on the monomer, the polymerization in the droplet takes place in homogeneous phase, and the polymer beads have a mean diameter of at least 150 μm.
1. Water-absorbing polymer beads having 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 5×10 −7 cm 3 s/g.
2. Polymer beads according to claim 1 , having a centrifuge retention capacity of at least 10 g/g.
3. Polymer beads according to claim 1 , having a mean diameter of at least 200 μm.
4. Polymer beads according claim 1 , wherein at least 90%, by weight, of the polymer beads have a diameter of from 100 to 800 μm.
5. Polymer beads according to claim 1 , comprising at least partly of polymerized ethylenically unsaturated acid-bearing monomers.
6. Polymer beads according to claim 5 , wherein the acid groups of the polymerized monomer are at least partly neutralized.
7. Polymer beads according to claim 5 , wherein the ethylenically unsaturated monomer is acrylic acid to an extent of at least 50 mol %.
8. Polymer beads according to claim 1 , which comprise at least 0.5%, by weight, of copolymerized crosslinker b).
9. Polymer beads according to claim 4 prepared by metering a monomer solution comprising
a) at least one ethylenically unsaturated monomer,
b) at least one crosslinker,
c) at least one initiator, and
d) water,
into a gas phase to form droplets and polymerizing the droplets of the monomer solution in the gas phase surrounding the droplets, the polymerization in the droplet taking place in a homogeneous phase, wherein the monomer solution comprises at least 0.5%, by weight, of the crosslinker b), based on the monomer a), and the polymer beads have a mean diameter of at least 150 μm.
10. Polymer beads according to claim 9 , wherein the monomer a) has at least one acid group.
11. Polymer beads according to claim 10 , wherein the acid groups of the monomer a) are at least partly neutralized.
12. Polymer beads according to claim 9 , wherein the monomer a) is acrylic acid to an extent of at least 50 mol %.
13. Polymer beads according to claim 9 , wherein the polymer beads have a mean diameter of at least 200 μm.
14. Polymer beads according to claim 9 , wherein at least 90%, by weight, of the polymer beads have a diameter of from 100 to 800 μm.
15. Polymer beads according to claim 9 , wherein a carrier gas flows through a reaction chamber.
16. Polymer beads according to claim 15 , wherein the carrier gas leaving the reaction chamber is recycled at least partly after one pass.
17. Polymer beads according to claim 15 , wherein an oxygen content of the carrier gas is from 0.001 to 0.15% by volume.
18. Polymer beads according to claim 9 , wherein the polymer beads are dried and/or postcrosslinked in at least one further process step.
19. A hygiene article comprising polymer beads according to claim 5 .
20. Polymer beads according to claim 9 wherein the monomer solution comprises at least 1.5%, by weight, of the crosslinker b).
21. Polymer beads according to claim 5 wherein the content of the hydrophobic solvents is less than 0.0005%, by weight.
22. Polymer beads according to claim 5 wherein the beads are essentially free of the hydrophobic solvents.
23. Polymer beads according to claim 5 wherein the beads are free of the hydrophobic solvents.
24. Polymer beads according to claim 5 , which comprise at least 1.5%, by weight, of copolymerized crosslinkers.