Process for Producing Water-Absorbent Polymer Particles by Polymerizing Droplets of a Monomer Solution

A process for producing water-absorbent polymer particles by polymerizing droplets of a monomer solution, comprising

• a) at least one ethylenically unsaturated monomer which bears an acid group and may be at least partly neutralized,
• b) at least one crosslinker,
• c) at least one 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 and flowing the gas cocurrent through a polymerization chamber, wherein a temperature of the gas leaving the polymerization chamber is from 90 to 150° C. and a gas velocity inside the polymerization chamber is from 0.1 to 2.5 m/s, followed by thermal postcrosslinking at a temperature of at least 60° C. with a postcrosslinker comprising groups which can form at least two covalent bonds with acid groups of the polymer particles in the presence of a gas stream comprising at least 0.01 kg of steam per kg of dry gas.