Method of reducing the content of residual monomers in aqueous polymer dispersions

A process for reducing the amount of residual monomer in an aqueous polymer dispersion prepared by free-radical polymerization, comprising:

• conducting an aftertreatment in the aqueous polymer dispersion which is heated to 60 to 120° C.;
• and
• adding of an initiator system which consists essentially of
• a) from 0.01 to 5% by weight, based on the overall amount of monomer used to prepare the polymer dispersion, of
• a1) a compound R1—OO—R2,
• where R1 and R2 are hydrogen, a C1-C8-alkyl or a C6-C12-aryl and are identical or different, with the proviso that at least one of R1 and R2 is hydrogen, and/or
• a2) a compound which in aqueous medium releases hydrogen peroxide, and
• b) from 0.01 to 30% by weight, based on the overall amount of monomer used to prepare the polymer dispersion, of an organic α-hydroxy carboxylic acid, which contains no polymerizable, olefinically unsaturated C—C double bond, or of a salt thereof, and
• c) from 10 to 1000 ppm by weight, based on the overall amount of monomer used to prepare the polymer dispersion, of a multivalent metal ion selected from the group consisting of an iron ion, a copper ion, a manganese ion, a nickel ion, a cerium ion, a chromium ion, a platinum ion and a palladium ion;
• wherein said initiator system contains no vanadium ions.