A process for preparing an aqueous polymer latex by polymerising a monomer composition M by a radical emulsion polymerisation, where the monomer composition M consists of a) 80 to 99.95% by weight, based on the total weight of the monomer composition M, of ethylenically unsaturated monomers M1, which are selected from mixtures of - at least one monomer M1a, selected from C 1 -C 20 -alkyl esters of acrylic acid and C 5 -C 20 -alkylesters of methacrylic acid; and - at least one monomer M1b, selected from vinyl aromatic monomers, and C 1 -C 4 -alkyl esters of methacrylic acid and mixtures thereof; b) 0.05 to 5% by weight, based on the total weight of the monomer composition M, of one or more monoethylenically unsaturated monomers M2, which are selected from monoethylenically unsaturated monocarboxylic acids having 3 to 6 carbon atoms and monoethylenically unsaturated dicarboxylic acids having 4 to 6 carbon atoms; c) 0 to 20% by weight of non-ionic monomers M3, which are different from monomers M1; where at least 95% of the monomer composition M to be polymerised are metered during a period P to the polymerisation reaction under polymerisation conditions, wherein during at least one period P(n) within said period P the relative amount of monomers M2, which are metered to the polymerisation reaction during the periods P(n), to the total amount of the monomer composition M, which is metered to the polymerisation reaction during said period P(n), is at least 1% by weight higher than the relative amount of monomers M2, which are metered to the polymerisation reaction outside of each of said periods P(n), to the total amount of the monomer composition M, which is metered to the polymerisation reaction outside of each of said periods P(n), where the first period P(n) starts at a point of time t(s) and the last period P(n) ends at a point of time t(e), where t(s) is at a point of time, where at least 10% of the total amount of the monomer composition M to be polymerised have been metered to the polymerisation reaction and where t(e) is at a point of time, where at most 90% of the total amount of the monomer composition M to be polymerised have been metered into the polymerisation reaction. a) 80 to 99.95% by weight, based on the total weight of the monomer composition M, of ethylenically unsaturated monomers M1, which are selected from mixtures of - at least one monomer M1a, selected from C 1 -C 20 -alkyl esters of acrylic acid and C 5 -C 20 -alkylesters of methacrylic acid; and - at least one monomer M1b, selected from vinyl aromatic monomers, and C 1 -C 4 -alkyl esters of methacrylic acid and mixtures thereof; - at least one monomer M1a, selected from C 1 -C 20 -alkyl esters of acrylic acid and C 5 -C 20 -alkylesters of methacrylic acid; and - at least one monomer M1b, selected from vinyl aromatic monomers, and C 1 -C 4 -alkyl esters of methacrylic acid and mixtures thereof; b) 0.05 to 5% by weight, based on the total weight of the monomer composition M, of one or more monoethylenically unsaturated monomers M2, which are selected from monoethylenically unsaturated monocarboxylic acids having 3 to 6 carbon atoms and monoethylenically unsaturated dicarboxylic acids having 4 to 6 carbon atoms; c) 0 to 20% by weight of non-ionic monomers M3, which are different from monomers M1;