Process for preparing aqueous polymer dispersions

A process for preparing an aqueous polymer dispersion, wherein the process comprises:

• a free-radically initiated emulsion polymerization in an aqueous medium, in the presence of at least one vinyl polymer A, to obtain a polymer B comprising a vinyl polymer phase B1 with a glass transition temperature, calculated by means of the Fox equation, of from −20 to +15° C. and a vinyl polymer phase B2 with a glass transition temperature, calculated by means of the Fox equation, of from +50 to +110° C., with the proviso that the difference in glass transition temperature between polymer B1 and polymer B2 is at least 40° C.,
• wherein:
• the weight ratio of polymer B1 to polymer B2 is from 80:10 to 10:20,
• the at least one polymer A is obtained by a process comprising:
• free-radically initiated bulk or solution polymerizing of:
• from 5 to 25 wt. % of at least one carboxylic acid functional ethylenically unsaturated monomer A1, and
• from 75 to 95 wt. % of at least one ethylenically unsaturated monomer A2 different than monomer A1,
• where the amounts are given relative to the total weight of monomers charged in the polymerization to prepare the vinyl polymer A,
• the glass transition temperature of the at least one vinyl polymer A, calculated by means of the Fox equation, is from 40 to 150° C.,
• the weight average molecular weight of the at least one polymer A, determined via SEC, is from 1000 to 70000 g/mol,
• the weight average molecular weight of the polymer A is lower than the weight average molecular weight of the polymer B, determined via SEC, and
• the weight ratio of polymer A to polymer B is from 10:90 to 40:60.