AQUEOUS BINDER COMPOSITION

The current invention relates to a process for preparing an aqueous binder composition, the process comprising at least the following steps A) and B): A) preparing carboxylic acid-functional vinyl polymer (Polymer A) by emulsion polymerisation of a monomer composition A comprising the following monomers: Ai) at least one carboxylic acid functional olefinically unsaturated monomer; and Aii) at least one olefinically unsaturated monomer different than Ai); B) preparing vinyl polymer (Polymer B) by emulsion polymerization of a monomer composition B in the presence of Polymer A where the monomer composition B comprises the following monomer(s): Bi) at least one olefinically unsaturated monomer; Bii) optionally carboxylic acid functional olefinically unsaturated monomer different than Bi); where at least part of the carboxylic acid groups of Polymer A are deprotonated to obtain ionic or potentially ionic water-dispersing groups; where the acid value of Polymer B is lower than the acid value of Polymer A; where the weight average molecular weight (Mw) of Polymer B is higher than the weight average molecular weight (Mw) of Polymer A; where the glass transition temperature of Polymer B is equal to or lower than that of Polymer A; where the weight ratio of Polymer A to Polymer B is in the range of from 5:95 to 95:5; and whereby step A) comprises at least a first polymerisation stage of a monomer composition A1 effected in the presence of a chain transfer agent C1 having a water solubility of at least 5 g/L and a subsequent second polymerisation stage of a monomer composition A2 effected in the presence of a chain transfer agent C2 having a water solubility of at most 0.5 g/L, and whereby the second polymerisation stage of step A) is effected in the optional further presence of chain transfer agent C1 having a water solubility of at least 5 g/L whereby the concentration of chain transfer agent C1 having a water solubility of at least 5 g/L in the second polymerisation stage of step A) is at most 35 mole-%, relative to the total molar concentration of chain transfer agent C1 and chain transfer agent C2 present in the second polymerisation stage of step A).