Polymer dispersion, its use and process for its preparation

An aqueous polymer dispersion comprising a styrene acrylate polymer obtained by free-radical polymerisation of

• 1-90 weight-% of a monomer (a) comprising at least one optionally substituted styrene,
• 10-99 weight-% of a monomer (b) comprising at least one alkyl (meth)acrylate, and
• 0-9 weight-% of a monomer (c) comprising at least one ethylenically unsaturated monomer, which is different from the monomer (b),
• in a polymerisation medium comprising a cationic prepolymer obtained by free radical polymerization, initiated by a presence of a first radical initiator comprising a peroxide, of at least
• 10-55 weight-% of a monomer (i) comprising at least one ethylenically unsaturated quaternary and/or tertiary amine,
• 35-90 weight-% of a monomer (ii) comprising at least one optionally substituted styrene, and
• 0-55 weight-% of a monomer (iii) comprising at least one alkyl (meth)acrylate;
• in a first polymerization stage with a polymerization solvent comprising at least one C1-C6 carboxylic acid anhydride, wherein a mole ratio of C1-C6 carboxylic acid anhydride to water in the polymerization medium is from 0.9:1.1 to 1.1:0.9, wherein oligomer content in the polymer dispersion is ≤1.4 weight-%, calculated from dry polymer content.