Cationic surface sizing agent

A process for preparation of an aqueous polymer dispersion (A), the method comprising:

• free radical polymerizing, in a polymerization solvent having C1-6-carboxylic acid and C1-6-carboxylic acid anhydride, in a presence of a second free radical initiator a second ethylenically unsaturated monomer blend having:
• (i) 5 to 50% by weight of at least one ethylenically unsaturated quarternary amine selected from quaternary salt of N,N,N-tri(C1-4-alkyl)amino C1-4-alkylmethacrylate or acrylate with a mineral acid or an organic acid and/or quaternary salt of N,N,N-tri(C1-4-alkyl)amino C1-4-alkyl(meth)acrylamide with a mineral acid or an organic acid,
• (ii) 0 to 40% by weight of at least one ethylenically unsaturated tertiary amine selected from N,N-di(C1-4-alkyl)amino C1-4-alkylmethacrylate or acrylate and/or N,N-di(C1-4-alkyl)amino C1-4-alkyl(meth)acrylamide,
• (iii) 10 to 95% by weight of at least one optionally substituted styrene,
• (iv) 0 to 50% by weight of at least one C1-C4-alkyl methacrylate or acrylate, and
• (v) 0 to 10% by weight of other ethylenically unsaturated copolymerizable monomers, wherein a sum of (i)+(ii)+(iii)+(iv)+(v) is 100%;
• adding water to an obtained polymer composition to obtain an aqueous prepolymer composition (B);
• and
• copolymerizing in a presence of said aqueous prepolymer composition (B) and a water-soluble redox system having a first free radical initiator for the free radical emulsion copolymerization of a first ethylenically unsaturated monomer blend having:
• (a) 0 to 75% by weight of at least one optionally substituted styrene,
• (b) 15 to 100% by weight of at least one C1-C4-alkyl methacrylate or acrylate, and
• (c) 0 to 10% by weight of other ethylenically unsaturated copolymerizable monomers, wherein a sum of (a)+(b)+(c) is 100%,
• to obtain the aqueous polymer dispersion (A).