Surface sizing composition, method of production, and use thereof

A method of producing a surface sizing composition, the method comprising:

• providing a metal salt, wherein the metal has at least 3 valence electrons, wherein the metal salt is selected from the group of aluminium sulfate, aluminium chloride, aluminium nitrate, polyaluminium sulfate (PAS), polyaluminium chloride (PAC), and polyaluminium chloride sulfate (PACS), polyaluminium formiate, polyaluminium nitrate, and any combination thereof;
• providing an aqueous polymer dispersion, wherein the aqueous polymer dispersion is an aqueous polymer dispersion (A) and an aqueous prepolymer composition (B),
• wherein the aqueous polymer dispersion (A) is obtainable by free radical emulsion copolymerizing a first ethylenically unsaturated monomer blend comprising
• (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 (meth)acrylate, and
• (c) 0 to 10% by weight of other ethylenically unsaturated copolymerizable monomers,
• wherein the sum (a)+(b)+(c) is 100%,
• in the presence of a water-soluble redox system comprising a first free radical initiator for the free radical emulsion copolymerization and
• wherein the aqueous prepolymer composition (B) is obtainable by free radical emulsion copolymerizing in a polymerization solvent comprising C1-6-carboxylic acid and C1-6-carboxylic acid anhydride, a second ethylenically unsaturated monomer blend comprising
• (i) 5 to 50% by weight of at least one ethylenically unsaturated quaternary amine selected from quaternary salt of N,N,N-tri(C1-4-alkyl)amino C1-4-alkyl(meth)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-alkyl(meth)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 (meth)acrylate, and
• (v) 0 to 10% by weight of other ethylenically unsaturated copolymerizable monomers,
• wherein the sum (i)+(ii)+(iii)+(iv)+(v) is 100%,
• in the presence of a second free radical initiator, and adding water to the obtained polymer dispersion to obtain the aqueous prepolymer composition (B);
• and
• mixing the metal salt and the aqueous polymer dispersion to provide a surface sizing composition to be used in papermaking.