An aqueous dispersion of a multistage polymer, wherein the multistage polymer comprises: (A) a polymer A comprises as polymerized units, based on the weight of the polymer A, (a1) from 75% to 92% by weight of a hydrophilic (meth)acrylic acid alkyl ester; (a2) from 2.1% to 10% by weight of a carbonyl-containing functional monomer; and (a3) from 5% to 15% by weight of an acid monomer comprising from 5% to 14% by weight of an α, β-ethylenically unsaturated carboxylic acid and from 1% to 9% by weight of a phosphorous-containing acid monomer; and wherein the polymer A has a number average molecular weight as determined by GPC according to the experimental part of the description of from 3,000 to 100,000; and (B) a polymer B comprises as polymerized units, based on the weight of the polymer B, (b1) from 81% to 99% by weight of a hydrophobic monomer selected from a vinyl aromatic monomer, a hydrophobic (meth)acrylic acid alkyl ester, or mixtures thereof; (b2) from 0.8% to 10% by weight of a carbonyl-containing functional monomer; and (b3) from 0 to 15% by weight of a hydrophilic monomer; wherein the polymer B has a glass transition temperature calculated using the Fox Equation according to the description at least 40°C lower than that of the polymer A, and the weight ratio of the polymer A to the polymer B is from 25:75 to 50:50. (A) a polymer A comprises as polymerized units, based on the weight of the polymer A, (a1) from 75% to 92% by weight of a hydrophilic (meth)acrylic acid alkyl ester; (a2) from 2.1% to 10% by weight of a carbonyl-containing functional monomer; and (a3) from 5% to 15% by weight of an acid monomer comprising from 5% to 14% by weight of an α, β-ethylenically unsaturated carboxylic acid and from 1% to 9% by weight of a phosphorous-containing acid monomer; and wherein the polymer A has a number average molecular weight as determined by GPC according to the experimental part of the description of from 3,000 to 100,000; and (a1) from 75% to 92% by weight of a hydrophilic (meth)acrylic acid alkyl ester; (a2) from 2.1% to 10% by weight of a carbonyl-containing functional monomer; and (a3) from 5% to 15% by weight of an acid monomer comprising from 5% to 14% by weight of an α, β-ethylenically unsaturated carboxylic acid and from 1% to 9% by weight of a phosphorous-containing acid monomer; and (B) a polymer B comprises as polymerized units, based on the weight of the polymer B, (b1) from 81% to 99% by weight of a hydrophobic monomer selected from a vinyl aromatic monomer, a hydrophobic (meth)acrylic acid alkyl ester, or mixtures thereof; (b2) from 0.8% to 10% by weight of a carbonyl-containing functional monomer; and (b3) from 0 to 15% by weight of a hydrophilic monomer; wherein the polymer B has a glass transition temperature calculated using the Fox Equation according to the description at least 40°C lower than that of the polymer A, and the weight ratio of the polymer A to the polymer B is from 25:75 to 50:50. (b1) from 81% to 99% by weight of a hydrophobic monomer selected from a vinyl aromatic monomer, a hydrophobic (meth)acrylic acid alkyl ester, or mixtures thereof; (b2) from 0.8% to 10% by weight of a carbonyl-containing functional monomer; and (b3) from 0 to 15% by weight of a hydrophilic monomer; <EOS>