Abstract: This invention relates to aqueous polymer dispersions obtained by free radical emulsion copolymerization of ethylenically unsaturated monomers comprising (a) 30 to 60% by weight of at least one optionally substituted styrene, (b) 60 to 30% by weight of at least one C1-C4-alkyl (meth)acrylate, and (c) 0 to 10% by weight of other ethylenically unsaturated copolymerizable monomers, in the presence of (d) 10 to 40% by weight of degraded starch having a molecular weight Mn of 500 to 10,000, and (e) a graft-linking, water-soluble redox system as free radical initiator for the free radical emulsion copolymerization, wherein the sum (a)+(b)+(c)+(d) is 100%.

Abstract: This invention relates to novel aqueous emulsion polymers obtained by free-radical initiated addition polymerization of at least one monomer in the presence of degraded starch and at least one polymeric emulsifier from the series of sulfo-containing maleic anhydride copolymers and/or sulfo-containing oligourethanes with a branched molecular structure. The invention further relates the use of such emulsion polymers as surface sizing agents for paper and paperlike materials.