A graft copolymer B constructed from: B1: 40 to 85 wt%, based on the solids content of the graft copolymer B, of a graft substrate obtainable by (a) polymerizing:: 10.5 to 24.5 wt%, based on the graft substrate B1, of at least one vinylaromatic, in particular styrene, and: 75.5 to 89.5 wt%, based on the graft substrate B1, of at least one diene, in particular butadiene, where and sum to 100 wt%; and (b) agglomerating the obtained graft substrate B1 by adding: 0.01 to 5 parts by weight, based on 100 parts by weight of the graft substrate B1, in each case based on the solids content, of an agglomerating copolymer of:: 80 to 99.9 wt% of one or more C 1 to C 12 alkyl acrylates or C 1 to C 12 alkyl methacrylates and: 0.1 to 20 wt% of one or more comonomers from the group consisting of acrylamide, methylacrylamide, ethylacrylamide and n-butylacrylamide, where and sum to 100 wt%; and B2: 15 to 60 wt%, based on the solids content of the graft copolymer B, of a graft sheath obtainable by reacting the agglomerated graft substrate B1 with a mixture of: 70 to 90 wt%, based on the graft sheath, of styrene and/or α-methylstyrene, in particular styrene, and 10 to 30 wt%, based on the graft sheath, of acrylonitrile and/or methyl methacrylate, in particular acrylonitrile, where the graft substrate B1 and the graft sheath B2 sum to 100 wt% in total; characterized in that (i) the agglomerating copolymer has a polydispersity U of from 0.26 to 0.20 and a d 50 value of from 110 to 140 nm, in particular 115 to 140 nm, and (ii) the agglomerated graft substrate B1 has a bimodal particle size distribution of a fraction x) of nonagglomerated particles having a d 50 value in the range of from 80 to 120 nm and a fraction y) of agglomerated particles having a d 50 value in the range of from 350 to 450 nm and a polydispersity U in the range of from 0.27 to 0.20. B1: 40 to 85 wt%, based on the solids content of the graft copolymer B, of a graft substrate obtainable by (a) polymerizing:: 10.5 to 24.5 wt%, based on the graft substrate B1, of at least one vinylaromatic, in particular styrene, and: 75.5 to 89.5 wt%, based on the graft substrate B1, of at least one diene, in particular butadiene,: 10.5 to 24.5 wt%, based on the graft substrate B1, of at least one vinylaromatic, in particular styrene, and: 75.5 to 89.5 wt%, based on the graft substrate B1, of at least one diene, in particular butadiene, and (b) agglomerating the obtained graft substrate B1 by adding: 0.01 to 5 parts by weight, based on 100 parts by weight of the graft substrate B1, in each case based on the solids content, of an agglomerating copolymer of:: 80 to 99.9 wt% of one or more C 1 to C 12 alkyl acrylates or C 1 to C 12 alkyl methacrylates and: 0.1 to 20 wt% of one or more comonomers from the group consisting of acrylamide, methylacrylamide, ethylacrylamide and n-butylacrylamide, where and sum to 100 wt%; and: 0.01 to 5 parts by weight, based on 100 parts by weight of the graft substrate B1, in each case based on the solids content, of an agglomerating copolymer of:: 80 to 99.9 wt% of one or more C 1 to C 12 alkyl acrylates or C 1 to C 12 alkyl methacrylates and: 0.1 to 20 wt% of one or more comonomers from the group consisting of acrylamide, methylacrylamide, ethylacrylamide and n-butylacrylamide, where and sum to 100 wt%; and: 80 to 99.9 wt% of one or more C 1 to C 12 alkyl acrylates or C 1 to C 12 alkyl methacrylates and: 0.1 to 20 wt% of one or more comonomers from the group consisting of acrylamide, methylacrylamide, ethylacrylamide and n-butylacrylamide, B2: 15 to 60 wt%, based on the solids content of the graft copolymer B, of a graft sheath obtainable by reacting the agglomerated graft substrate B1 with a mixture of: 70 to 90 wt%, based on the graft sheath, of styrene and/or α-methylstyrene, in particular styrene, and 10 to 30 wt%, based on the graft sheath, of acrylonitrile and/or methyl methacrylate, in particular acrylonitrile, where the graft substrate B1 and the graft sheath B2 sum to 100 wt% in total; 70 to 90 wt%, based on the graft sheath, of styrene and/or α-methylstyrene, in particular styrene, and 10 to 30 wt%, based on the graft sheath, of acrylonitrile and/or methyl methacrylate, in particular acrylonitrile, (i) the agglomerating copolymer has a polydispersity U of from 0.26 to 0.20 and a d 50 value of from 110 to 140 nm, in particular 115 to 140 nm, and (ii) the agglomerated graft substrate B1 has a bimodal particle size distribution of a fraction x) of nonagglomerated particles having a d 50 value in the range of from 80 to 120 nm and a fraction y) of agglomerated particles having a d 50 value in the range of from 350 to 450 nm and a polydispersity U in the range of from 0.27 to 0.20.