The invention relates to thermoplastic moulding materials based on acrylonitrile-butadiene-styrene, comprising a SAN-polymer matrix and a graft copolymer, structured from an agglomerated butadiene rubber-graft base and a SAN-graft envelope. The agglomerated graft base has a bimodal particle size distribution of a fraction x) of non-agglomerated particles having a d50-value in the range from 80 to 120 nm and a fraction y) of agglomerated particles having a d50-value in the range from 350 to 550 nm and a polydispersity U of less than 0.28. The invention further relates to a method for producing the graft copolymer, wherein an acrylate copolymer having a polydispersity U of less than 0.27 and a d50-value of 100 to 150 nm is used for agglomeration. The invention further relates to the use of thermoplastic moulding materials for producing moulded parts.