Graft copolymer B composed of: B1: 50 to 70 wt.-%, preferably 55 to 65 wt.-%, based on B, of at least one graft base B1 obtained by emulsion polymerization of the following monomers: b11: 70 to 99.9 wt.-%, based on B1, of at least one acrylate; b12: 0 to 29.9 wt.-%, based on B1, of at least one further comonomer selected from: styrene, α-methylstyrene, acrylonitrile, methacrylonitrile, methylmethacrylate, and N-phenylmaleimide (N-PMI); b13: 0.1 to 10 wt.-%, based on B1, of one or more bi- or polyfunctional, crosslinking monomers; where the sum of b11, b12 and b13 adds up to 100 wt.-%; B2: 30 to 50 wt.-%, preferably 35 to 45 wt.-%, based on B, of a graft shell B2, which is obtained via emulsion polymerization in the presence of at least one graft base B1, which graft shell B2 comprises a first layer B21 made of at least one vinylaromatic monomer, and a second layer B22 made of a mixture comprising at least one vinylaromatic monomer B21-1 and at least one nitrile monomer B22-2; where the sum of B1 and B2 adds up to 100 wt.-%; characterized in that i. the weight average particle diameter d w of the graft base B1 is in the range of from 255 to 290 nm; ii. the fraction of B21 - based on the total amount of B2 - is 10 to 26 wt.-%; and iii. the amount of B22-2, based on the total amount of B22, is 20 to 30 wt.-%, preferably 24 to 26 wt.-%. B1: 50 to 70 wt.-%, preferably 55 to 65 wt.-%, based on B, of at least one graft base B1 obtained by emulsion polymerization of the following monomers: b11: 70 to 99.9 wt.-%, based on B1, of at least one acrylate; b12: 0 to 29.9 wt.-%, based on B1, of at least one further comonomer selected from: styrene, α-methylstyrene, acrylonitrile, methacrylonitrile, methylmethacrylate, and N-phenylmaleimide (N-PMI); b13: 0.1 to 10 wt.-%, based on B1, of one or more bi- or polyfunctional, crosslinking monomers; where the sum of b11, b12 and b13 adds up to 100 wt.-%; b11: 70 to 99.9 wt.-%, based on B1, of at least one acrylate; b12: 0 to 29.9 wt.-%, based on B1, of at least one further comonomer selected from: styrene, α-methylstyrene, acrylonitrile, methacrylonitrile, methylmethacrylate, and N-phenylmaleimide (N-PMI); b13: 0.1 to 10 wt.-%, based on B1, of one or more bi- or polyfunctional, crosslinking monomers; B2: 30 to 50 wt.-%, preferably 35 to 45 wt.-%, based on B, of a graft shell B2, which is obtained via emulsion polymerization in the presence of at least one graft base B1, which graft shell B2 comprises a first layer B21 made of at least one vinylaromatic monomer, and a second layer B22 made of a mixture comprising at least one vinylaromatic monomer B21-1 and at least one nitrile monomer B22-2; where the sum of B1 and B2 adds up to 100 wt.-%; characterized in that i. the weight average particle diameter d w of the graft base B1 is in the range of from 255 to 290 nm; ii. the fraction of B21 - based on the total amount of B2 - is 10 to 26 wt.-%; and iii. the amount of B22-2, based on the total amount of B22, is 20 to 30 wt.-%, preferably 24 to 26 wt.-%. i. the weight average particle diameter d w of the graft base B1 is in the range of from 255 to 290 nm; ii. the fraction of B21 - based on the total amount of B2 - is 10 to 26 wt.-%; and iii. the amount of B22-2, based on the total amount of B22, is 20 to 30 wt.-%, preferably 24 to 26 wt.-%.