Graft copolymer B produced from emulsion polymerization with a core-shell structure comprising a core B1 and three layers B2 to B4, in the sequence B1 to B4 from inside to outwards, and in each case based on the sum of B1 to B4: B1: 6 to 19 wt.-% of a core (= graft base) B1 comprising at least one copolymer consisting of monomers B11 and B12: B11: 95.0 to 99.9 wt.-%, preferably 96.0 to 98.0 wt.-%, of at least one vinylaromatic monomer, and B12: from 0.1 to 5.0 wt.-%, preferably 2.0 to 4.0 of at least one bi- or polyfunctional, crosslinking monomer; where the sum of B11 and B12 adds up to 100 wt.-%; B2: 41 to 54 wt.-%, preferably 42 to 50 wt.-%, more preferably 43 to 45 wt.-%, of a first layer B2 consisting of at least one copolymer comprising monomers B21 and B22: B21: up to 99.0 wt.-%, preferably up to 98.5 wt.-%, of at least one C2-C8-alkyl acrylate; B22: 1.0 to 5.0 wt.-%, preferably 1.5 to 3.5 wt.-%, of one or more bi- or polyfunctional, crosslinking monomers; where - based on B2 - the sum of the total monomers adds up to 100 wt.-%; B3: 1 to 19 wt.-% of a second layer B3 consisting of at least one vinylaromatic polymer; and B4: 21 to 39 wt.-% of a third layer B4 consisting of at least one copolymer comprising at least one vinylaromatic monomer B41 and at least one nitrile monomer B42, wherein the amount of B42, based on B4, is 20 to 33 wt.-%, preferably 22 to 28 wt.-%, and where the sum of the total monomers (in particular B41 and B42) adds up to 100 wt.-%; where the sum of B1, B2, B3 and B4 adds up to 100 wt.-%; and wherein the weight average particle diameter d w - determined by turbidity measurement - of the graft copolymer B is in the range of from 280 to 450 nm. B11: 95.0 to 99.9 wt.-%, preferably 96.0 to 98.0 wt.-%, of at least one vinylaromatic monomer, and B12: from 0.1 to 5.0 wt.-%, preferably 2.0 to 4.0 of at least one bi- or polyfunctional, crosslinking monomer; where the sum of B11 and B12 adds up to 100 wt.-%; B2: 41 to 54 wt.-%, preferably 42 to 50 wt.-%, more preferably 43 to 45 wt.-%, of a first layer B2 consisting of at least one copolymer comprising monomers B21 and B22: B21: up to 99.0 wt.-%, preferably up to 98.5 wt.-%, of at least one C2-C8-alkyl acrylate; B22: 1.0 to 5.0 wt.-%, preferably 1.5 to 3.5 wt.-%, of one or more bi- or polyfunctional, crosslinking monomers; where - based on B2 - the sum of the total monomers adds up to 100 wt.-%; B3: 1 to 19 wt.-% of a second layer B3 consisting of at least one vinylaromatic polymer; and B4: 21 to 39 wt.-% of a third layer B4 consisting of at least one copolymer comprising at least one vinylaromatic monomer B41 and at least one nitrile monomer B42, wherein where the sum of B11 and B12 adds up to 100 wt.-%; B2: 41 to 54 wt.-%, preferably 42 to 50 wt.-%, more preferably 43 to 45 wt.-%, of a first layer B2 consisting of at least one copolymer comprising monomers B21 and B22: B21: up to 99.0 wt.-%, preferably up to 98.5 wt.-%, of at least one C2-C8-alkyl acrylate; B22: 1.0 to 5.0 wt.-%, preferably 1.5 to 3.5 wt.-%, of one or more bi- or polyfunctional, crosslinking monomers; where - based on B2 - the sum of the total monomers adds up to 100 wt.-%; B21: up to 99.0 wt.-%, preferably up to 98.5 wt.-%, of at least one C2-C8-alkyl acrylate; B22: 1.0 to 5.0 wt.-%, preferably 1.5 to 3.5 wt.-%, of one or more bi- or polyfunctional, crosslinking monomers; where - based on B2 - the sum of the total monomers adds up to 100 wt.-%; B3: 1 to 19 wt.-% of a second layer B3 consisting of at least one vinylaromatic polymer; and B4: 21 to 39 wt.-% of a third layer B4 consisting of at least one copolymer comprising at least one vinylaromatic monomer B41 and at least one nitrile monomer B42, wherein the amount of B42, based on B4, is 20 to 33 wt.-%, preferably 22 to 28 wt.-%, and where the sum of the total monomers (in particular B41 and B42) adds up to 100 wt.-%; where the sum of B1, B2, B3 and B4 adds up to 100 wt.-%; and wherein the weight average particle diameter d w - determined by turbidity measurement - of the graft copolymer B is in the range of from 280 to 450 nm.