after this 40 wt parts of a mixture of 74 5 wt styrene and 25 5 wt acrylonitrile based on the total amount of the monomers are fed continuously within 4 hours to the provided rubber latex starting with the monomer feed an aqueous sodium rosin soap solution is fed in 5 hours and an aqueous solution of tert butylhydroperoxide as well as an aqueous solution of sodium ascorbate is fed in 9 hours the temperature is increased within 4 hours from 59 u00b0 c to 81 u00b0 c and kept in the range of 78 to 81 u00b0 c for further 8 hours the graft latex b 1 is stabilized with 0 8 wt of a phenolic antioxidant and coagulated with a mixture of magnesium sulfate and acetic acid washed with water and dried at 70 u00b0 c in a drying cabinet to obtain the graft polymer b 1 according to the invention a mixture of 15 wt parts of the rubber latex b1 and 45 wt parts of the rubber latex b2 corresponding to a weight ratio of b1 to b2 of 25 75 based on solids evaporation sample at 180 u00b0 c for 25 min in drying cabinet is mixed and adjusted with water to a solid content of ca 30 wt and heated to 59 u00b0 c after this 40 wt parts of a mixture of 74 5 wt.