Method for preparing ABS graft copolymer

A method of preparing an acrylonitrile-butadiene-styrene (ABS) graft copolymer by graft-copolymerizing a vinyl aromatic monomer and a vinyl cyan monomer onto rubber latex, the method comprising the steps of:

• (1) adding, based on 100 parts by weight of the total weight of the rubber latex, the vinyl aromatic monomer and the vinyl cyan monomer, 0.0001-0.1 parts by weight of an aqueous polymerization initiator to 50-80 wt % (on a solid basis) of the rubber latex, and mixing the polymerization initiator with the rubber latex to prepare a first mixture;
• (2) adding 5-10 wt % of the total amount of a monomer emulsion mixture, prepared by emulsifying 20-50 wt % of a second mixture of the vinyl aromatic monomer and the vinyl cyan monomer with, based on 100 parts by weight of the total weight, 0.01-10 parts by weight of an emulsifier, to the first mixture, initiating polymerization of the monomers, and then adding the remaining amount of the monomer emulsion mixture, and performing polymerization of the monomers;
• and
• (3) adding, based on 100 parts by weight of the total weight, 0.0001-0.5 parts by weight of an assistant polymer initiator during polymerization of the monomers at a time point when a degree of conversion of the monomers reaches 80-90%, and performing polymerization of the monomers.