Method of preparing thermoplastic resin

A method of preparing a thermoplastic resin composition, the method comprising:

• i) preparing a graft copolymer latex by graft-polymerizing a) a conjugated diene rubber with b) at least one selected from the group consisting of an aromatic vinyl compound, a vinyl cyan compound, and an acrylate-based compound;
• ii) performing coagulation by adding a polymeric antimicrobial agent having 1) a primary, secondary, tertiary, or quaternary amine group, or 2) poly(N-vinylpyrrolidone), to the graft copolymer latex;
• iii) obtaining a thermoplastic resin from the coagulated graft copolymer;
• and
• iv) melt-kneading 20 to 80% by weight of the thermoplastic resin (A) with 20 to 80% by weight of a copolymer resin (B) formed by polymerizing at least one selected from the group consisting of an aromatic vinyl compound, a vinyl cyan compound, and an acrylate-based compound,
• wherein the antimicrobial agent polymer of ii) is added as a dissolved solution of 0.1 to 10% by weight in an aqueous acidic solution having a pH of 2 to 5.