The present invention relates to a method of preparing an ASA graft
copolymer, a method of preparing a
thermoplastic resin composition including the ASA graft
copolymer, and a method of manufacturing a molded article using the
thermoplastic resin composition. More particularly, the present invention provides an ASA graft
copolymer having improved
thermal stability through addition of an emulsifier in a specific amount range in a seed preparation step and introduction of a multifunctional
carboxylic acid having 20 or more carbon atoms or a salt thereof, as an emulsifier, in a shell preparation step and a high-quality
thermoplastic resin composition exhibiting excellent
impact resistance, such as
impact strength and tensile strength, and excellent appearance, such as surface gloss, whiteness, and retention-associated heat discoloration, and causing considerable reduction in the amount of gas generated on a surface of a resin during a high-temperature
thermoforming process due to inclusion of the ASA graft copolymer.