Patent ID: 8129469

Claim:
A method for preparing a graft copolymer comprising the steps of (i) preparing a composite rubber polymer (A-1) latex having an average particle diameter of 250-600 nm and a gel content of 50-95% by adding 0.1-10 parts by weight of acid solution having a concentration of 0.5-10 wt% after being diluted as a solid content in the range of 20-50 wt% by adding water in a mixture of 20-80 parts by weight based on solid content of a diene rubber polymer (a-1) latex having an average particle diameter of 50-150 rim and 20-80 parts by weight based on solid content of a core-shell structured, cross-linked alkyl acrylate rubber polymer (b-1) latex having an average particle diameter of 50-250 nm per 100 parts by weight of the total monomers comprising the composite rubber polymer (A-1), wherein the core-shell structured, cross-linked alkyl acrylate polymer (b-1) comprises a core polymerized or copolymerized from 5-40 parts by weight of an aromatic vinyl compound and 0-10 parts by weight of a monomer copolymerizable with the aromatic vinyl compound and a shell copolymerized from 60-95 parts by weight of an alkyl acrylate monomer and 0.02-3.0 parts by weight of a cross-linking agent, per 100 parts by weight of the total monomers comprising the core-shell structured, cross-linked alkyl acrylate rubber polymer (b-1); (ii) polymerizing 5-60 parts by weight of an alkyl acrylate monomer in the presence of 5-60 parts by weight based on solid content of the above-prepared composite rubber polymer (A-1) latex, per 100 parts by weight of the total monomers comprising the graft copolymer, to obtain an acrylic rubber polymer latex; and (iii) graft polymerizing the acrylic rubber polymer latex prepared in the step (ii) with 20-80 parts by weight of at least one monomer selected from a group consisting of an aromatic vinyl compound, a vinyl cyanide compound and an alkyl (meth)acrylate compound, per 100 parts by weight of the total monomers used in preparation of the graft copolymer, to obtain an ASA (acrylonitrile-styrene-acrylate) graft copolymer for thermoplastic resin composition having impact strength at low temperature, pigmentation property, gloss, and weathering resistance.