Polyester resin composition and production method thereof

A polyester resin composition comprising:

• 1) a polyester consisting essentially of polyethylene terephthalate,
• 2) an ionic liquid selected from the group consisting of tetrabutyl ammonium benzoate, tetrabutyl ammonium salicylate and a combination thereof, and
• 3) a copper compound selected from the group consisting of copper(I) oxide, copper(II) oxide, lithium dimethylcuprate, lithium dibutylcuprate, copper(I) acetate, copper(II) acetate, copper benzoate, copper carbonate, copper chloride, copper bromide and copper iodide;
• wherein a surface roughness Ra of a glass plate having scattered products derived from the polyester resin composition attached thereto is less than 0.150 μm as measured when a sample of the polyester resin composition is melted at 300° C. for 60 minutes under a nitrogen atmosphere, cooled and thermally treated at 220° C. for 8 hours to form the scattered products attached to the glass plate, the glass plate having an initial surface roughness Ra of 0.010 μm, and
• a gelation ratio after a 6-hour melting treatment at 300° C. under an atmosphere with an oxygen concentration of 1% is 5% or less.