Thermoplastic polyester compositions and films formed using same

A thermoplastic polyster composition prepared by adding a slurry of the following components (a), (b) and (c) in any polyester production stage:

• (a) colloidal silica particles having a ratio of a particle diameter (D.sub.1 m.mu.) measured by a dynamic light scattering method to a particle diameter (D.sub.2 m.mu.) measured by a nitrogen gas absorbing method, D.sub.1 /D.sub.2, or more, D.sub.1 being in the range of 60 to 600 m.mu., further having a thickness in the range of 41 to 100 m.mu. is observation using an electron microscope, and having a straight-chain or branched shape, said silica particles being substantially free of agglomerations, (b) 50 to 3,000 ppm (weight ratio) as SO.sub.3, relative to the SiO.sub.2 which constitutes said colloidal silica particles, of a sulfur atom-containing compound, (c) 1,000 to 20,000 ppm (weight ratio) as Na.sub.2 O, relative to the SiO.sub.2 which constitutes said collidal silica particles, of Na atom.