Abstract: A biaxially stretched polyester film is disclosed.The biaxially stretched polyester film comprises substantially spherical silica particles of colloidal state having average diameters ranging from approximately 20 nm to approximately 200 nm in amounts ranging from approximately 0.005% to approximately 1.0% by weight based on the total weight of the polyester; and alumina particles having average diameters ranging from approximately 50 nm to approximately 500 nm in amounts ranging from approximately 0.05% to approximately 5.0% by weight, the average diameter of the alumina particles being larger than that of the silica particles.The biaxially stretched film prepared according to the present invention has excellent surface properties such as slip property and abrasion resistance, so that the film may be used for manufacturing final products without scratches and white powders occurring thereon.

Abstract: A method for the production of polyester suitable to be molded into a film with excellent surface properties.The method comprises passing dispersion-in-water type silica sol with pH 8 to 11 including sodium oxide as a stabilizer and silica particles with average diameters 0.01 to 0.3 .mu.m through a column of cation exchange resin having the following formula 1 and the strong basic anion exchange having the following formula 2, in sequence to adjust the pH value to 2 to 4, adding an hydrogen bonding agent and ethylene glycol thereto, heating the resultant silica sol under a reduced pressure to remove water, so as to obtain dispersion-in-ethylene glycol type silica sol, and adding 0.01 to 5% by weight of the dispersion-in-ethylene glycol type silica sol, as based on the weight of polyester to be produced, at any step during the ester interchange reaction.