Polyester composition

Claim 2

A polyester composition which comprises a polyester with a terminal carboxyl group content of 40 to 80 equivalent/10⁶ g and 0.01 to 5 wt%, based on the total amount of the polyester composition, of aluminum oxide particles of 0.005 to 5 µm in average particle diameter, and which has a melt resistivity of 6 x 10⁶ to 5 x 10⁸ Ω.cm.

Claim 3

A polyester composition according to claim 1, wherein the particle size distribution index γ of the aluminum oxide particles represented by the following formula: wherein D₇₅ represents the particle diameter of the particle when the cumulative weight calculated from the minimum particle diameter amounts to 75% of the total weight, and D₂₅ represents the particle diameter of the particle when the cumulative weight calculated from the minimum particle diameter amounts to 25% of the total weight, is not more than 2.

Claim 4

A polyester composition according to claim 1, wherein the volumetric shape parameter φ v of the aluminum oxide particles represented by the following formula: φ v = V D³ wherein V represents the volumes (µm³) of the particles and D the maximum diameter in the projection of the particles, is 0.1 to π 6 .

Claim 5

A process for producing a biaxially oriented polyester film for long hours in succession, which comprises the steps of: extruding a polyester composition from an extruder into a form of a sheet at 280 to 310°C while maintaining the lip portion of the die of the extruder at a tempera­ture of 280 to 330°C and a relative humidity of 30 to 100%, the polyester composition comprising a polyester with a terminal carboxyl group content of 40 to 80 equivalents/10⁶ g and 0.01 to 5 wt%, based on the total amount of the polyester composition, aluminum oxide particles 0.005 to 5 µm in average particle diameter and having a melt resistivity of 6 x 10⁶ to 5 x 10⁸ Ω.cm; cooling the sheet to 40 to 70°C by an electrostatic cooling method to produce an amorphous sheet; stretching the amorphous sheet in the machine and transverse directions to 4 to 20 times by area at 80 to 140°C; and subjecting the stretched sheet to heat treatment at 150 to 240°C for 1 to 600 seconds.

Claim 6

A process for producing a biaxially oriented polyester film for long hours in succession, which comprises the steps of: extruding a polyester composition from an extruder into a form of a sheet at 280 to 310°C while maintaining the lip portion of the die of the extruder at a tempera­ture of 280 to 330°C and a relative humidity of 30 to 100%, the polyester composition comprising a polyester with a terminal carboxyl group content of 40 to 80 equivalents/10⁶ g and 0.01 to 5 wt%, based on the total amount of the polyester composition, aluminum oxide particles 0.005 to 5 µm in average particle diameter and having a melt resistivity of 6 x 10⁶ to 5 x 10⁸ Ω.cm; cooling the sheet to 40 to 70°C by an electrostatic cooling method to produce an amorphous sheet; stretching the amorphous sheet in the machine and transverse directions to 4 to 20 times by area at 80 to 140°C; and subjecting the stretched sheet to heat treatment at 150 to 240°C for 1 to 600 seconds.