Polyester film and method for producing same

A biaxially successively stretched single-layer polyester film consisting of:

• a polyester (A) which is polybutylene terephthalate consisting of
• 88 mol % or more of butylene terephthalate units and 12 mol % or less of sebacic acid units,
• a polyester (B) which is of polyethylene terephthalate consisting of
• 88 mol % or more of ethylene terephthalate units and 12 mol % or less of isophthalic acid units,
• or
• 96.5 mol % or more of ethylene terephthalate units and 3.5 mol % or less of 1,4-cyclohexane dimethanol units,
• an inorganic lubricant in an amount of up to about 0.08 mass %, and
• optionally one or more additives selected from the group consisting of:
• a catalyst;
• a reaction inhibitor;
• an antioxidant;
• a heat stabilizer;
• an ultraviolet absorber;
• and an antistatic agent;
• wherein a mass ratio (A/B) between the polyesters (A) and (B) is 70/30 to 55/45,
• a dry heat shrinkage rate (A) through heat treatment at 160° C. for 30 minutes is 20% or less in any of four directions on a film surface consisting of a 0° direction as an arbitrary direction, and a 45° direction, a 90° direction and a 135° direction clockwise from the 0° direction, and a difference between a maximum value and a minimum value of these dry heat shrinkage rates is 5% or less,
• a dry heat shrinkage rate (B) through heat treatment at 200° C. for 15 minutes is 35% or less in any of the above four directions, and a difference between a maximum value and a minimum value of these dry heat shrinkage rates is 5% or less, and
• a thickness variation calculated from the following equation in the above four directions is 10% or less:
• Thickness variation (%)=(Tmax−Tmin)/Tave×100
• Tmax:
• maximum thickness in the above four directions of polyester film
• Tmin:
• minimum thickness in the above four directions of polyester film
• Tave:
• average thickness in the above four directions of polyester film,
• wherein the polyester film is manufactured by a method comprising;
• stretching an unstretched sheet in a flow direction of the sheet (MD stretching) and then stretching the same in a width direction (TD stretching), and
• heat relaxation treating the stretched film at a lateral stretching magnification of 1% to 10% and at a temperature of about 140° C. to about 200° C.,
• wherein the MD stretching is performed in two or more stages such that the MD stretching magnification (X) represented by a product of stretching magnifications in the respective stages in the MD stretching and the TD stretching magnification (Y) satisfy the following conditions:
• a stretching magnification ratio (X/Y) of 0.82 to 1.10, and
• an area magnification (X×Y) of 12.00 to 16.00.