Biaxially oriented polyester film

1. A biaxially oriented polyester film prepared by biaxially drawing a polyester sheet which has a glass transition temperature (Tg) of 60° C. or higher, in the longitudinal and transverse directions, wherein the biaxially oriented film has a specific gravity of 1.38 or less and a dead-fold retainability of at least 60% defined as below:

Dead-fold retainability (%)=(180−θ)×100/180

wherein θ is an angle formed when a polyester film sheet is folded under a pressure of 30 psi for 3 seconds at room temperature and the folded sheet is kept at a vertical position for 1 hr.

2. The biaxially oriented polyester film of claim 1 , wherein the polyester sheet is prepared from one or more resins obtained by polycondensation of a dicarboxylic acid component and an alkylene glycol component.

3. The biaxially oriented film of claim 2 , wherein the dicarboxylic acid component comprises dimethyl terephthalate or terephthalic acid.

4. The biaxially oriented film of claim 3 , wherein the dicarboxylic acid component further comprises at least one aromatic dicarboxylic acid selected from the group consisting of isophthalic acid, naphthalene dicarboxylic acid, dimethyl isophthalate, dimethyl-2,6-naphthalene dicarboxylate.

5. The biaxially oriented film of claim 2 , wherein the alkylene glycol component comprises ethylene glycol.

6. The biaxially oriented film of claim 5 , wherein the alkylene glycol component further comprises at least one alkylene glycol selected from the group consisting of diethylene glycol, trimethylene glycol, tetramethylene glycol, 1,4-cyclohexane dimethanol and 2,2-dimethyl(−1,3-propane)diol.

7. The biaxially oriented film of claim 1 , wherein the crystallization degree of the film is less than 40%.

8. The biaxially oriented film of claim 1 , wherein the product of drawing ratios in the longitudinal and transverse directions is in the range of 4 to 20.

9. The biaxially oriented film of claim 1 , wherein the drawing in the longitudinal direction is conducted at a temperature in the range of Tg of the polyester sheet and Tg+30° C.

10. The biaxially oriented film of claim 1 , wherein the drawing in the transverse direction is conducted at a temperature in the range of Tg of the polyester sheet +10° C. and Tg+40° C.

11. The biaxially oriented film of claim 1 , wherein the refractive indexes of the film in the longitudinal and transverse directions are each 1.6 or higher.

12. The biaxially oriented film of claim 1 , wherein the film is used for twist wrapping.