To facilitate the dying of biaxially oriented polyester film by a method wherein five specified conditions are satisfied by the film.
A biaxially oriented polyester film satisfies the following five conditions; (1) 3.295≤NMD+NTD≤3.340, here: NMD means the index of refraction in the transversal direction of the film and NTD means the index of refraction in the lateral direction of the film. (2) surface degree of orientation ≤0.740. in this case, the degree of surface orientation is the average value of square of cosine of an angle between the vertical surface of a benzine ring surface in the biaxially oriented film of polyester and is shown by a formula I. In the formula I, NMD is the index of refractory in the thickness direction of the film and-n is shown by a formula II. (3) crystallinity (%) ≤54. Further, the crystallinity is shown by a formula III. ρn=1.336, ρc=1.457. (4) 3≤ longitudinal heat shrinkage percentage (%) ≤7. (5) 0≤ traverse heat shrinkage percentage (%) ≤3. Here; the heat shrinkage percentage is a shrinkage index at 200°C for 10min.
HAN KYUN-HEUMU
PARK JON-CHAN
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