BIAXIALLY ORIENTED POLYESTER FILM

摘要

PURPOSE:To obtain the title film excellent in mechanical strength and improved in slitting properties by setting Young's moduli in longitudinal and lateral directions to specific values or more and specifying the difference between refractive indice in the longitudinal and lateral directions and a crystal orientation index. CONSTITUTION:A biaxially oriented polyester film is characterized by that the Young's moduli in the longitudinal and lateral directions thereof are respectively 500kg/mm<2> or more, a crystal orientation index A exceeds 1.50 and the difference between the refractive index nx in the longitudinal direction and the refractive index ny in the lateral direction is 0.045 or less. Polyester to be used is crystalline polyester such as polyethylene terephthalate or polyalkylene naphthalate and 80mol.% or more of the repeating unit thereof is pref. composed of ethylene terephthalate. When the crystal orientation index A is 1.50 or more, no pref. effect is obtained because napping is generated or a slit end surface is cut at the time of slitting. When the difference between the refractive indice nx, ny in the longitudinal and lateral directions exceeds 0.045, a wavy phenomenon is generated in a roll end surface at the time of slitting.