| 摘要 |
PURPOSE:To obtain a biaxially oriented polyester film whose strength of a longitudinal direction, that is, a running direction is high, by a method wherein a range of a refractive index after longitudinal orientation and lateral orientation is specified. CONSTITUTION:Polyethylene terephthalate raw materials are melt-extruded with, for example, a normal method, cooled, solidified and an unoriented film is obtained. Then the film is oriented laterally so that a lateral and thicknesswise refractive indexes nY, nZ satisfy a formula. Then longitudinal orientation is performed so that a mean refractive index n shown by the formula satisfies the formula. It is further preferable if the mean refractive index is at least 1.593. The film obtained like this may be oriented longitudinally at need. When the biaxially oriented film obtained like this is heat-treated at 190-240 deg.C, longitudinal strength is increased. |
