| 摘要 |
PURPOSE:To obtain a film with higher Young's modulus and excellent slitting property by a method wherein the title film consists of polyester containing a specified repeated base unit and having the variations of the thicknesses in the longitudinal direction and in the width direction, the refractive index in the direction of the thickness and the melt viscosity, all of which are brought in the specified ranges. CONSTITUTION:Polyester must contain 80 mole-percent or ore of a repeated base unit represented by the formula (1). A film consisting of polyester containing less than 80 mole- percent of the repeated base unit has lower Young's modulus. The film is obtained by biaxially orienting said polyester. The refractive index in the direction of the thickness of the film after being oriented must lie within the range of 1.50-1.57. A film, the refractive index in the direction of the film thickness of which is smaller than the range just mentioned above, has poor slitting propery. In addition, the variations of the thicknesses both in the longitudinal direction and in the width direction of the oriented film must be 20% or less. A film with the variations of the thicknesses both in the longitudinal direction and in the width direction of the film exceeding 20% has poor slitting property. The melt viscosity of the oriented film must be in the range of 500-20,000 poises at 290 deg.C and 200 sec<-1>. A film with the melt viscosity smaller than 500 poises has poor slitting property, while a film with the melt viscosity larger than 20,000 poises has lower Young's modulus. |
