| 摘要 |
PURPOSE:To obtain the titled film used in magnetic disc or the like by a process wherein said light screening film with omnidirectional homogeneous solid state property is provided with near infrared ray absorption property and the difference between the surface roughness of the front and back sides of said film is eliminated and further its linear expansion coefficient due to temperature or humidity is made small. CONSTITUTION:Polyester such as polyethylene terephthalate or the like is added with pigment such as carbon black or the like and thoroughly kneaded and, after that, molten at an extrusion former. Next, the polyester is orientated longitudinally at the temperature of 95-100 deg.C and, after that, further orientated crosswise under the condition so as to give said orientated polyester film the same degree of orientation as that given in the longitudinal orientation. Furthermore, the crosswise orientated film is cooled below the transition point of glass and then heat-treated under the state being kept stretched, resulting in obtaining the desired biaxially orientated film, which has the absorptivity of near infrared rays, the wave lengths of which are 800-900mmu, of more than 90%, the difference between the central line mean roughnesses of the front and the back sides of the film of less than 0.005mu and yet the isotropy index of the film plane formulated in the formula (where A represents the maximum value of linear expansion coefficient due to temperature or humidity and B represents the minimum value at the time when A is employed as the maximum value) of within 10%. |
