| 摘要 |
PURPOSE:To obtain the title film having a flat surface excellent in slidability and abrasion resistance, a large lengthwise strength, suited as a base film of a magnetic recording medium, by forming a number of irregularities of a special shape on the surface. CONSTITUTION:A fibrous substance (e.g., calcium titanate) of an average fiber diameter d<=0.5mu, an average fiber length l<=5mu and an l/d of 3-10 is added to a polyester so that the relationship of the formula [wherein W is the amount (wt%) of the fibrous substance added] may be satisfied, and the mixture is melt-extruded and quenched to obtain an unstretched film. This film is stretched at 80-100 deg.C and at a lengthwise stretch ratio of 3.7-4.5 and a crosswise stretch ratio of 3.0-3.8 and heat-set to obtain a 4-35mu-thick polyester film having at least 200/mm<2> irregularity units each comprising an anisotropic protrusion 21 ascribable to the fine fibrous substance and a recess 24 of a major diameter >=2mu, adjacent to the major axis of the protrusion 21 of the surface of a film 23, a lengthwise F-5 value >=13kg/mm<2> which is larger than the crosswise F-5 value. |
