| 摘要 |
PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film for a magnetic recording medium useful as a large capacity digital data storage tape excellent in the durability after the repeated use, in which no error is generated by the deviation of track, and in which the output characteristics are excellent. SOLUTION: The biaxially oriented polyester film for the magnetic recording medium is characterized in that (1) a film is formed by a polyethylene-2,6- naphthalate of which the concentration of the terminal carboxyl group is 10-70 equivalent/10<6> g, (2) the thermal expansion coefficientαt with respect to the width direction of the film is from +15×10<-6> to -10×10<-6> / deg.C, the moisture expansion coefficientαh is from +15×10<-6> to +5×10<-6> /%RH, and the value of (αt+2αh) is equal to or less than 45×10<-6> , (3) the variation of the width between before and after the treatment with the load of 22 MPa in the longitudinal direction of the film for 72 hours at 49 deg.C and 90%RH is equal to or less than 0.35%, and (4) the thickness of the film is 2-7μm. |
