| 摘要 |
PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film useful for manufacturing a magnetic recording medium excellent in the running property, the electromagnetic transducing characteristics, the preservation durability, and so forth, in particular, for a metal thin film magnetic recording medium suitable for the recording for a long period of time. SOLUTION: The biaxially oriented polyester film for the magnetic recording medium is characterized in that (1) a primer for a magnetic layer consisting of a continuous thin film including fine particles having the mean particle diameter of 0.005-0.1μm on either side of a polyester film substantially not including inert particles, (2) the surface roughness WRa after the coating of the primer for the magnetic layer is 0.1-5 nm, (3) 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> , (4) 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 (5) the thickness of the film is 2-7μm. |
