Biaxially oriented film of polyethylene-2,6-naphthalate

Claim 1

A biaxially oriented film of polyethylene-2,6-naphthalate having a. a thickness of 1 to 25 micrometers, b. a Young's modulus of at least 650 kg/mm 2 in a given direction on the film plane and a direction crossing it at right angles, the difference between the Young's moduli in the two directions being at most 200 kg/mm 2 , the difference between the thermal shrinkages in the two directions being 0 to 0.1%, c. a surface roughness (Ra) of at least 0.003 micrometer but less than 0.01 micrometer, the distribution curve representing the relation between the number of protrusions on the film surface and the heights of the protrusions crossing the equation log.sub.10 y=12x+3.7 wherein y represents the number of protrusions per mm 2 , and x is the protrusion height (micrometers), at least in an area where the number of protrusions is 30/mm 2 and the protrusion height exceeds the maximum value of the distribution curve, the biaxially oriented film containing 0.005 to 3% by weight of fine particles in which the long diameter/short diameter ratio of the particles is from 1.0 to 1.2, a relative standard deviation of not more than 0.5, and an average particle diameter of 0.005 to 0.6 micrometer.