Biaxially - oriented film of polyethylene - 2,6 - naphth enable dicarboxylate

摘要

A biaxially oriented film of a polyethylene-2,6- naphthalenedicarboxylate, which (A) has a Young's modulus of at least 800 kg/mm.sup.2 in the longitudinal direction and a Young's modulus of at least 500 kg/mm.sup.2 in the transverse direction, the Young's modulus in the longitudinal direction being greater than the Young's modulus in the transverse direction, (B) satisfies the following relationships between an elongation S.sub.M (%) in the longitudinal direction per a load of 1 kg/mm.sup.2 at 120° C. and the Young's modulus Y.sub.M (kg/mm.sup. 2) in the longitudinal direction,PP-0.001Y.sub.M +0.89 S.sub.M -0.001Y.sub.M +1.57PPwhen Y.sub.M is 1, 200 kg/mm.sup.2 or less,PPandPP-0.31 S. sub.M 0.37PPwhen Y.sub.M is greater than 1,200 kg/mm. sup.2, and (C) has a heat shrinkage percentage of 0.1% or less in the longitudinal direction when the biaxially oriented film has been heat- treated under no load at 70° C. for 1 hour. This film is almost free from a dimensional change and has high strength in the longitudinal direction, is excellent in flatness and is useful as a base film for a magnetic recording medium capable of recording for a long time with a high density.