Biaxially oriented film of polyethylene-2,6-naphthalenedicarboxylate

A biaxially oriented film of a polyethylene- 2,6-naphthalendicarboxylate having a longer length in the longitudinal direction than width in the transverse direction, which

• (A) has a Young's modulus of at least 900 kg/mm.sup.2 in the longitudinal direction and a Young's modulus of at least 550 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 by at least 250 kg/mm.sup.2,
• (B) satisfies the following relationships between an elongation S.sub.M (%) in the longitudinal direction and the Young's modulus Y.sub.M (kg/mm.sup.2) in the longitudinal direction,
• -0.001Y.sub.M +1.05<S.sub.M <-0.001Y.sub.M +1.48
• when Y.sub.M is 1,200 kg/mm.sup.2 or less,
• and
• -0.15<S.sub.M <0.28
• when Y.sub.M is greater than 1,200 kg/mm.sup.2,
• and
• (c) has a heat shrinkage percentage of 0.08% or less in the longitudinal direction when the biaxially oriented film has been heat-treated under no load at 70.degree. C. for 1 hour, and the biaxially oriented film satisfies the following relationship between a plane orientation coefficient NS and an average refractive index nA,
• NS.gtoreq.1.607nA-2.434 (C)
• 1.658.ltoreq.nA.ltoreq.1.665 (D).
• 13.

A biaxially oriented film of a polyethylene- 2,6-naphthalenedicarboxylate, which

• (A) has a Young's modulus of at least 1000 kg/mm.sup.2 in the longitudinal direction and a Young's modulus of at least 600 kg/mm.sup.2 in the traverse direction, the Young's modulus in the longitudinal direction being greater than the Young's modulus in the transverse direction by at least 250 kg/mm.sub.2,
• (B) satisfies the following relationships between an elongation S.sub.M (%) in the longitudinal direction and the Young's modulus Y.sub.M (kg/mm.sup.2) in the longitudinal direction,
• -0.001Y.sub.M +1.14<S.sub.M <-0.001Y.sub.M +1.40
• when Y.sub.M IS 1200 kg/mm.sup.2 or less, and
• -0.06<S.sub.M <0.20
• when Y.sub.M is greater than 1,200 kg/mm.sup.2, and
• (C) has a heat shrinkage percentage of 0.05% or less in the longitudinal direction when the biaxially oriented film has been heat-treated under no load at 70.degree. C. for 1 hour, and the biaxially oriented film satisfies the following relationship between a plane orientation coefficient NS and an average refractive index nA,
• NS.gtoreq.1.607nA-2.434 (C)
• 1.658.ltoreq.nA.ltoreq.1.665 (D).