Abstract: 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.degree. C. and the Young's modulus Y.sub.M (kg/mm.sup.2) in the longitudinal direction,-0.001Y.sub.M +0.89<S.sub.M <-0.001Y.sub.M +1.57when Y.sub.M is 1,200 kg/mm.sup.2 or less,and-0.31<S.sub.M <0.37when 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.degree. C. for 1 hour.

Abstract: A biaxially oriented film of a polyethylene-2,6-naphthalenedicarboxylate, which (1) has a Young's modulus of at least 600 kg/mm.sup.2 in the transverse direction and a Young's modulus of at least 500 kg/mm.sup.2 in the longitudinal direction, the Young's modulus in the transverse direction being greater than the Young's modulus in the longitudinal direction, (2) satisfies the following relationships between an elongation S.sub.T (%) in the transverse direction per kg/mm.sup.2 of a load at 120.degree. C. and the Young's modulus Y.sub.T (kg/mm.sup.2) in the transverse direction,-0.001Y.sub.T +0.89<S.sub.T <-0.001Y.sub.T +1.57when Y.sub.T is at least 1,200 kg/mm.sup.2,and-0.31<S.sub.T <0.37when Y.sub.T is greater than 1,200 kg/mm.sup.2, and (3) has a heat shrinkage percentage of 0.1% or less in the longitudinal direction when heat-treated under no load at 70.degree. C. for 1 hour.

Abstract: There is provided a biaxially oriented polyester base film for a magnetic tape for use in a cassette for a digital audio tape recorder. The film has (a) a stress when the biaxially oriented polyester base film is extended in the longitudinal direction by 5% of at least 12 kg/mm.sup.2, (b) a residual extension in the longitudinal direction of 0.2% or less and a residual extension in the transverse direction of 0.3% or less, (c) a heat shrinkage in the longitudinal direction of 1.5% or less, (d) at least one surface of which has a surface roughness, Ra, of 25 nm or less, and (e) a film thickness of in the range of from 3 to 12 .mu.m. A magnetic tape for use in a cassette for a digital audio tape recorder is also provided.

Abstract: There is provided a biaxially oriented polyester base film for a magnetic tape for use in a cassette for a digital audio tape recorder. The film has (a) a stress when the biaxially oriented polyester base film is extended in the longitudinal direction by 5% of at least 12 kg/mm.sup.2, (b) a residual extension in the longitudinal direction of 0.2% or less and a residual extension in the transverse direction of 0.3% or less, (c) a heat shrinkage in the longitudinal direction of 1.5% or less, (d) at least one surface of which has a surface roughness, Ra, of 25 nm or less, and (e) a film thickness of in the range of from 3 to 12 .mu.m. A magnetic tape for use in a cassette for a digital audio tape recorder is also provided.

Abstract: There is provided a biaxially oriented polyester base film for a magnetic tape for use in a cassette for a digital audio tape recorder. The film has (a) a stress when the biaxially oriented polyester base film is extended in the longitudinal direction by 5% of at least 12 kg/mm.sup.2, (b) a residual extension in the longitudinal direction of 0.2 % or less and a residual extension in the transverse direction of 0.3% or less, (c) a heat shrinkage in the longitudinal direction of 1.5% or less, (d) at least one surface of which has a surface roughness, Ra, of 25 nm or less, and (e) a film thickness of in the range of from 3 to 12 .mu. m. A magnetic tape for use in a cassette for a digital audio tape recorder is also provided.

Abstract: A biaxially oriented polyester film containing fine particles of insoluble inert substance, has a kinetic coefficient of friction of not more than 0.3, and has microscopic protrusions integrated thereto, said protrusion having an peak-to-valley height of not more than 1 micron mostly range of 0.2 to 0.5 micron. The film having improved slipperiness is useful as the base film of magnetic recording tape for a video tape recorder or a computer data storage.