Oriented polyester film

A magnetic recording tape comprising a magnetic film layer and a base film

• which comprises modified polyethylene-2,6-naphthalenedicarboxylate containing a 4,4'-diphenyldicarboxylic acid component as a polymerized unit in its polymer chain, in an amount of 1 to 5 mol% based on a total acid component content of the modified polyethylene-2,6-naphthalenedicarboxylate, and having a heat of crystallization of 16 to 25.5 Joule/g, and
• which is biaxially oriented.

An electric insulating film having electric insulating properties

• which comprises modified polyethylene-2,6-naphthalenedicarboxylate containing a 4,4'-diphenyldicarboxylic acid component as a polymerized unit in its polymer chain, in an amount of 1 to 5 mol% based on a total acid component content of the modified polyethylene-2,6-naphthalenedicarboxylate, and having a heat of crystallization of 16 to 25.5 Joule/g, and
• which is biaxially oriented.

A magnetic recording tape comprising a magnetic film layer and a base film

• which comprises modified polyethylene-2,6-naphthalenedicarboxylate containing a glycol component derived from a compound represented by the following formula, ##STR6## wherein each of n and m is independently a positive number of 1 to 9 as an average provided that m+n equals 2 to 9 as an average, and having a heat of crystallization of 6 to 20 Joule/g, and
• which is biaxially oriented.

An electric insulating film having electric insulating properties

• which comprises modified polyethylene-2,6-naphthalenedicarboxylate containing a glycol component derived from a compound represented by the following formula, ##STR7## wherein each of n and m is independently a positive number of 1 to 9 as an average, provided that m+n equals 2 to 9 as an average, and having a heat of crystallization of 6 to 20 Joule/g, and
• which is biaxially oriented.