Patent ID: 7022407

Claim:
A polyester film for capacitors which is a biaxially oriented film comprising polyethylene-2,6-naphthalenedicarboxylate as a principal component and having a thermal strain ratio RMD (150) of the film in the machine direction at a temperature of 150° C. of −1.5%≦ RMD (150)≦0.0% when the thermal strain ratio RMD ( T ) in the transverse machine direction at a temperature T° C. based on the length LMD ( t ) in the machine direction ( MD ) of the film at a temperature t° C. is defined by RMD ( T )={[ LMD ( T )− LMD (35)]/ LMD (35)}×100(%), further wherein the thermal strain ratio RTD (150) in the transverse direction of the film at a temperature of 150° C. is −1.0%≦ RTD (150)≦0.0% when the thermal strain ratio RTD ( T ) in the transverse direction of the film at a temperature T° C. based on the length LTD ( t ) in the transverse direction ( TD ) of the film at a temperature t° C. is defined by RTD ( T )={[ LTD ( T )− LTD (35)]/ LTD (35)}×100(%); and wherein the thermal strain ratios RMD (210) and RTD (210) at a temperature of 210° C. are −3.5%≦ RMD (210)≦0.0% −3.5%≦ RTD (210)≦0.0%