Patent ID: 7833614

Claim:
A biaxially stretched polyester film comprising a polyester layer (layer A) which comprises polybutylene terephthalate homopolymer and at least one of a copolyester and an amorphous polyester, said biaxially stretched polyester film satisfying any one of the following requirements: (1) the polyester layer (layer A) exhibiting one melting point as measured by temperature-ride DSC method, and the polyester film having an edge tear resistance of not more than 100 N in each of longitudinal and width directions thereof; and (2) an average number of uncompatilized polybutylene terephthalate masses having a maximum length of not less than 1 μm which are present in a section of the layer A, being not more than 1.0 per 100 μm 2 of the section of the layer A, and the polyester film having an edge tear resistance of not more than 100 N in each of longitudinal and width directions thereof.