BIAXIALLY ORIENTED POLYESTER FILM

摘要

Disclosed is a biaxially oriented polyester film having a polyester layer (a P1 layer) comprising a polyester (A1) mainly composed of either one of ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate, a thermoplastic resin (D1) and inorganic particles (C1), wherein the content (WD1) of the thermoplastic resin (D1) in the P1 layer is 2 to 40 mass% inclusive relative to the amount of the P1 layer, the weight average molecular weight (MwA1) of the polyester (A1), the weight average molecular weight (MwD1) of the thermoplastic resin (D1), the weight average molecular weight (MwA1') of the polyester (A1) as determined after treating the polyester (A1) at 125°C and 100%RH for 72 hr, and the weight average molecular weight (MwD1') of the thermoplastic resin (D1) as determined after treating the thermoplastic resin (D1) at 125°C and 100%RH for 72 hr meet the relationship represented by the following formula: 1.5×MwA1'/MwA1 = MwD1'/MwD1, the thermoplastic resin (D1) is present in the polyester (A1) as a dispersion phase in the P1 layer, and a dispersion phase having a length in a longer axis direction of larger than 30,000 nm (30 µm) is contained at a density of 2 pieces/3×109 nm2 (2 pieces/3,000 µm2) or less. The biaxially oriented polyester film has excellent wet heat resistance and excellent ultraviolet ray resistance. Therefore, when the biaxially oriented polyester film is used as a solar cell back sheet, a solar cell having high durability can be provided.