| abstract |
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 (W D1 ) 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 (Mw A1 ) of the polyester (A1), the weight average molecular weight (Mw D1 ) of the thermoplastic resin (D1), the weight average molecular weight (Mw A1 ') 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 (Mw D1 ') 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×Mw A1 '/Mw A1 ≤ Mw D1 '/Mw D1 , 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/3×10 9 nm 2 (2 pieces/3,000 μm 2 ) or less.nThe 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. |