TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The polyester of at least one of primary polyester layer and secondary polyester layer is PTFE. The intrinsic viscosity of the polyester of at least one of the primary and secondary polyester layers is at least 0.61 preferably no more than 0.85. The primary and secondary polyester layers comprise glycidyl ester of formula (I).R1, R2 = alkyl; andR3 = H or alkyl (preferred) and total number of carbon atoms in the groups in R1-R3 is 3-48, preferably 3-13, preferably R1 is CH3, total number of carbon atoms in the alkyl in R2 and R3 is 2-47, preferably 7.The glycidyl ester in at least one of the primary and secondary polyester layers is reacted with the polyester by injecting the glycidyl ester into the molten polyester prior to the polymer being cast into a film. The total amount of glycidyl ester present in the multilayer film is 0.2-5 mol.%, based on the amount of polyester in the film. The glycidyl ester is present in each of the primary and secondary polyester layers in an amount that is 0.1-5 mol.%, preferably 0.15-1.2 mol.%, based on the amount of polyester in the respective layer. The glycidyl ester is present in the secondary polyester layer in an amount of at least 0.65 mol.%, preferably no more than 1 mol.%. The glycidyl ester is present in the primary polyester layer in an amount of at least 0.1 mol.%, preferably no more than 0.65 mol.%. The concentration of the glycidyl ester in the secondary layer (GE)s is greater than the concentration of the glycidyl ester in the primary layer (GE)p, preferably such that the ratio (GES)s:(GEP)s is at least 1.2:1. The concentration of the glycidyl ester in the secondary layer, the concentration of the glycidyl ester in the primary layer and overall concentration of glycidyl ester in the multilayer film (GE)o is such that ((GE)s/(GE)s)x(GE)o is at least 0.7, preferably at least 1. At least one, preferably both, of the primary and secondary polyester layers further comprises at least one metal cation comprising group I or group II metal cation. The reaction product of the glycidyl ester with the end groups of the polyester in at least one, preferably both, of the primary and secondary polyester layers is obtained by reacting the glycidyl ester with the end-groups of the polyester in the presence of at least one metal cation comprising group I or group II metal cations. The metal cations are group I metal cations, preferably sodium. The primary polyester layer is optically clear or translucent. The secondary polyester layer comprises a particulate filler material. The particulate filler is particulate inorganic fillers, preferably alumina, titania, talc, silca, calcined china clay, calcium carbonate or barium sulfate. The secondary polyester layer comprises 0.3-30 wt.% particulate filler. The secondary polyester layer is an opaque white layer, preferably exhibiting transmission optical density of 0.3-1.75, preferably at least 0.7. The particulate filler is titanium dioxide and/or barium sulfate. The amount of filler incorporated into the secondary polyester layer is 2-30 wt.%, preferably 10-20 wt.%. The primary polyester layer is free of particulate filler or comprises no more than 2.5 wt.%, preferably no more than up to 1 wt.%. The amount of particulate filler in the primary polyester layer is less than 50%, preferably less than 10 wt.% of the amount of particulate filler in the secondary polyester layer. The primary and secondary polyester layers contain the same polyester material. The primary polyester layer preferably accounts for greater than 50%, preferably 75-95% of the total thickness of the primary and secondary polyester layers. The primary and secondary polyester layers comprises a UV-absorber, preferably a UV-absorber comprising benzophenones, benzotriazoles, benzoxazinones or triazines. The amount of UV-absorber in the primary and/or secondary polyester layer is 0.1-10% relative to the total weight of the layer. The primary and secondary polyester layers comprises an antioxidant. The film has at least one additional layer disposed on one or both surfaces to form a multilayer composite structure. The multilayer biaxially oriented polyester film is useful as a component of a photovoltaic cell. The improvement in delamination resistance is exhibited over a prolonged period in humid conditions at elevated temperatures, as measured after 60 hours at 121 degrees C and 1.2 bar pressure and 100% relative humidity. EXAMPLE - No suitable example given.