Multilayer polyester film with high optical density with a total thickness of max. 60 μm with two white outer layers and a carbon-black containing base layer

A biaxially oriented polyester film comprising at least three layers, said film comprising two white outer layers and a carbon-black-filled base layer, wherein

• the thickness of each of the outer layers is from 4 to 15 μm,
• the thickness of the base layer is ≧the total of the thicknesses of the outer layers and at least 50% of the film total thickness,
• the white outer layers comprise polyester and at least 15% by weight of TiO2,
• the base layer comprises polyester and at least 4% by weight of TiO2,
• the carbon-black-filled base layer comprises from 1 to 8% by weight of carbon black, and
• the standard viscosity value of the film is at least 600, where all % by weight values are based on the weight of the respective layer,
• the film has a Berger whiteness of at least 80 on at least one side, and
• the two white outer layers are external layers,
• the film has a total thickness of 20 to 60 μm
• and said film further comprises a flame retardant.