Biaxial oriented polyethylene terephthalate (BOPET) is a transparent film that has been stretched along its length and width during manufacture. This aligns the polymer structure within the film, resulting in a material with improved mechanical properties, low elongation under load and good thermal stability. Some well-known trade names include Mylar, Melinex and Hostaphan. It is available in a huge range of finishes and laminated constructions, from transparent to opaque, matte to glossy, insulating to conductive, UV-stabilised, heat-sealable, siliconised and recycled. Applications are as widespread, from packaging (like crisp packs and stand-up pouches) to high-performance yacht racing sails reinforced with super fibres such as carbon fibre, aramid fibre and ultra-high-molecular-weight polyethylene (UHMWPE).

It is coated with a thin layer of aluminium, by vacuum metallising, to produce packaging films that provide a barrier to oxygen and light, as well as a bright metallic backdrop to printed graphics and branding.

In addition, like regular PET, is has excellent dielectric strength, which combined with its other properties makes it an ideal substrate for printed electronics. It is used for in-mould labelling (IML) and in mould decoration (IMD) – the film is printed, thermoformed and back injection moulded, providing the graphics and haptics on the surface of an injection moulded part straight out of the mould.

The same as polypropylene (PP) film, PET film is used to make self-reinforced composites (SR-PET, SRPET). Polyester film with different melting points is stacked and slit into yarn, which is woven into fabric. With heat and pressure, the lower melting point polymer melts and forms a matrix around the higher melting point material, which remains oriented with all the associated mechanical properties.