BoPET (biaxially oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical stability, dimensional stability, transparency reflectivity, and electrical insulation. . When metallized, it has gas and moisture barrier properties, The film is "biaxially oriented", which means that the polymer chains are oriented parallel to the plane of the film, and therefore oriented in two axes. A variety of companies manufacture boPET and other polyester films under different brand names. In the UK and US, the best-known trade names are Mylar, Melinex, Lumirror and Hostaphan. It was the first biaxially oriented polymer to be manufactured on a mass commercial scale.

History

BoPET film was developed in the mid-1950s, originally by DuPont, Imperial Chemical Industries (ICI), and Hoechst. In 1953 Buckminster Fuller used Mylar as a skin for a geodesic dome, which he built with students at the University of Oregon. In 1955 Eastman Kodak used Mylar as a support for photographic film and called it "ESTAR Base". The very thin and tough film allowed 6000 ft reels to be exposed on long-range U-2 reconnaissance flights. In 1964, NASA launched Echo II, a 40 m diameter balloon constructed from a 9 μm thick mylar film sandwiched between two layers of 4.5 μm thick aluminium foil bonded together.

Manufacture and properties

The manufacturing process begins with a film of molten polyethylene terephthalate (PET) being extruded onto a chill roll, which quenches it into the amorphous state. It is then biaxially oriented by drawing. The most common way of doing this is the sequential process, in which the film is first drawn in the machine direction using heated rollers and subsequently drawn in the transverse direction, i.e., orthogonally to the direction of travel, in a heated oven. It is also possible to draw the film in both directions simultaneously, although the equipment required for this is somewhat more elaborate. Draw ratios are typically around 3 to 4 in each direction. Once the drawing is completed, the film is "heat set" and crystallized under tension in the oven at temperatures typically above 200 C. The heat setting step prevents the film from shrinking back to its original unstretched shape and locks in the molecular orientation in the film plane. The orientation of the polymer chains is responsible for the high strength and stiffness of biaxially oriented PET film, which has a typical Young's modulus of about 4 GPa. Another important consequence of the molecular orientation is that it induces the formation of many crystal nuclei. The crystallites that grow rapidly reach the boundary of the neighboring crystallite and remain smaller than the wavelength of visible light. As a result, biaxially oriented PET film has excellent clarity, despite its semicrystalline structure. If it were produced without any additives, the surface of the film would be so smooth that layers would adhere strongly to one another when the film is wound up, similar to the sticking of clean glass plates when stacked. To make handling possible, microscopic inert inorganic particles, such as silicon dioxide, are usually embedded in the PET to roughen the surface of the film. Biaxially oriented PET film can be metallized by vapor deposition of a thin film of evaporated aluminium, gold, or other metal onto it. The result is much less permeable to gases (important in food packaging) and reflects up to 99% of light, including much of the infrared spectrum. For some applications like food packaging, the aluminized boPET film can be laminated with a layer of polyethylene, which provides sealability and improves puncture resistance. The polyethylene side of such a laminate appears dull and the boPET side shiny. Other coatings, such as conductive indium tin oxide (ITO), can be applied to boPET film by sputter deposition.

Applications

Uses for boPET polyester films include, but are not limited to:

Flexible packaging and food contact

Covering over paper

Insulating material

Solar, marine, and aviation

Science

Electronic and acoustic

Printing media

Other