BoPET

BoPET (biaxially oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties, and electrical insulation. 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. 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 reels to be exposed on long-range U-2 reconnaissance flights. In 1964, NASA launched Echo II, a diameter balloon constructed from a thick mylar film sandwiched between two layers of thick aluminium foil bonded together. 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" or crystallized under tension in the oven at temperatures typically above . 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.