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 and electrical insulation. When metallized, it has gas and moisture barrier properties and is reflective. The film is "biaxially oriented", which means that the polymer chains are oriented parallel to the plane of the film, and therefore oriented over 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. Buckminster Fuller used Mylar in 1953 as a skin for a geodesic dome, which he built with students at the University of Oregon. Eastman Kodak used Mylar in 1955 as a support for photographic film and called it "ESTAR Base". The very thin and tough film allowed 6,000-foot (1,800 m) reels to be exposed on long-range U-2 reconnaissance flights. NASA launched Echo II in 1964, a 40-metre (131 ft) diameter balloon constructed from a 9-micrometre (0.00035 in) thick boPET film sandwiched between two layers of 4.5-micrometre (0.00018 in) 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 (392 °F). 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 (0.58×10^6 psi). 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
Laminates containing metallized boPET foil (in technical language called printin or laminate web substrate) protect food against oxidation and aroma loss, achieving long shelf life. Examples are coffee "foil" packaging and pouches for convenience foods. Pop-Tarts toaster pastries are sold in pairs wrapped in silver boPET. They were previously wrapped in foil. White boPET web substrate is used as lidding for dairy goods such as yogurt. Clear boPET web substrate is used as lidding for fresh or frozen ready meals. Due to its excellent heat resistance, it can remain on the package during microwave or oven heating. boPET is used as a material for roasting bags Metallised film is related to boPET Laminated sheet metal (aluminium or steel) used in the manufacture of cans (bisphenol A-free alternative to lacquers) has some relationship with boPET
Covering over paper A clear overlay on a map, on which notations, additional data, or copied data, can be drawn without damaging the map Metallized boPET is used as a mirror-like decorative surface on some book covers, T-shirts, and other flexible cloths. Protective covering over buttons/pins/badges The glossy top layer of a Polaroid SX-70 photographic print As a backing for very fine sandpaper boPET film is used in bagging comic books, in order to best protect them during storage from environmental conditions (moisture, heat, and cold) that would otherwise cause paper to slowly deteriorate over time. This material is used for archival quality storage of documents by the Library of Congress (Mylar type D, ICI Melinex 516 or equivalent) and several major library comic book research collections, including the Comic Art Collection at Michigan State University. While boPET is widely (and effectively) used in this archival sense, it is not immune to the effects of fire and heat and could potentially melt, depending on the intensity of the heat source, causing further damage to the encased item. Similarly, trading card decks (such as Pokémon, Magic: The Gathering, and Yu-Gi-Oh!) are packaged in pouches or sleeves made of metallized boPET. It can also be used to make the holographic artwork featured on some cards, typically known as "holos", "foils", "shinies", or "holofoils". For protecting the spine of important documents, such as medical records.
Insulating material
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