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Metallized boPET film, 32 layers of ~14 µm thickness each
Metallized boPET film, 32 layers of ~14 µm thickness each

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, and Hostaphan.[1]

History

BoPET film was developed in the mid-1950s,[2][3] originally by DuPont,[2] Imperial Chemical Industries (ICI), and Hoechst.

In 1955 Eastman Kodak used Mylar as a support for photographic film and called it "ESTAR Base".[4] The very thin and tough film allowed 6,000-foot (1,800 m) reels to be exposed on long-range U-2 reconnaissance flights.[5]

In 1964, NASA launched Echo II, a 40-metre (131 ft) diameter balloon constructed from a 9-micrometre (0.00035 in) thick mylar film sandwiched between two layers of 4.5-micrometre (0.00018 in) thick aluminium foil bonded together.[6]

Manufacture and properties

Chemical structure of polyethylene terephthalate
Chemical structure of polyethylene terephthalate

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.[7] 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 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 are usually embedded in the PET to roughen the surface of the film such as silicon dioxide.[8]

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[citation needed], 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

References

  1. ^ Mark T. DeMeuse (2011). Biaxial Stretching of Film: Principles And Applications. Elsevier. p. 48. ISBN 9780857092953.
  2. ^ a b Izard, Emmette Farr, "Production of polyethylene terephthalate", U.S. patent no. 2,534,028 (filed: 1948 May 13; issued: 1950 December 12).
  3. ^ Adams, John Francis Edward; Gerber, Kenneth George; Holmes-Walker, William Anthony, "Process for the production of biaxially oriented polyethylene terephthalate film", U.S. patent no. 3,177,277 (filed: 1957 May 10 ; issued: 1965 April 6).
  4. ^ "Kodak HCF Film/ESTAR Base" (PDF). www.kodak.com. Eastman Kodak Company. April 2015. Retrieved 2018-08-24.
  5. ^ Eyes in the Sky, Dino A. Brugioni 2010, Naval Institute Press, ISBN 978 1 59114 082 5, pp. 102, 115.
  6. ^ Staugaitis, C. & Kobren, L. (1966) "Mechanical And Physical Properties of the Echo II Metal-Polymer Laminate (NASA TN D-3409)", NASA Goddard Space Flight Center.
  7. ^ "Process Flow". Ampef.com. Archived from the original on 2017-11-20. Retrieved 2018-08-24.
  8. ^ Thiel, Ulrich. "Polyester Additives" (PDF). Dr. Thiele Polyester Technology. Retrieved 4 January 2019.
  9. ^ "Specifications for Polyester: Poly(ethylene-terephthalate)". Preservation. Library of Congress. Archived from the original on June 23, 2004.
  10. ^ "What is Mylar Paper - More Than Just Decoration". Jampaper.com. 23 October 2013. Retrieved 2015-07-02.
  11. ^ Scott, Randall W. (1998). "A Practicing Comic-Book Librarian Surveys His Collection and Craft". Serials Review. 24 (1): 49–56. doi:10.1080/00987913.1998.10764429.
  12. ^ Kristen Heinichen (June 17, 2008). "Albany library's entire collection exposed to smoke". Athens Messenger. Archived from the original on 2015-07-03. Retrieved 2015-07-02 – via Athens County Public Libraries.
  13. ^ "How to Convert Mylar Aerospace Drawings to 3D CAD". CAD / CAM Services. 31 January 2018.