Editing BoPET (section)

== Manufacture and properties ==
{{more citations needed section|date=January 2023}}
[[Image:Polyethylene terephthalate.svg|thumb|upright=1.35|Chemical structure of polyethylene terephthalate]]

The manufacturing process begins with a film of molten [[polyethylene terephthalate]] (PET) being [[Plastics extrusion|extruded]] onto a chill roll, which quenches it into the amorphous state. It is then biaxially oriented by [[Drawing (manufacturing)|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., [[orthogonality|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 [[crystal]]lized under tension in the oven at temperatures typically above {{convert|200|C}}.<ref name=":3">{{Cite book |last=DeMeuse |first=Mark T. |url=https://books.google.com/books?id=BXpwAgAAQBAJ |title=Biaxial Stretching of Film: Principles and Applications |date=2011-07-18 |publisher=Elsevier |isbn=978-0-85709-295-3 |language=en}}</ref> 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.{{Citation needed|date=June 2024}} 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  {{cvt|4|GPa|e6psi}}. 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.<ref>{{cite web |last1=Thiel |first1=Ulrich |title=Polyester Additives |url=https://www.polyester-technology.com/fileadmin/media/literature/70-Thiele_POLYESTER-ADDITIVES-LONGVERSION-2007.pdf.pdf |website=Dr. Thiele Polyester Technology |access-date=4 January 2019}}</ref>

Biaxially oriented PET film can be [[Metallised film|metallized]] by [[vapor deposition]] of a [[Thin-film deposition|thin film]]  of [[evaporation|evaporated]] [[aluminium]], [[gold]], or other metal onto it. The result is much less permeable to gases (important in [[Modified atmosphere|food packaging]]) and reflects up to 99% of light{{citation needed|date=October 2021}}, 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 [[Heat sealer#Heat sealing process|sealability]] and improves [[puncture resistance]]. The polyethylene side of such a laminate appears dull and the boPET side shiny.{{Citation needed|date=June 2024}} Other coatings, such as conductive [[indium tin oxide]] (ITO), can be applied to boPET film by [[sputter deposition]].{{Citation needed|date=June 2024}}