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PET film (biaxially oriented)
Biaxially-oriented polyethylene terephthalate ('''boPET''') is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical stability|chemical and Shape|dimensional strength of materials|stability, Transparency (optics)|transparency, reflectivity, gas and aroma barrier properties and electricity|electrical Electrical insulation|insulation.
A variety of Company (law)|companies manufacture boPET and other polyester films under different Brand|trade names. In the US and Britain, the most well-known trade names are '''Mylar''', '''Melinex''' and '''Hostaphan'''.
History
BoPET film was developed in the mid-1950s, originally by DuPont and Imperial Chemical Industries (ICI). In 1960 and 1964, NASA launched the Echo satellites, diameter balloons of metallized thick boPET film.
Manufacture and properties
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.[manufacturing process] 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" or 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 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.
Biaxially oriented PET film can be Metallised film|metallized by vapor deposition of a Thin-film deposition|thin film of evaporation|evaporated aluminum, gold, or other metal onto it. The result is much less permeable to gasses (important in Modified atmosphere|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 PET side shiny.
Other coatings, such as conductive indium tin oxide (ITO), can be applied to boPET film by sputter deposition.
Metallized nylon (or "foil") balloons used for floral arrangements and parties are often mistakenly called "Mylar", one of the trade names for boPET film.
Applications
Uses for boPET polyester films include, but are not limited to:
Flexible packaging and food contact applications
- 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.
- Attractive glossy or matte surfaces on the outside of packages are achieved using boPET.
- White boPET web substrate is used as lidding for dairy goods such as yoghurt.
- 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.
- Roasting bags
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 Corporation|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[Library of Congress] and several major library comic book research collections, including the Comic Art Collection at Michigan State University.[Scott, Randall W. “A Practicing Comic-Book Librarian Surveys His Collection and Craft.” ''Serials Review'' 24.1 (1998): 49-56.] 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.
- For protecting the spine of important documents, such as medical records.
Insulating material
- An Electrical insulation|electrical insulating material
- Thermal insulation|Insulation for houses and tents in ''cold'' environments, covering the inner walls with the metallized surface facing ''inward'', thus reflecting heat ''back into'' the space
- Thermal insulation|Insulation for houses and tents in ''hot'' environments, covering the outer walls with the metallized surface facing ''outward'', thus reflecting heat ''away from'' the space
- Five layers of metallized boPET film in NASA's spacesuits make them radiation resistant and keep astronauts warm.
- Metallized boPET film "Space blanket|emergency blankets" conserve a shock (circulatory)|shock victim's body heat.
- As a thin strip to form an airtight seal between the control surfaces and adjacent structure of aircraft, especially Glider aircraft|gliders.
- Light insulation for indoor gardening.
- Wildland fire shelters.
- Proximity (aluminized) suits used by AR-FF fire fighters for protection from the high amount of heat release from fuel fires.
- Aluminized boPET films are no longer used as thermal and acoustic insulation in aircraft since they were found to have been a factor in the Swissair flight 111|Swissair Flight 111 crash. The accident investigation showed that the aluminum layer prevents the film from self-extinguishing in a fire under the conditions in aircraft.[accident investigation]
Solar and marine applications
- Metallized boPET is intended to be used for solar sails as an alternative spacecraft propulsion|means of propulsion for spacecraft such as Cosmos 1|Cosmos 1
- Metallized boPET Sun|solar curtains reflect sunlight and heat away from windows.
- Aluminized, as an inexpensive solar eclipse viewer, although care must be taken, because invisible fissures can form in the metal film, reducing its effectiveness.
- High performance sails for sailboats and hanggliders
- Use boPET films as the back face of the PV modules in solar panels
Science
- Amateur and professional visual and telescopic solar filters. BoPET films are often annealed to a glass element to improve thermal conductivity, and guarantee the necessary flat surface needed for even telescopic solar observation. Manufacturers will typically utilise films with thicknesses of between 280 and 500 microns, in order to give the films better resilience. 250 Micron thickness films with a heavy aluminium coating are generally preferred for naked-eye Solar observation during eclipses.
- Films in annular ring mounts on gas-tight cells, will readily deform into spherical mirrors. Photomultiplier cosmic-ray observatories often make use of these mirrors for inexpensive large (1.0m and above), lightweight mirror surfaces for sky-sector low and medium energy cosmic ray research.
- As a light diaphragm material separating gases in hypersonic shock and expansion tube facilities.
- As a beamsplitter in Fourier transform infrared spectroscopy, typically with Laser applications. Film thicknesses are often in the 500 micron range.
- Coating around hematocrit tubes.
Electronic and acoustic applications
- Carrier for flexible printed circuits
- Very thin boPET film is often used as the diaphragm material in electrostatic loudspeakers and Microphone#Electret capacitor microphones|electret microphones.
- boPET film has been used in the production of banjo & drumheads since 1958 due to its durability and acoustical properties when stretched over the bearing edge of the drum. They are made in single- and double-ply versions, with each ply being between 2 Thou (unit of length)|mils and 10 mils (0.05 – 0.25 mm) in thickness, with a clear or opaque surface, originally used by the company Evans.
- boPET film is used as the substrate in practically all magnetic tape|magnetic recording tapes and floppy disks.
- Metallized boPET film, along with other plastic films, is used as a dielectric in foil capacitors.
- Clear boPET bags are used as packaging for audio media such as compact discs and vinyl records.
Graphic arts
- Often engineering plans or architectural drawings are plotted onto sheets of boPET film. The boPET sheets become legal documents from which copies or blueprints are made. boPET sheets are more durable and can withstand more handling than bond paper.
- Overhead transparency film for photocopiers or laser printers (boPET film withstands the high heat).
Other
- Route information signs, called rollsigns or rollsign|destination blinds, displayed by public transport vehicles
- For materials in kite flying|kites
- Covering glass to decrease probability of shattering
- In theatre effects as confetti.
- As the adhesive strip to attach the string to a teabag
- One of the many materials used as Windsavers or valves for valved harmonicas
- On farmland, highly reflective aluminized PET film ribbons are tied to the plants to create shimmers from the sun for an effect similar to a scarecrow.
- In comic book Comic book collecting|collecting, Mylar bags are preferred for long-term storage or protection of high-value items
References
External links
- ''Plastics: Background Information for Teachers''. by The American Plastics Council (HTML format) or (PDF format) - 1.9MB), which includes the "Recycling Symbol|chasing arrow" recycling symbols (PET is #1) and a description of plastics.
- An interesting toy has been developed using boPET and a stick-shaped Van de Graff generator.
Category:Dielectrics
Category:Plastics
Category:Reflective building components
Related Images- Chemical structure of polyethylene terephthalate.
Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL
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