BoPET

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 most well-known trade names are Mylar, Melinex and Hostaphan.^[1]

History

BoPET film was developed in the mid-1950s, originally by DuPont, Imperial Chemical Industries (ICI) and Hoechst. In 1964, NASA launched Echo II (the second mission of Project Echo). It was a 40 m (131 ft) diameter balloon constructed from "a 0.35mil (9µm) thick mylar film sandwiched between two layers of 0.18 mil (4.5µm) thick aluminum foil and bonded together."^[2]

Manufacture and properties

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.^[3] 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. 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.

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 PET 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 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.
• 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.
• Roasting bags
• Metallised films
• Laminated sheet metal (aluminum or steel) used in the manufacture of cans (bisphenol A-free alternative to lacquers)

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^[4] (specifically Mylar® type D)^[5] and several major library comic book research collections, including the Comic Art Collection at Michigan State University.^[6] 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.^[7]
• For protecting the spine of important documents, such as medical records.

Insulating material

• An electrical insulating material
• Insulation for houses and tents, reflecting thermal radiation
• Five layers of metallized boPET film in NASA's spacesuits make them radiation resistant and help regulate temperature.
• Metallized boPET film "emergency blankets" conserve a shock victim's body heat.
• As a thin strip to form an airtight seal between the control surfaces and adjacent structure of aircraft, especially gliders.
• Light insulation for indoor gardening.
• Fire shelters, used by wildland firefighters.
• 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 crash. The accident investigation showed that the aluminum layer prevents the film from self-extinguishing in a fire under the conditions in aircraft.^[8]
• Used in sock and glove liners to lock in warmth

Solar, marine and aviation applications

• Metallized boPET is intended to be used for solar sails as an alternative means of propulsion for spacecraft such as Cosmos 1
• Metallized boPET 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, hanggliders, paragliders and kites
• Use boPET films as the back face of the PV modules in solar panels
• Metallized boPET as a reflector material for solar cooking stoves
• To bridge control surface gaps on sailplanes, reducing profile drag

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 280–500 micrometres (0.011–0.020 in), in order to give the films better resilience. 250-micrometre (0.0098 in) 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 (1.0 m 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 micrometre range.
• Coating around hematocrit tubes.
• Insulating material for a cryocooler radiation shield.
• As a window material to confine gas in detectors and targets in nuclear physics.
• In CT scanners it acts as a physical barrier between the xray-tube, detector ring and the patient allowing negligible attenuation of the xray beam when active.
• Spacecraft are insulated with a metallized BoPET film.
• The material for solar sails of spacecraft.

Electronic and acoustic applications

• Carrier for flexible printed circuits.
• Mylar coated aluminum, also known as a 'Foil Shield', is commonly found in ethernet and other multi-conductor cables.
• boPET film is often used as the diaphragm material in electrostatic loudspeakers and 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 2–10 mils (0.051–0.254 mm) in thickness, with a transparent or opaque surface, originally used by the company Evans.
• boPET film is used as the substrate in practically all 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.
• Clear and white boPET films are used as core layers and overlays in Smart Cards.

Graphic arts

• Before the widespread adoption of CAD, engineering drawings or architectural drawings were plotted onto sheets of boPET film, known as drafting 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. This is no longer necessary, as traditional diazo "blueprint" duplication has fallen out of use.
• Overhead transparency film for photocopiers or laser printers (boPET film withstands the high heat).

Other

• Balloons
• Route information signs, called rollsigns or destination blinds, displayed by public transport vehicles
• For materials in 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.
• Measuring tape
• Protecting pinball machine playfields from wear
• Used in dentistry when restoring teeth with composite.
• Recently, shredded BoPET has found widespread usage in the Cosmetics industry. Several leading brands of nail polish have marketed varieties of nail polish containing shreds of different coloured BoPET, often using the common name mylar.

References

External links

• History of Polymers & Plastics for Teachers. by The American Chemistry Council (HTML format) or (PDF format) - 1.9MB, which includes the "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 Graaff generator.