Metallised films (or metalized films) are polymer films coated with a thin layer of metal, usually aluminium. They offer the glossy metallic appearance of an aluminium foil at a reduced weight and cost. Metallised films are widely used for decorative purposes and food packaging, and also for specialty applications including insulation and electronics.
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Manufacture
Metallisation is performed using a physical vapour deposition process. Aluminium is the most common metal used for deposition, but other metals such as nickel or chromium are also used. The metal is heated and evaporated under vacuum. This condenses on the cold polymer film, which is unwound near the metal vapour source. This coating is much thinner than a metal foil could be made, in the range of 0.5 micrometres. This coating will not fade or discolour over time. While oriented polypropylene and polyethylene terephthalate (PET) are the most common films used for metallisation, nylon, polyethylene and cast polypropylene are also used.
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Properties
Metallised films have a reflective silvery surface similar to aluminium foil and are highly flammable. The coating also reduces the permeability of the film to light, water and oxygen. The properties of the film remain, such as higher toughness, the ability to be heat sealed, and a lower density at a lower cost than an aluminium foil. This gives metallised films some advantages over aluminium foil and aluminium foil laminates. It was thought that metallised films would become a replacement for aluminium foil laminates, but current films still cannot match the barrier properties of foil. Some very high barrier metallised films are available using EVOH, but are not yet cost effective against foil laminates.
Table 1. Comparison of Metallised PET and aluminium foil
Uses
Decoration
Metallised films were first used for decorative purposes as Christmas tinsel, and continue to be used for items such as wrappers and ribbons. The metallic helium filled novelty balloons given as gifts are made of metallised BoPET and often called Mylar balloons commercially.
Packaging
Both metallised PET and PP have replaced foil laminates for products such as snack foods, coffee and candy, which do not require the superior barrier of aluminium foil. Metallised nylon and polyethylene are used in the meat export market. The controlled permeation extends shelf life.
Metallised films are used as a susceptor for cooking in microwave ovens. An example is a microwave popcorn bag.
Many food items are also packaged using metallised films for appearance only, as these produce a package with greater sparkle when compared to competing products that use printed paper or polymer films.
Insulation
Metallised PET films are used in NASA's spacesuits to make them radiation resistant and keep astronauts warm, and in proximity (aluminised) suits are used by AR-FF fire fighters for protection from the high amount of heat released from fuel fires. Silvered emergency blankets are also used to conserve a shock victim's body heat. These reflective materials might also be used as part of industrial and commercial insulation (for example, insulated shipping containers).
Insulating blankets made with metallised PET were also used as insulation in aircraft until safety concerns resulted in their deprecation. In particular, it was found to be a major contributor to the in-flight fire of Swissair Flight 111 on September 2, 1998.
Electronics
Metallised films are used as a dielectric in the manufacture of a type of capacitor used in electronic circuits, and as a material in some types of antistatic bags.
Tragedies
The inflammable nature of Metallised PET led to the deaths of 229 people aboard Swissair Flight 111 off Nova Scotia Canada.
Source of the article : Wikipedia
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