Metallized polyethylene terephthalate

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.

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.

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

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.

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.

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