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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Metallisation is performed using a physical vapor 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 vapor 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 discolor 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.
Metallised films used for balloons
Metallised films were first used for decorative purposes as Christmas tinsel, and continue to be used for items such as wrappers and ribbons. 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.
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.
Metallised PET films are used in NASA spacesuits to reflect heat radiation, keeping astronauts warm, and in &#;proximity suits&#; used by firefighters for protection from the high amount of heat released from fuel fires. Aluminized emergency blankets ("space blankets") are also used to conserve a shock victim's body heat. MPET has been used as an antistatic container for other heat and sound insulating materials used in aircraft, to prevent the insulation from leaking into the passenger cabin, but is not itself the insulator in that use.
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.
METALIZED PET (PET MET OR MPET)
PET met is a polyester film metallized with aluminum powder. Polyester film (short: PET film) is already characterized by high strength and surface hardness. Especially with regard to the diffusion density however, the favorable properties can be improved through the use of aluminum powder.
A PET film metallized with aluminum powder has advantages and disadvantages compared to conventional aluminum foils. The advantages of PET met are in particular, a higher tensile strength and lower production costs. The possible processing speed is significantly higher than that of aluminum foil in the composite material produced by vaporizing aluminum.
This makes PET met, the metallized polyester film particularly useful for applications such as:
With PET met, a composite material made of a wafer-thin aluminum layer and a PET film, diffusion values of less than 0.5 cm³/m²/d for oxygen and less than 0.3 g/m²/d for water vapor even at material thicknesses of 10 μm can be achieved. These values are already close to the measurement limits which are indicated for the identification of the absolute diffusion density.
For an absolute diffusion density, however, a classic aluminum foil or a different metal foil has to be used.
In contrast to a laminated composite material, the aluminum coating in PET met only causes a change in the diffusion, optical and electrical properties. The favorable strength values and the low specific weight of the PET film however, remains practically unchanged.
Winding of PET met film
The thickness of the aluminum layer in PET met is so small that the unit, Ångström, which is derived from the atomic diameter is usually used instead of a measurement in μm (micrometers).
A measurement in Ångström can therefore be regarded as a corresponding number of atoms stacked directly above one another. This of course, does not reflect the actual physical conditions, but illustrates the extremely small thickness of the metal support layer.
The typical range of aluminum layer thicknesses for a metallized PET film of about 30 Å &#; 500 Å is about 3 to 50 nm. For comparison, pure gold can be reshaped into Å of thin gold leaf. At this thickness, gold is already slightly translucent and appears green against the light.
Even metallized PET film is not completely opaque. In the case of incident light, this effect cannot be seen due to the silvery reflecting surface. This effect can be varied by printing, dyeing or varnishing as well as by brushing or embossing the PET film.
The highly reflective surface of a metallized PET film can be used to create an attractive optical effect in packaging as well as a barrier against electromagnetic radiation.
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Aluminum vapor-coated PET foil reflects not only visible and infrared light, but also UV radiation. The aluminum coating differs from the silver or gold coatings used for similar purposes. Compared to these metals, the degree of reflection of aluminum is largely independent of the wavelength. The minimum which is still above 85%, is slightly above 800 nm in the near infrared range.
Packaging made of metallized PET
The reflection factor and the optical density of metallized PET film increases proportionally with the thickness of the aluminum coating. The optical density (OD) measured as the logarithm of the incident optical radiation divided by the radiation energy transmitted is typically between 0.25 and 3.5 for PET met.
The degree of reflection is thus about 66-99% because, due to the logarithmic scaling, an optical density of 0.3 corresponds to half of the transmitted radiation intensity.
Just as it is the case with solid pure aluminum, a superficial oxidation takes place in the aluminum coated PET film. It protects the metal from further chemical conversion, both on the side exposed to air as well as the substrate side.
This self-passivation resulting from the formation of transparent aluminum oxide leads to a reduction in the optical density and the electrical conductivity, especially in the first 24 hours after the production of the metallized PET film. It must therefore be taken into account during production if customer specifications are to be observed with regard to these values.
Since the oxide layer grows to a maximum of 3 to 4 nm irrespective of the thickness of the aluminum coating, the effect is particularly strong with thin coatings. With an approximately 10 nm thick aluminum coating, the optical density can drop from 0.2 to 0.28 within 24 hours and to about half of the original value in the long term.
Twist closure for candies
Metallized PET film is suitable, for example, in packaging technology as a direct replacement for aluminum foil. For the same film thickness, PET met is characterized by a higher tensile strength and flexural rigidity such that the composite material produced by vaporization of the metal is not as wrinkled as aluminum foil.
This can give the package a more presentable look. Despite this, individual packages for candies, which are closed by twisting the film ends (twist closure) are also possible with metallized PET film.
However, aluminum foil also has advantages in certain packaging because, unlike PET met film, it is opaque and guarantees an absolute diffusion density. In general, food packaged with PET met film is therefore not as durable as those packaged with aluminum foil. PET met film is therefore more suitable for foodstuffs with short shelf life, which only stay in the supermarket for a relatively short time before a buyer gets it.
Further application possibilities for metallized PET film in the field of food packaging result from the fact that the extremely fine metal coating changes the electrical conductivity at the surface of the film. This makes PET met visible in metal detectors, but can also be used, for example in the packaging of microwave dishes with low water content, so that the food can absorb more energy and heat up quickly.
In the case of building insulation, the positive influence of the aluminum vaporization on the diffusion resistance as well as on the reflection of thermal radiation is important. Compared to aluminum foil, PET met offers a smaller degree of reflection and no absolute vapor barrier, but a significantly higher mechanical resistance.
Courtesy: Wikipedia and AlfiPa
Tekra has been selling polyester plastic film in sheets and rolls to diverse markets since the s, and our experience shows. Advancements in the printing industry led to Tekra seeking strategic alliance partners to support those changes as well as meet the technical and commercial needs of our customers. Today, we continue our proud association with globally recognized Melinex® and Mylar® polyester brands as an authorized distributor of Mylar® Specialty Films (formerly DuPont Teijin Films&#;).
PET plastic film material is known as biaxially oriented polyethylene terephthalate or BOPET because of the way the film is produced, which also gives it superior properties to other plastic films. Tekra sources Melinex® and Mylar® polyester films in large roll form from Mylar® Specialty Films (formerly DuPont Teijin Films&#;) which can be converted into custom sized slit rolls or sheets according to customer specifications. Tekra offers hardcoating to help PET film materials resist scratches and abrasions while offering enhanced chemical resistance.
As a result of the biaxial orientation process, Melinex® and Mylar® polyester films offer superior properties and provide exceptional versatility when compared to other thermo-engineered plastic films. These properties include:
Tekra inventories Melinex® and Mylar® polyester film in a broad range of product types and gauges. We also apply functional custom coatings to polyester PET films in our clean room coating facility.
Tekra inventories Melinex® and Mylar® polyester PET films in sheets and rolls in a broad range of product types and thicknesses ranging from 0.48 mil to 14 mil. Tekra can offer clear polyester film materials for applications requiring excellent optical properties, hazy polyester films for various industrial end uses, and opaque white polyester films that work well with pressure sensitive labels, cards, and general printing applications. We also apply functional custom coatings to polyester PET films in our clean room coating facility.
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