Metallized PET film: What can PET metal be used for?
PET met is a polyester film metallized with aluminum powder. Polyester film (short: PET film) is inherently characterized by high strength and surface hardness. However, especially in terms of diffusion density, its excellent properties can be increased by the use of aluminum powder.
Aluminum-coated PET film has both advantages and disadvantages compared to conventional aluminum foil. The advantages of PET film are the higher tensile strength and lower production costs. In aluminum-coated composites, the processing speed is significantly higher than that of aluminum foil.
This makes PET met (metallized polyester film) particularly suitable for the following applications:
Packaging, especially for foods that are sensitive to moisture or oxidation, such as coffee or instant soups. PET-met stand-up pouches
Relative vapor barriers in building insulation.
Tapes and labels with relatively high chemical and heat resistance for use in vehicle technology or outdoor areas.
High diffusion density, but not absolute diffusion density
PET met, a composite material made of a thin aluminum layer and PET film, has 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 a material thickness of 10 μm. These values are already close to the measurement limits for identifying the absolute diffusion density.
However, in order to obtain the absolute diffusion density, classic aluminum foil or other metal foils must be used.
PET met combines the advantages of aluminum and PET film.
Compared to the laminated composite, the aluminum coating in the PET film only causes changes in the diffusion, optical and electrical properties. However, the excellent strength values and low specific gravity characteristics of the PET film remain almost unchanged.
The thickness of the aluminum layer in the PET film is so small that the unit Ångström, which is derived from the atomic diameter, is often used instead of μm (micrometer).
The measured values in angstroms can therefore be regarded as the corresponding number of atoms stacked directly on top of each other. Of course, this does not reflect the actual physical conditions, but illustrates the very small thickness of the metal support layer.
Metallized PET film layer thickness absolute values and comparison
The thickness of the aluminum layer of metallized PET film is usually about 30 Å – 500 Å, which is about 3 to 50 nm. In comparison, pure gold can be reshaped into a thin gold foil of 2000 Å. At this thickness, gold is already slightly transparent and appears green in the light.
PET metal foils are always slightly transparent.
Even metallized PET films are not completely opaque. In the case of incident light, this effect is not visible due to the silvery reflective surface. This effect can be altered by printing, dyeing or varnishing as well as brushing or embossing the PET film.
Effective protection against UV and thermal radiation
The highly reflective surface of metallized PET films can be used to create attractive optical effects in packaging as well as to block electromagnetic radiation.
Aluminum vapor-coated PET foils reflect not only visible and infrared light, but also UV light. Aluminum coatings differ from silver or gold coatings, which are used for similar purposes. In contrast to these metals, the degree of reflection of aluminum is largely independent of wavelength. The minimum value is still above 85%, slightly above 800 nm in the near-infrared range.
Effect of film thickness on the reflectivity of PET films
PET metal packaging industry
Packaging made with metallized PET
The reflectance and optical density of metallized PET films increase in proportion to the thickness of the aluminum coating. The optical density (OD) is measured as the logarithm of the incident optical radiation divided by the energy of the transmitted radiation and for metallized PET films its values are typically between 0.25 and 3.5.
The thicker the aluminum layer, the higher the optical density and the lower the transparency.
The reflectivity is therefore about 66-99%, since according to the logarithmic scaling an optical density of 0.3 corresponds to half the intensity of the transmitted radiation.
Changes in properties due to aging
Just like solid pure aluminum, aluminized PET films also undergo surface oxidation. It prevents further chemical transformation of the metal on the side exposed to air and on the substrate side.
This self-passivation caused by the formation of transparent aluminum oxide leads to a decrease in optical density and conductivity, especially in the first 24 hours after the production of the metallized PET film. This must therefore be taken into account during production if customer specifications for these values are to be adhered to.
PET films become more transparent over time.
Since the maximum thickness of the oxide layer is independent of the thickness of the aluminum coating and can only grow up to 3 to 4 nanometers, the effect of the oxide layer is particularly pronounced for thinner coatings. For an aluminum coating with a thickness of approximately 10 nanometers, the optical density drops from 0.2 to 0.28 within 24 hours and in the long term to about half of the original value.
Typical applications for metallized PET films
Metallized PET as food packaging
Metallized PET films are suitable, for example, as a direct replacement for aluminum foil in packaging technology. At the same film thickness, metallized PET films have a higher tensile strength and bending stiffness, so that composites produced by metal evaporation do not wrinkle like aluminum foil.
PETmet as an aluminum substitute with a front and a bottom.
This results in a more aesthetically pleasing packaging. Nevertheless, individual packaging of sweets can also be achieved with metallized PET films, i.e. sealing by twisting the film ends (twist seal).
But aluminum foil also has advantages in certain packaging areas, because, unlike PET film, it is not transparent and guarantees an absolute diffusion density. Therefore, in general, foods packaged in PET film are not as durable as those packaged in aluminum foil. PET film is therefore more suitable for foods with a shorter shelf life, which remain in the supermarket for a relatively short time until the buyer picks it up.
A further application possibility of metallized PET film in the field of food packaging is that the extremely fine metal coating changes the conductivity of the film surface. This makes the PET metal visible in metal detectors, but can also be used to package microwaves with a low moisture content, so that the food can absorb more energy and heat up quickly.
In terms of building insulation, the positive influence of aluminum vaporization on diffusion resistance as well as on the reflection of thermal radiation is very important. Compared with aluminum foil, PET film does not offer an absolute vapor barrier, but the mechanical resistance and reflectivity are significantly higher.
