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Aluminium foil is rolled in two layers in order to achieve a high degree of thinness. The machine separating the two layers uses an innovative plasma method for removing the greasy separating agent.
© Kampf Schneid- und Wickeltechnik GmbH & Co. KG
Plasma method of foil production
Projektinfo 04/2013

Dr. Wilfried Lehmann, head of technology at Kampf, shows the sketches of the pilot system construction.
© Christina Geimer, BINE Informationsdienst
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Degreasing aluminium foil using corona treatment

Rolls of aluminium foil commonly used in households have already been in the oven for up to one hundred hours before coming into contact with food. The heat causes the separating agent needed during production to evaporate. Without the grease, the foil cannot be rolled so thinly, and the consumption of aluminium, which requires a considerable amount of energy, would be greater. The aim is to replace the costly degreasing procedure with a corona treatment. During this procedure, high-energy electrons come into contact with the foil. The method can be integrated inline, in other words directly, into the production flow.

Aluminium which is almost completely airtight is used in households in order to keep food fresh and warm. The material is also widely used for industrial purposes. For example, it is added to the packaging used for drink cartons. A foil thickness of less than 0.007 millimetres – even thinner than a human hair – already offers sufficient barrier properties.

In order to achieve this degree of thinness, the aluminium foil is rolled in two layers. During this process, an inner matt side and a shiny outer side are created. In order to ensure that the two sides aren’t glued together, they are moistened with a separating agent which is not suitable for consumption. During the standard production procedure, the aluminium foil is then heated in a chamber furnace for up to 100 hours so that the separating agent diffuses out.

As an alternative to this annealing process, which requires a great deal of time and energy, a corona treatment approach has been forwarded since the 1970s. The mechanical engineering company Kampf from Wiehl, North Rhine-Westphalia, has now successfully developed a prototype. This enables the inline degreasing of the aluminium foil following the rolling process.

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What is corona treatment?

Corona treatment is a plasma method under atmospheric pressure. Essentially, the physical plasma is an ionised gas mixture. In nature, it occurs in lightning, for example. In industry, plasma which is generated under controlled conditions is used as a pollutant-free alternative to chemical methods. It is suitable for modifying, cleaning and disinfecting surfaces.

The types of plasma used include glow discharging like that used in fluorescent lamp bulbs. The method requires a low pressure environment. Another option, known as barrier discharging or dielectrically impeded discharging or silent discharging, is similar to glow discharging. It has the advantage that it can also ignite under atmospheric pressure. The term corona discharging is used to describe directly applied discharging in foil surface processing.

Until now, corona treatment has been used above all for processing synthetic materials. Yoghurt pots and other products made of plastic have a water-resistant, electrically insulated and non-polar surface. However, corona treatment is helpful in enabling them to be glued or printed. For example, it is used upstream in offset printing machines. The method increases the tension in the surface and in so doing, improves moisture penetration.

The corona treatment is also used with other synthetic, metal and paper surfaces to ensure that the products bond better with the dye, adhesive and coatings.

Prototype for inline degreasing put to the test

Thanks to the continuous degreasing during running production, the lengthy annealing process is no longer required. The developers are also hoping to make energy savings. The chamber furnaces consume around 1.3 kWh per kilogram of aluminium foil during degreasing. As a result, this process uses up to ten per cent of the entire energy consumed during production. This lies between 12.9 and 17.7 kWh per kilogram of aluminium foil.

In the chamber furnaces, the rolled aluminium foil is subjected to temperatures of between 250 °C and 400 °C. The temperature can only be increased slowly, since the rolls, which are up to one metre thick, will otherwise crack. It takes up to five days to diffuse out the separating agent. By contrast, the corona method can be incorporated inline into the production process. After the double foil has been separated, it runs through the corona station in the Sepamat (cover image).

Plasma method functions under atmospheric pressure

The aluminium foil is degreased by an atmospheric air plasma. It runs over an earthed roller above which are high-voltage electrodes. The plasma is generated by a voltage in a range of up to around 20 kV.

Ions from the plasma come into contact with the foil with a high level of energy. This causes the separating grease to oxidise. By-products of this process include ozone, nitrogen oxide and carbon dioxide. The gases are suctioned out. The cold plasma method functions under atmospheric pressure.

The degreasing depends on several parameters of the corona. Staff at the University of Applied Sciences and Arts (HAWK) in Göttingen have researched the optimum settings. In this way, the power density, and therefore the energy which acts on the surface of the foil, can be controlled.

Here, the aim is to achieve the highest possible degree of degreasing without damaging the foil. In practice, the foil does not always lie completely flat on the roller. It could get snagged and tear. At higher speeds, the lower residence time of the foil under the electrodes must be compensated. For this purpose, a larger number of electrode rods can be used, for example.

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Manufacturer of the plasma and corona systems
Tigres GmbH

Scientific monitoring
HAWK HHG

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BINE-Projektinfo 04/2013
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