What is Corona treatment?

Plastic surfaces are mostly hydrophobic, i.e. wetting with a liquid is difficult, if impossible. If ink or adhesive is to adhere, the surface must be activated. The process is called plasma or corona treatment.

A corona system essentially consists of two main components, the generator (high frequency) and the corona station.

The generator takes an incoming AC voltage from the power grid, converts it and thus drives a resonance circuit that consists of a transformer and electrodes in the corona station. This creates a high voltage of approx. 30 kV at a frequency around 20 to 30 kHz, which leads to a plasma discharge between the electrodes. The dielectric between the electrodes prevents high voltage flashover and causes the plasma to spread over the entire electrode surface. Correspondingly, the technical term for the process is a dielectric barrier discharge (DBD).

Different electrode configurations are possible in the station in order to optimally serve the respective applications.

Any surface can be pretreated with the plasma generated in this way; the corresponding plasma discharge is referred to as corona. The most widespread application is the corona pretreatment of sheet materials, e.g. Foils. Accordingly, the counter electrode is shown as a roller in our diagram. In order to achieve good process results, the distance between the electrode and the roller is set as accurately as possible to between 1.5 to 2 mm across the widthdepending on the electrode / roller configuration.

The corona discharge causes a change in the microscopic roughness on the surface, which results in an increased contact area in the following process, e.g. for coatings, printing inks or laminates. In addition, the surface is oxidized. This in turn changes the wettability of the surface and enables direct chemical crosslinking with conventional coatings, printing inks and laminates.

Since our customers usually carry out a pretreatment in ambient air, ozone is created as a by-product, which must be extracted to the outside or removed via a catalyser.

Since each substrate reacts differently to the action of the corona pretreatment, it has to be adjusted to the respective process. For this purpose, the specific energy input, called dose, required for the process is determined. In addition to the dose (D), the parameters web or corona width (CB) and maximum process speed (v) are included in the calculation of the power required for the design of the corona system per treatment side (P):

P [W] = D [Wmin/m²]  x  CB [m]  x  v [m/min]

With our decades of experience, we will be happy to advise you on process issues related to your application.

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