Corona discharge


In electrical engineering a corona discharge is a discharge that occurs on high-voltage conductors. Already thirty years ago it was recognised that the targeted application of such a discharge can be used to improve adhesion on plastic surfaces. Its effect is shown below in a simplified illustration, corresponding to a dielectric barrier discharge (DBD).
A HF corona treatment plant consists generally of the generator, the high-voltage transducer and the treatment station. Today's high-voltage generators are almost exclusively produced using semiconductor technology. The advantages compared to the old tube-based equipment, which is still in use occasionally, is a significantly improved efficiency together with a much greater power output, as well as greater reliability and a lower level of stray radiation, provided that suitable power modules are used. To provide a continuous high-voltage plasma, the high voltage is applied at a high frequency (25 - 50 kHz), thus the name "high-frequency corona".
A high-voltage electrode (several kV) is brought into close proximity with the substrate's surface to be treated. An earthed counter electrode is in contact with the back of the substrate. A discharge zone is formed in the gap between the electrodes, which has a high concentration of different activated atoms and molecules that strike the substrate surfaces to be treated. The closed molecule chains of the surface are cracked open, which allows activated species, primarily oxygen radicals, to be deposited. Polar molecules are now generated on the surface, which was previously non-polar, to which colour molecules can bind chemically.

Since the air gap between the electrodes is usually only 1 - 2 mm, the main field of application for corona is in the sector of relatively flat surfaces and thin substrates, such as packaging film and labels, but also cups and tubes. Conductive as well as non-conductive substrates can be treated.
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