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.