Corona discharge treatment (CDT) is typically used to improve adhesion of ink or coatings on plastic films. In the CDT, an electrode is connected to a high voltage source. The film travels on a roller that is covered with a dielectric layer and is grounded. When a voltage is applied, the electrical discharge causes ionization of air, and a plasma is formed. In doing so, the surface of the film is oxidized, thus improving wetting and adhesion. Additionally, the discharge reacts with molecules of the adherend to form free radicals, which react with oxygen and eventually form polar groups that increase the surface energy of the adherend. Another way CDT improves bonding is that it roughens the adherend by removing the amorphous regions of the surface, which increases the surface area and improves adhesive bonding. Depending on the type of adherend being treated with CDT, the treatment times may differ. Some adherends may require longer treating times to achieve the same surface energy.