Corona treatment. In the case of corona treatment, the surface is exposed to a discharge between a grounded and powered electrode at high voltage. A low-frequency (typically 10-20 kHz) generator and step up transformer usually provide the high voltage to the electrode. In each half-cycle the applied voltage (20 kV peak) increases until it exceeds the threshold value for electrical breakdown of the air gap, causing the atoms and molecules to become ionized and creating an atmospheric plasma discharge. The voltage eventually peaks and falls below the conducting threshold. Each cycle consists of two such events involving current flow in each direction. In continuous operation the discharge appears to be a random series of faint sparks in a blue-purple glow (uv radiation). The point discharge generated across the pair of electrodes ionizes the gas present in the gap, which subsequently induces changes in the chemistry of the surface. Researchers (10) have demonstrated through derivatization reactions that carbonyl, enol, and carboxylic acid groups are formed on polyolefin materials after corona treatment. The most likely mechanism is free-radical in nature. The corona discharge contains ions, electrons, excited neutrals (atoms and molecules, and photons. All of these have sufficient energy to cause bond cleavage in the polymer surface. The resulting polymer chain radicals react extremely rapidly with 02.. Chain scission is involved in the formation of many of these groups, leading to a progressive reduction in the average molecular weight and finally to the production of CO, CO2, and H2O. In addition to oxidative degradation, there will also be direct degradation by ion-induced sputtering. These changes can have dramatic effects on the surface energy and functionality of polymer materials. Both dielectric polymer and conductive substrates can be treated with this method as illustrated in Figure 2. With nonconducting polymer films, the grounded roller is covered with a dielectric insulating material and a linear electrode is used. However, with conductive metallic substrates, the process is simply reversed by using a rotating electrode covered with a dielectric insulating material to prevent short-circuiting to ground. In either case, the electrode is always connected to a source of high voltage power, and the roller always remains grounded. However, the corona is a shower of arcs or sparks and each discharge point has the capability of causing localized damage and is difficult to apply consistently on three-dimensional components or structures. With corona treatment the effect on many materials is reported to be short-lived. This can represent a problem in some packaging applications where treatment stability is important.