Corona plasma

Particularly in mass spectrometry, where discharges are used to enhance or produce ions from sample materials, mostly coronas, plasmas, and arcs are used. The gas pressure is normally atmospheric, and the electrodes are arranged to give nonuniform electric fields. Usually, coronas and plasmas are struck between electrodes that are not of similar shapes, complicating any description of the discharge because the resulting electric-field gradients are not uniform between the electrodes. 

This chapter should be read in conjunction with Chapter 6, Coronas, Plasmas, and Arcs. A plasma is defined as a gaseous phase containing neutral molecules, ions, and electrons. The numbers of ions and electrons are usually almost equal. In a plasma torch, the plasma is normally formed in a monatomic gas such as argon flowing between two concentric quartz tubes (Figure 14.1). 

The exact conditions of gas pressure, current flow, and applied voltage under which the discharge occurs determine if it is of the corona, plasma, or arc type. The color of the emitted light may also change, depending not only on the type of gas used but also on whether it is a corona, plasma, or arc discharge. 

While polymeric surfaces with relatively high surface energies (e.g. polyimides, ABS, polycarbonate, polyamides) can be adhered to readily without surface treatment, low surface energy polymers such as olefins, silicones, and fluoropolymers require surface treatments to increase the surface energy. Various oxidation techniques (such as flame, corona, plasma treatment, or chromic acid etching) allow strong bonds to be obtained to such polymers. 

If corona, plasma, or flame treatment is chosen as the surface treatment, it is important to bond quickly after the treatment. Waiting several hours will reduce the effectiveness of the treatment. In some cases, attempts to bond 24 h after the treatment can give the same poor bonding results as if the plastic had never been surface treated. If surface oxidation is not possible, priming the surface with a chlorinated polyethylene primer is a second choice [95]. 

Some physical techniques can be classified into flame treatments, corona treatments, cold plasma treatments, ultraviolet (UV) treatment, laser treatments, x-ray treatments, electron-beam treatments, ion-beam treatments, and metallization and sputtering, in which corona, plasma, and laser treatments are the most commonly used methods to modify silicone polymers. In the presence of oxygen, high-energy-photon treatment induces the formation of radical sites at surfaces these sites then react with atmospheric oxygen forming oxygenated functions. 

Georgescu, N., Minea, R. and Lungu, C.P. (2005) Depollution experiments with repetitive pulsed corona plasmas, J. Optoel. Adv. Materials 7, 2501-7. 

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