Corona process

Corona process Method for surface pretreatment of plastics, based on the entrapment of reactive atoms from the gas phase in the surface of plastics by high-voltage discharge. 

Certain surfaces require some form of pretreatment prior to printing. Foil usually requires primer wash, and polyethylenes and polypropylenes need to have the surface oxidised. The corona process is almost invariably used for films and flame treatment for bottles. Whether a surface has received treatment or not can be detected by immersing it in water and observing whether or not the water runs off. An oxidised surface has a lower wetting angle. If the surface is not printed soon after treatment another treatment may be necessary. Inks will not key onto non-oxidised PE and PP and will be removed when a self-adhesive tape test is employed. 

Figure 11-74. Electron beam and pulsed corona processing of 100 ppm CH2CI2 in dry airat25°C.

As shown in Table 20.1, corona discharges are dominated by 0-atom reactions involving, for example, atomic oxygen and ozone that lead to scission of the PP chains and the formation of water-soluble LMWOM. The extensive chain scission associated with corona processes affects the wettability and stability of the treated-PP films. By contrast, as shown in Table 20.1, flame-surface oxidations are dominated by OH radical reactions that lead to oxidation without LMWOM formation. The more limited chain scission associated with flame treatment leads to formation of more polar groups on the PP surface. Due to the lack of LMWOM formation, it was concluded the flame-treated PP is more highly oxidized after washing with water, more wettable, and more stable than corona-treated PP [103]. 

Gas, or chemical, corona treatment is electrically similar to corona processes that ionize a process gas other than air, such as nitrogen, to increase the surface tension of primarily non-porous substrates. Because of the potentially lower breakdown voltage of substituted process gases, gas corona treating systems can operate at an electrical voltage much less than 10 kV. 

Allows for treatment of difficult-to-treat surfaces. Atmospheric plasma treatment is a viable alternative for a variety of substrates for which corona treating is ineffective. For example, fluoropolymer-based materials, such as Teflon , do not respond to the corona process, but will respond to atmospheric plasma treatment. 

The various stages of this process depend critically on the type of gas, its pressure, and the configuration of the electrodes. (Their distance apart and their shapes control the size and shape of the applied electric field.) By controlling the various parameters, the discharge can be made to operate as a corona, a plasma, or an arc at atmospheric pressure. All three discharges can be used as ion sources in mass spectrometry. 

All three types of discharge involve the formation of ions as part of the process. For various reasons, most of the ions are positive. The ions can be examined by mass spectrometry. If small amounts of a sample substance are introduced into a corona or plasma or arc, ions are formed by the electrons present in the discharge or by collision with ions of the discharge gas. 

Opera.tingPrinciples. No collection will occur in a precipitator, and no current will flow in the secondary circuit, until gas ionization starts around the discharge electrode. This process is known as corona formation. The starting voltage for corona in air in a tube precipitator is given as... 

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