Electrification process

Contact electrification. A second static electrification process is contact electrification. Here electrons migrate from clean, dry surfaces of dis-... 

We now consider the interface between carrier and toner, as these components interact in a dynamic development system. As we have noted, toner is commonly fabricated by "jet impacting" a brittle pigmented thermoplastic to form irregularly shaped particles of about 12/u m cross-section--particles whose surface comprises resin and carbon black (2, J 3, 58). The carrier cores are magnetic (ferrite or steel), typically about 250/4 m in size and therefore relatively massive. They are generally coated with a dielectric oxide or resin, which participates in the tribo-electrification process (2, it , 59). ... 

EXPERIMENTS ON THE ELECTRIFICATION PROCESS IN SOLID-SOLID CONTACTING... 

Table 1 tries to organize the four electrification mechanisms in terms of the factors observed to influence the electrification process in transformer oil (Crofts, 1986). 

Couette Mixer - To further investigate the physics of the electrification process in transformer oil/cellulosic systems, a compact Couette flow system of coaxial cylinders for electrification measurements is shown in Fig. 4, where the inner cylinder can rotate at speeds giving laminar and turbulent flows as well as the transition cellular convection regime (Melcher et al., 1986 Lyon, 1987). The cylinder walls on either side of the oil gap can be bare metal or can be covered with oil-impregnated paper used in transformers. 

Electrification is the process of producing an electric charge on an object. If the charge is confined to the object it is said to be electrostatic. The term static electricity refers to accumulated, immobile, electrical charges in contrast to charges in rapid flow, which is the subject of electrodynamics. 

Napier, D. H. and Russell, D. A. (1974) Proc. First Int. Sym. on Loss Prevention (Elsevier). Hazard assessment and critical parameters relating to static electrification in the process industries. 

It should be noted that the above phenomena all involve relative motion or charge segregation but no static electrification or true charge separation. The latter can only occur when the liquid is separated from the solid or is broken up. The above phenomena consequently can only create conditions that will permit charge separation. Any net charging process itself will, therefore, be very similar to that involved in liquid breakup except that the initial charge segregation will be influenced by double layers at the solid surface as well as by those at a liquid-gas interface. 

Guyton concludes that with nonconductors the charging process is due to particle electrification either upon impact or during passage through the... 

When solid particles are exposed to a high-temperature environment, typically for T > 1,000 K, charging by thermal electrification becomes important. The electrons inside the solid can acquire the energy from the high-temperature field and be freed by overcoming the energy barrier or the work function. By losing electrons in such a thermionic emission process, the particles are thermally electrified. 

Flame ionization. A final static electrification method is the ionization of particles in a flame. This effect was first observed as early as 1600, and it has recently become the subject of much interest because of potential application in such diverse areas as direct generation of electricity, control of combustion processes by applied electric fields, and the like (Lawton... 

During the first half of the nineteenth century one of the few significant researchers of static was Helmholtz (1821—1894), who introduced a comprehensive theory of electricity and also developed the contact potential theory of the electrification of insulators by contact. Also, P. E. Shaw showed the influence of chemical and physical conditions of surfaces on static generation. Many problems have arisen in recent decades in the textile, paper, and plastic industries because of the introduction of new insulating materials, particularly synthetic fibers and plastics, as well as increased processing speeds. This has led to an accelerated pace of research into the generation and discharge of static electricity in insulators. 

Most antistatic additives are polar waxes the alkane chain part of the molecule is attracted to the polymer, while the hydrophilic end attracts water. This moisture forms a thin conductive film on the surface of the plastic. A charge decay half time of 0.1 s or less provides adequate protection against static electrification. To achieve this, the surface resistivity must be less than 3 x 10 fl/square. Although surface films are worn away by abrasion, they are replenished by the additive slowly diffusing to the polymer surface. They will not function adequately when the relative humidity is less than 15% (not a problem in the UK ), and cannot be used for specialised polymers with melt processing temperatures exceeding 300 °C. The use of conducting fillers (see the next section) is a more permanent solution to static electrification. 

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