Charge generation

For materials which are poor conductors of electricity (insulators), as are most textiles and polymers, the causes of charging are very complex. In good conductors, the charging is largely electronic in nature, but the surface oftextiles is usually contaminated with additives, finishes, dirt and moisture, in all of which resides an abundance of ions (Wilson, 1987). In this case, charging may comprise electrons, ions and charged particles of the bulk materials, or any combination of these (Taylor and Secker, 1994). They also stated that because there might be little information on the ionic population of surfaces before contact is made, it may not be possible to predict the magnitude of the transferred charge.

Charges may be generated between a non-conductor and a conductor by induction. Consider a negatively charged rubber rod (non-conductor) brought near a neutral (uncharged) conducting sphere insulated from ground. The region of the sphere nearest the negatively charged rod will obtain an excess of positive charge, while the region of the sphere farthest from the rod will obtain an equal excess of negative charge. If the sphere is grounded, some of the electrons will be conducted to earth. When the grounding connection is removed, the sphere will contain an excess of induced positive charge (Haase, 1977).

Charging that occurs when two solids come into contact has been referred to as contact charging, fractional charging, tribo-electric charging and tribo-electrification. Usually, but by no means exclusively, contact charging is used to describe simple contacts between surfaces (i.e., contacts in which no sliding or rubbing occurs between the contacting surfaces). Thus, static electricity is generated when almost any pair of surfaces is separated, unbalancing the molecular configuration in the case of relatively non-conductive materials (Sello and Stevens, 1983). Henry (1953) reported that when the two surfaces are separated, either with or without obvious rubbing, charged particles are found to have crossed the boundary, with the usual result that the two surfaces have gained equal and opposite charges.

Although rubbing is not necessary for charge generation, it usually increases the amount of charge produced. 'Triboelectrification' is the term that applies when an electrical charge is generated on a body by frictional forces and is probably the major mechanism for the generation of electrostatic charge in textile materials (Wilson and Cavanaugh, 1972). Experiments have shown that, when an insulating surface is rubbed either by a conductor or another insulator, charge transfer may be several orders of magnitude greater than in a simple touching contact. This may be rationalized by noting that rubbing increases the intimacy of the contacting surfaces. Unless the electrical states of the two materials are extremely well balanced, there will be a large transfer of charge when their surfaces are brought into contact (Morton and Hearle, 1975).

Hersh and Montgomery (1955) found that the manner of rubbing, the length of material rubbed, the normal force, and rubbing velocity (in some cases) affected the amount of charge generated. Haenan (1976) showed that charge transfer increased with rubbing pressure, and Coste and Pechery (1981) reported that charge transfer was greatest when surface roughness was small. Some researchers (Montgomery et al., 1961; Zimmer, 1970; Ohara, 1979) have found some instances where the charging goes through a maximum value. Such effect has been related to local temperature gradients appearing across the contact, resulting in the enhanced diffusion of electrons from the hotter to the cooler surface (Taylor and Secker, 1994).

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