The triboelectric series

Haenen (1975) found that when the decay of surface charge was not exponential, the time constant was not a good measure of the actual decay time. In the case of a bed of particles, charge transport can take place between particles when one particle contacts another in the presence of an electric (self) field. For an isolated spherical particle of uniform surface resistivity y placed in an electric field, the relaxation time constant x for adjustment of charge over the surface of the particle was given by as Johnson and Melcher (1975) 

They calculate a range of x values (10 3 — 100 s) corresponding to increasing relative humidity 0— 100% with sand particles (d=0.5 mm, Kp=3.3) and assumed values of the contact cap radius (r/a= 10 and 104). 

An alternative expression for the charge relaxation time can be approximated for a multiphase system by an extension of continuum theory of solids. Applying conservation of charge V-J + dpqldt = 0, the equation of Poisson 

Masuda et. al. (1995) employ this equation to infer the time constant for a packed beds of polymer powder. 

It is natural to extend Equation 3-20 to an isotropic-homogenous multiphase system using an appropriate mixture resistivity and permittivity such as Equations 3.7 and 3.15 as 

---

In facilities that handle or manufacture sensitive electronic devices, static control is a primary concern. The two important aspects of static control are control of static on personnel and control of static in the facility. Both these issues are part of a composite static control strategy. Static control in personnel starts with attention to the clothing and shoes worn by people working in the environment. Use of cotton clothing is essential, as cotton is neutral in the triboelectric series. Leather-soled shoes are preferred to shoes with composite or crepe soles. Shoe straps made of semiconductive material can be wrapped around a person s ankles and attached to... 

Tribocharging is the process of electricity generation when two different materials rub against each other. Some materials easily give up or accept electrons from other materials imder friction. The tri-boelectric series lists materials that give up electrons in order from easiest to hardest. For many materials, the dielectric constant determines the position of that material in the triboelectric series. The further apart the two materials are that rub against each other, the more charge is transferred. 

To take advantage of this process for powder coating, a spray gun can be constructed without a high-voltage power supply. For most coating materials, the ideal material for gun construction would be PTFE since it is positioned at one extreme of the triboelectric series. However, for fluorpolymers, a gun constucted of nylon is best. Nylon is the engineering plastic that is furthest away from PTFE in the triboelectric series, so it provides the best charging of a fluoropolymer powder. The interior of the powder gun is constructed of nylon such that there... 

Methods for determining the position of a fabric in the triboelectric series are based on determining the charge that it acquires by friction with another fabric that is taken as the standard. 

Mixtures of marble with quartz, coal with sand, and clay with coal may be separated in this way. The separation process takes from a few seconds to several minutes. The quality of the separation depends on the material and the shape of the charged surfaces, as well as on the temperature and humidity, i.e., on just those factors which determine the adhesion of the powder to the surface [412], In choosing the material of the particles used as insulators we may make use of the triboelectric series. 

The further apart the carrier and developer materials stand in the triboelectric series, the greater is the charge which they acquire, and hence the greater is the charge density a 2. 

The weak electrostatic contribution is illustrated with one of the typical examples of a triboelectric pair. Polymers as well as other materials can be classified into a triboelectric series, i.e. a list in which the sign of the charge developed on a given surface by contact with another can be predicted from the relative positions of the two materials in the series (Fig. 11) . If we pick a triboelectric pair from the two extreme positions of the series, we should expect a strong electrostatic attraction between the pair. For example, nylon and teflon are the two extremes on the triboelectric series, and if electrostatic attraction is the major resistance, we should observe a high sliding friction between these two polymers. 

Materials differ in the energy required to remove an electron from the sphere of influence of its nucleus. This can be quantified based on a work function, which is the mini-mmn energy that must be given to an electron to hberate it from the surface of a particular material. The work function of different materials is the basis of a material s position in the triboelectric series (Table 1.1). 

---