Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 171 of 331

Fig. 2 Tribo charging process. timate contact and subsequent separation of the powder particles from the gun walls. The word "tribo" comes from the Greek word tribein meaning to rub. The tribo charging process is illustrated in Figure 2. When two different materials are brought into contact, there will be a trans- fer of charge from one material to the other in order to eliminate the imbalance of charge. The magnitude and direction of the charge transfer depends on many factors including the chemical and electronic structure of both materials. Over the years, a lot of testing has been done contacting one material against anoth- er and measuring the resulting magnitude and polarity of the charge attained on each of the materials. The results of one such study, presented in the form of a so-called "triboelectric series," are shown in Table I. The farther two materials are away from each other in the series, the greater the triboelectric charge that should be produced when these two materials come in contact. Note that polytetraflu- oroethylene (PTFE) is shown at one end of the series as being the strongest elec- tron acceptor and nylon 6/6 is shown at the other end as being the strongest elec- 170

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