Charge control agents (CCA's) are frequently added to toners to create a desired charge level and polarity. Triboelectric charging tests on model, CCA-surface-treated toners and carriers indicate that CCA transfer between toner particles and carrier beads can be a controlling factor for the magnitude and polarity of toner charge — in extreme cases, toner polarity can even be reversed as a result of CCA transfer. The tests also indicate that the transfer process (and hence the final charging properties of a toner) can be significantly affected by the concentration of the toner in a xerographic developer.
Robert J. Nash, Michael L. Grande, Richard N. Muller, "CCA Effects on the Triboelectric Charging Properties of a Two-Component Xerographic Developer" in Proc. IS&T Int'l Conf. on Digital Printing Technologies (NIP17), 2001, pp 358 - 364, https://doi.org/10.2352/ISSN.2169-4451.2001.17.1.art00079_1