The Effect of Carrier Properties on the Admix Performance of a Xerographic Developer

Abstract

During xerographic development, charged toner particles are removed from a xerographic developer, and to balance this loss an equivalent amount of fresh toner is dispensed into the developer. Since uncharged or poorlycharged toner particles tend to develop in the non-image “background” areas of a latent xerographic image, the dispensed toner particles must be rapidly brought to a charged state in order to avoid “background” development. Normally, the “added” toner and the “incumbent” toner (i.e., the charged toner particles already present in the developer) merge to a common level of charge, and the rate at which this merging process occurs (the so-called “admix rate”) is an important functional property of any xerographic developer design, with a rapid rate being especially desirable.In principle, the polarity, charge level, and the charge admix performance of any toner are functions of the chemical composition of the toner particles and of the xerographic carrier particles. However, even for a single, apparently optimized xerographic toner/developer design, charge admix performance may also vary as a result of extrinsic factors. For example, for some developer designs, the admix rate can vary from acceptably fast to effectively zero under certain conditions; paradoxically, this latter admix failure mode actually occurs as a result of an ultrarapid admixing process. In such a case, the added toner acquires a level of charge higher than that of the incumbent toner, and this increased charge is mirrored by an equivalent decrease in charge for the incumbent toner. In an extreme failure condition, the populations of “added” and “incumbent” toner particles scarcely show any tendency to merge to a common intermediate level of charge, and the developer “stabilizes” with the carrier beads in charge equilibrium with coexisting high-charged added toner particles and low-charged incumbent toner particles.For a single developer, the charging difference between the “aged” incumbent toner and the “fresh” added toner is the root cause for the above process, and normally such toner differences are reflected in toner charge values that decline with developer mixing time. However, in any two component developer, toner charge is also a function of carrier charging properties, and to illustrate the effect of carrier charge properties on toner charge and on toner charge admix, the present report details experimental data for a single “poor-admixing” toner paired with a range of carriers created by mixtures of “medium” and “high” charging carriers. Although the carrier changes created systematic changes in the charge level of the test toner, the toner admix performance remained as non-merging in all cases, indicating that this form of admix failure is indeed a function only of intrinsic toner properties. For the test developers, carrier mixtures designed to produce a high level of toner charge also produced a high level of overall charge stability as a function of developer mixing, and from normal charge-to-mass measurements such a result might be assumed to reflect a lack of carrier and toner aging. However, for the test toner, the subsequent admix performance remained as non-merging — apparently, for the high-charging developers, the mixing-induced declines in toner charging properties were coincidentally balanced by mixing-induced increases in carrier charging properties.