Nanosize silica, titania and alumina metal oxide particles are common xerographic toner additives for control of flow, charge, development and transfer. Typically the oxides are rendered hydrophobic by reaction of the surface hydroxyl groups with organosilanes. This treatment leads to improved flow and charging properties but also typically gives unacceptably long admix charging times. It is shown that admix times on alumina and titania can be dramatically reduced by adsorption of amines such as triethylamine. The most acidic hydroxyl groups of alumina and titania undergo a proton transfer to form a salt with the triethylamine: it is this species that gives rise to improved admix. In effect, the formation of the quaternary salt on the surface mimics the performance of traditional ionic charge control agents. Alternatively, the same decrease on admix can be achieved by incorporating covalently bound sulfate species on the oxide surface. The sulfate groups increase the ionic character through an enhancement of the Bronsted acidity of the oxide. Only a relatively few hydroxyl groups need to be converted to an ionic form (either by amine addition or sulfate incorporation) to dramatically improve admix. A majority of the hydroxyl groups remain unaffected. These remaining groups can then be reacted with an organosilane to yield an oxide with increased negative charge and improved flow. The decrease in admix time with amine pretreatment is independent of the effects of subsequent hydrophobic treatments. In this case, the shorter admix times generated by the quaternary amine salt is retained.
Richard P. N. Veregin, Maria N. V. McDougall, Carl P. Tripp, "Chemical Surface Modification of Alumina and Titania for Improved Admix of Xerographic Toners" in Journal of Imaging Science and Technology, 2001, pp 174 - 178, https://doi.org/10.2352/J.ImagingSci.Technol.2001.45.2.art00009