THE ROLE OF WATER IN THE TRIBOELECTRIC CHARGING OF ALKYLCHLOROSILANE-TREATED SILICAS AS TONER SURFACE ADDITIVES

Fumed silicas are commonly used to modify the charge and flow of toner s. However, Little work has been done to understand the origin of rela tive humidity (RH) sensitivity of toner charging with silica surface a dditives. In this work, toner triboelectric charging using a hydrophil ic silica and silicas modified by a surface-attached long-chain hydroc arbon or fluorocarbon silane was studied as a function of silica water content. The silica water content was measured by infrared spectrosco py. It is shown that the toner charging is controlled by the silica wa ter distribution. Water is necessary for charging of hydrophobic silic a; the charge increases with increasing water content up to 20% RH. Ab ove 20% RH, the charge decreases linearly with water content, and this transition at 20% RH coincides with a change in the water adsorption isotherm. In contrast, the charge of hydrophilic silica decreases with relative humidity. Linear extrapolation of the toner charge with all silicas reaches a common point of zero charge at about 2.4 +/- 0.4 mol ecules H2O/nm(2). The drop in charge with relative humidity is consist ent with a leaky dielectric model, where charge is dissipated by the f ormation of a contiguous mater layer on the silica. On the hydrophobic silicas, this transition to a contiguous water network occurs near 20 % RH.