EFFECTS OF SURFACE-AREA AND SURFACE MODIFICATION ON THE CHARGING AND HUMIDITY SENSITIVITY OF FUMED SILICAS IN TONERS

The effects of surface area and surface modification of amorphous fume d silicas on the charging properties of styrene-butadiene toner, again st polymer-coated metal beads, were studied systematically. Without ex ception, silica additives though used at very low concentration (0.5% by weight on the toner surface) dominated the charging of the resultin g toners. At 20% RH (relative humidity), our data showed that (1) all silicas impart negative charging in the styrene-butadiene toner, (2) h ydrophobic silicas charge more negative than hydrophilic silicas, (3) the charge level is primarily governed by the surface area of the sili ca, and (4) the rate of charge exchange as measured by the admix time increases as the hydrophobicity of the silica increases. The effect of RH on toner charging was also examined. Results indicated that chargi ng is universally sensitive to RH when silicas are used in toners. Spe cifically, tribe decreases as RH increases. Hydrophobic silicas were m ore resistant to RH changes, presumably because of their smaller moist ure uptake at high RH. Accompanying the decrease in tribe, a reduction in admix time was generally observed at high RH. Use of the knowledge gained in this work for future toner formulation is suggested.