| Non-aqueous ink jet inks, suspensions in a nonpolar solvent, and their long-time stability at different temperatures were studied using single-particle optical sensing (SPOS), dynamic light scattering (DLS), and viscometry. Samples ground for different periods and having different amounts of free polymer and different amphiphilic polymer additives used for steric stabilization were prepared.; SPOS characterizes the particle size distribution at the high end of the particle diameter (>0.5 mum). For two formulations of the inks tested, the longer the grinding time, the fewer the large particles that may clog the print heads. In the aging at different temperatures, the high end of the size distribution exhibited changes ascribed to sedimentation and aggregation. In the absence of aggregation, the decrease of the particle volume was identified as originating from sedimentation; the dependence on particle diameter and viscosity agreed with theoretical prediction. The aggregation appeared as a hump in the tail of the main peak. The hump was more clearly observed for inks aged at a higher temperature, having more free polymers, and having more loosely adsorbed polymer additives.; DLS studies verified that the mean particle diameters are smaller than the lower limit of the detection range of SPOS. The studies revealed strong repulsive interactions between particles, which were ascribed to nearly bare Coulomb interactions between charged ink particles in a medium of a low dielectric constant and a low density of counter charges.; The rheology of the fresh inks showed a shear thinning and rheopectic behavior. The increase in viscosity with aging time, which indicates a buildup of the flocculated structure with time, was observed at elevated temperatures. The results of viscometry agree with the findings in the SPOS studies.; Changing the hydrophilicity of the amphiphilic polymer additives had a drastic effect on the particle size distribution and stability. The right degree of hydrophilicity, generally accepted for use as emulsifier in water-in-oil emulsions, brought the stability to the inks. |
