Rheological Studies of Emulsion Ink Stability

Rheological measurements of emulsion inks were done using a stress-controlled rheometer with a cone and plate geometry and a rate-controlled rheometer with a concentric cylinder geometry. The results from both experiments were consistent. The emulsion inks can be classified into stable, metastable, and unstable emulsions according to the shape of flow curves for shear stresses of up to a few thousand dynes per square centimeter. Stable emulsion inks are characterized by flow curves that are very similar to those of fresh inks without added fountain solution. These curves are typical of pseudo-plastic systems and feature a yield value at zero shear rate and a gradual decrease in the viscosity with increasing shear rate. Stable emulsion inks are characterized by flow curves that are very similar to those of fresh inks without added fountain solution. These curves are typical of pseudo-plastic systems and feature a yield value at zero shear rate and a gradual decrease in the viscosity with increasing shear rate. Flow curves of metastable emulsions become irregular at higher shear rates where the viscosity changes randomly with increasing shear rate. These observations are consistent with the assumption of incipient water separation and a concurrent reformation of emulsion. Unstable emulsion inks are characterized by gross water separation. The apparent loss of viscosity results in the shear rate jumping to infinity at a certain shear stress with the stress-controlled rheometer. Correspondingly, the shear stress drops to zero as the applied shear rate exceeds a certain value with the rate-controlled rheometer. These emulsions are believed to undergo shear induced water separation, and slippage between the moving surfaces takes place solely in the newly formed water phase. The stability characteristics of emulsion inks were found to be dependent upon the physico-chemical nature of the ink and fountain solution the composition of emulsion ink, and the temperature. A given system may behave as a metastable or a stable emulsion depending upon the shear history of the sample. These factors will be discussed in this paper.