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Summary:
The rheology of lithographic printing inks containing emulsified fountain solutions was studied with a stress-controlled rheometer using a cone-and-plate sensor system. An unusually rapid increase in the rate of shear was observed as the applied shear stress approached a critical value. A hypothesis presented applies to this abnormal behavior. The critical shear stress value and the shape of shear stress versus shear rate curve of emulsion inks varied with water content and with temperature. These also varied with fountain solution and ink composition. Rheological measurements of this kind provide a useful technique for studying the shear stability of fountain solution emulsified in lithographic ink. Implications of these emulsion ink shear stability factors are discussed relative to press performance.