Rheological analysis of highly pigmented inks: Flocculation at high temperatures

Continous flow and oscillatory measurement results of a series of highly pigmented inks, containing a bimodal distribution of solid particles and dissolved polymer in the ink vehicle, are presented. Viscosity versus shear rate data is adjusted to a "Bingham Exponential Decrease" model, showing that the viscosity at high shear rates decreases with temperature following an Arrhenius-like equation. However, yield stress, viscosity taken at a shear stress of 250 Pa, and thixotropy, increase with temperature displaying an unusual behavior. It is assumed that a reinforcement of the cohesive interactions between components takes place on heating. Time, temperature, and frequency effects on dynamic viscoelastic functions are analyzed, confirming the existence of flocculation due to interparticle attractions. The results are characterized by the following features: (i) Contrary to what is usually observed in liquids, both storage and loss moduli increase with temperature. (ii) Both viscoelastic functions increase in a convex way with time, following an adapted Cheng-Evans model. (iii) The increase of viscoelastic functions is not altered by periods of resting time. The experimental results indicate that at rest the dispersions flocculate to a greater extent at higher temperatures (typically at 40 and 60 degrees C) than at 20 degrees C. To explain these results we assume that heteroflocculation, through bridging of small particles, is promoted by phase separation of polymer solution forming the ink vehicle, which takes place at 36 degrees C. (C) 1998 The Society of Rheology. [S0148-6055(98)00302-2].