RHEOLOGICAL ANALYSIS OF HIGHLY PIGMENTED INKS - FLOCCULATION AT HIGH-TEMPERATURES

Continous flow and oscillatory measurement results of a series of high ly pigmented inks, containing a bimodal distribution of solid particle s and dissolved polymer in the ink vehicle, are presented. Viscosity v ersus 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 stre ss, viscosity taken at a shear stress of 250 Pa, and thixotropy, incre ase with temperature displaying an unusual behavior. It is assumed tha t a reinforcement of the cohesive interactions between components take s place on heating. Time, temperature, and frequency effects on dynami c viscoelastic functions are analyzed, confirming the existence of flo cculation due to interparticle attractions. The results are characteri zed by the following features: (i) Contrary to what is usually observe d in liquids, both storage and loss moduli increase with temperature. (ii) Both viscoelastic functions increase in a convex way with time, f ollowing an adapted Cheng-Evans model. (iii) The increase of viscoelas tic functions is not altered by periods of resting time. The experimen tal results indicate that at rest the dispersions flocculate to a grea ter extent at higher temperatures (typically at 40 and 60 degrees C) t han at 20 degrees C. To explain these results we assume that heteroflo cculation, through bridging of small particles, is promoted by phase s eparation of polymer solution forming the ink vehicle, which takes pla ce at 36 degrees C. (C) 1998 The Society of Rheology. [S0148-6055(98)0 0302-2].