LINEAR VISCOELASTIC BEHAVIOR OF COPPER PHTHALOCYANINE DISPERSIONS USED IN PRINTING INKS

In this paper we describe the linear viscoelastic properties of copper phthalocyanine (CuPCN) dispersions that are used in the manufacturing of offset lithographic printing inks. Transmission electron microscop y shows that the primary pigment particles are rod-like and have sizes in the range of 10 to 300 nm. Steady shear measurements show that the dispersions are Newtonian at a pigment volume fraction of 0.073 and b ecome increasingly shear thinning as the pigment volume fraction is in creased. The strong shear-thinning nature of these dispersions can be attributed to the highly flocculated nature of the dispersions, which is due to interparticle attractions. The structural complexity of the dispersions also results in an unexpected linear viscoelastic response . While at low frequencies (0.1 and 1.0 Hz) the extent of the linear r egion decreases with increasing pigment concentration, at a higher fre quency (10 Hz) the extent of the linear region increases with increasi ng pigment concentration. This increase in the linear region with incr easing pigment concentration suggests that at higher frequencies the d ispersion is less brittle, and that the rheological behavior is domina ted by intra-aggregate associations. In addition, frequency sweeps sho w that the dispersions behave like a viscoelastic liquid at low pigmen t concentrations. However, at higher pigment concentrations (yet signi ficantly lower than the maximum packing fraction) the dispersions beha ve like a cross-linking polymer at its gel point.