Linear viscoelastic behavior of copper phthalocyanine dispersions used in printing inks

Abstract

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 microscopy 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 become increasingly shear thinning as the pigment volume fraction is increased. 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 ex-tent of the linear region decreases with increasing pigment concentration, at a higher frequency (10 Hz) the extent of the linear region increases with increasing pigment concentration. This increase in the Iinear region with increasing pigment concentration suggests that at higher frequencies the dispersion is less brittle, and that the rheological behavior is dominated by intra-aggregate associations. In addition, frequency sweeps show that the dispersions behave like a viscoelastic liquid at low pigment concentrations. However, at higher pigment concentrations (yet significantly lower than the maximum packing fraction) the dispersions behave like a cross-linking polymer at its gel point.

Rheology of Concentrated Dispersions of Copper Phthalocyanine Pigments in Organic Solvents

“…This shrinkage may be the cause of the slip shown by Figures 5 and 6. The sample at the lower polymer (Z-39) concentration showed no such time dependence, presumably because it had reached an equilibrium structure and smaller floc volume (as found in the dilute systems 25 ) and because the fewer interfloc contacts contributed to the lower G′ and G′′ of the dispersion.…”

“…No time dependence of the rheological parameters was noticed under these conditions, indicating that slip at the plate walls was not a problem. In addition, measurements 25 on similar systems using a parallel plate geometry with different gaps showed no evidence of slip. However, if, after loading, the samples were subject to steady preshear (at 100 Pa stress for 1 min), G′ and G′′ were found to be time dependent.…”

“…21,22 The a-and b-forms of CuPc are the only polymorphs widely used, 22 though other forms, such as d-, e-, and p-, have been generally described as distorted a-forms. 23,24 The rheology of dispersions of CuPc pigment in solvents containing polymers has been reported, 25 showing that with increasing pigment concen- tration both storage and loss modulii increased, whereas tan δ decreased. In a previous paper, 26 the colloid stability of dilute dispersions (0.3 w/v %) of two CuPc pigments (where the interparticle separation was several microns) in organic solvents was reported, showing electrostatic repulsion to be the main factor contributing to stability, even in low dielectric solvents.…”

“…The rheology of organic pigments dispersed in organic solvents is of importance to paints and inks, though much less work has been reported for these systems than for aqueous solvents. Copper phthalocyanine (CuPc) pigments are the most ubiquitous and commercially valuable class of organic pigments. , The a- and b- forms of CuPc are the only polymorphs widely used, though other forms, such as d-, e-, and p-, have been generally described as distorted a-forms. , The rheology of dispersions of CuPc pigment in solvents containing polymers has been reported, showing that with increasing pigment concentration both storage and loss modulii increased, whereas tan δ decreased.…”

Correlation of ink viscosity and printability in offset lithography process on paperboard used in packaging

“…Printing is the application of a thin film of a dye or pigment dispersion in liquid with desired viscosity to paper or polymer film [4]. As the concentration of ink increases, the flocculated nature also increases that results into high shear thinning which occurs due to interparticle attraction [5]. Inks with higher concentration of pigments are short inks having shorter ink filament size and possess higher yield value.…”