Rheological Properties of Printing Inks

Abstract

This paper describes the rheology of printing inks. Several kinds of pigments-linseed oil suspensions and commercial inks were used as the samples.For many years flow requirements of printing inks for various types of printing processes have been determined by skilled workers through experience rather than through experiments. However, it is very important for advances in printing or coating engineering that technical skills obtained emperically are confirmed theoretically or by scientific methods. The ink on a printing machine undergo various types of shear strain and complex deformations and flow processes. The optimum printing conditions and printability of inks are closely associated with their rheological properties. Therefore, we investigated the non-linear viscoelastic phenomena of printing ink under complicated rheological processes such as stress relaxation under large deformation, stress growth and relaxation following initiation and cessation of steady flow, non-linear creep and creep recovery.The equilibrium modulus in the long time region was observed in the stress relaxation curve. This phenomenon may be attributed to the formation of temporary network structure due to the association of suspended particles. Shear stress development after sudden initiation of steady shear flow showed an overshoot and approached an equilibrium value. The transient phenomena seem to be caused by the break down of the structure of dispersed particles. Disperse systems of pigments such as printing inks show remarkable thixotropy, rheopexy, pseudo-plastic flow, and structural viscosity attributed to a denser network structure of suspended pigments. In order to provide new concepts for these phenomena, creep compliance and creep recovery were measured under the conditions of various flow histories. The elastic properties of the network structure of suspended particles can be evaluated also from the creep experiments. The yield stress can also be determined from the relationship between recoverable strain and shear stress under steady flow. The formation of the temporary network structure of suspended particles were also investigated by an acoustical technique.