Study of Ink Structure by Creep Technique

Creep is a technique of studying the change in deformation of a fluid with time when a constant shear stress is applied to the sample. This technique is very useful for studying stress- controlled rheological phenomena in inks such as pigment sedimentation, ink leveling, and sagging, and ink feed mechanism by open fountain systems. It can also be used to characterize the structure of a viscoelastic fluid. As stress is applied to a viscoelastic fluid, the deformation or strain initially increases very rapidly with time and gradually approaches asymptotically to a linear region. It was observed in this work that a typical Japanese newsink exhibits a critical strain below which a normal creep is obtained. Abnormal creep behavior occurs as the strain becomes larger than the critical strain. The creep curve departs upward from the steady, linear region and eventually approaches another linear asymptote. This infers that a structural change has occurred during the deformation process. For most American newsinks, however, a related structural change takes place almost as soon as the stress is applied. These results indicate that the characteristic internal structure of Japanese inks is quite different from that of American inks. Structural differences and their effect on rheological printing inks are discussed in this paper.