COATING IMBIBITION RATE STUDIES OF OFFSET INKS: a novel determination of ink-on-paper viscosity and solids concentration using the ink tack force-time integral

摘要

Comparison of structure relaxation of a presheared offset ink with observed ink tack development on both non-absorbent substrates of different smoothness and surface free energy, and on a representative absorbent multi-coated paper, determined as a function of time, is achieved using oscillatory and shear rheometry and the SeGan Ink Surface Interaction Tester (ISIT), respectively. By incorporating the extrapolated viscosity properties of the ink toward increasing concentration, a mutual relation is sought and developed to establish the viscosity character of the ink on the substrate such that ink-on-paper viscosity and solids content can be derived from the tack force measurement. The viscosity extrapolation is derived initially from viscosity data obtained under progressive dilution with mineral oil and then compared with the individual force-time integrals of the ISIT, constant acceleration, ink tack pull-off curves during tack rise (Gane et al, 1994) to obtain a predicted volume loss of fluid from the ink. The role of intrinsic ink/rubber blanket adhesion is assumed inherently as a continuous boundary condition but the "sealing" effect at the substrate surface, which is dependent on ink amount, roughness and ink mobility, strongly influences the evidence of cavitation, which contributes an important component of the observed tack force especially on very smooth and non-absorbent substrates. An earlier proposed model adopting a geometry of parallel plate separation in a fluid medium (Xiang and Bousfield, 1998) is compared with tack results and a considerable mismatch is found for the method under study. Better agreement is found using an analysis of the tack force-time integrals, footprint area and an uniaxial extensional viscous term from which an improved model of the tack-force mechanism is presented. This novel approach allows the solids and viscous parameters of the ink to be analysed as the initial ink setting actually occurs on real papers.