Offset ink concentration profile during vehicle imbibition into coated paper, Part 3: proposing a viscous gradient model.

Application: This study contributes to the understanding of ink-paper interactions, and in particular provides a model for the setting of offset paste inks during the vehicle absorption phase.

The individual force-time integrals of the ISIT device under constant separation acceleration are known as the ink tack pull-off curves during tack rise. We used the integrals to obtain data about the state of ink as a function of elapsed time after paper contact. We derive a predicted volume loss of fluid from the ink on a coated paper sample. Intrinsic ink/rubber blanket adhesion is assumed to be a continuous boundary condition. However, the "sealing" effect at the substrate surface, which is dependent on ink amount, roughness, and ink mobility, strongly influences the likelihood of cavitation. Cavitation, if present, contributes to the observed tack force, and occurs especially on very smooth and nonabsorbent substrates. In comparing the tack results with a model previously proposed by Xiang and Bousfield in 1998, which adopts a geometry of parallel plate separation in a fluid medium, we found a considerable mismatch. We develop a new relation between the viscous character of the ink at a given concentration on the substrate, which is effectively the ink-on-paper viscosity and solids content, and the tack force measurement. better agreement results from using an analysis of the tack force-time integrals, footprint area, and a uniaxial extensional viscous term from which an improved model of the tack-force mechanism is presented. This novel approach allows analysis of the solids and viscous parameters of the ink as the initial ink setting actually occurs on real papers.

Schoelkopf is senior scientist and Gane is vice president, research and development, paper and pigment systems, OMYA AG, Oftringen, Switzerland. Email Schoelkopf at [e-mail].