Offset ink tack and rheology correlation Part 2: determining in real time the solids content of ink-on-paper using the ink tack force-time integral.

Application: Controlling the dynamics of a preferred network absorption pathway in relation to the total pore volume is a major factor in controlling the press runnability of coated paper in relation to the development of tack.

We embarked on these studies having already laid the groundwork through experimentation and modelling to better understand the absorption of liquid into porous media, such as paper coatings. Two series of studies resulted. The first series is titled Imbibition Behavior Of Offset Inks; the second, more specialized series is titled Offset Ink Tack and Rheology Correlation.

Our challenge was to explain why an ink applied to a fine glossy coated paper begins to set much faster than when applied to a highly porous coating, such as a matte surface, containing coarse pigments. We questioned the maxim that porous structures can be adequately considered as a bundle of capillaries. We quickly realized that the capillary picture leads to the false assumption of continuity of flow and the established equilibrium between the wetting force and the retarding viscous force in the stricture of the capillary radius.

Realizing that pores interconnect and result in connections of different sizes and geometries was the key to finding a way forward. Equilibrium simply cannot exist in a network structure of pores. Each time a geometrical change is encountered the liquid is accelerated or decelerated over a vanishingly short timescale. The whole process inside a network structure is, in fact, a series of short timescale phenomena all joined together in a macroscopic observation and timescale via the permeability of the structure--this relation only looks like [square root of (t)] but the constants do not remain invariable as a function of pore size. We had to abandon the Lucas-Washburn equation. The model answered our question.

To bring all this into the real world we needed to consider what actually happens to an offset ink as it contacts a coating. Primarily, oils are removed, creating a progressive concentrating of the ink. Last month, Part 1 of the first series of papers Imbibition Behavior Of Offset Inks showed our experiments and findings when considering typical ink oils as they absorb into porous network structures. This month, we take the further step to reality in Part 2 by studying the actual rate of absorption from an offset ink.

Studying sample structures and pure oils still leaves a gap for some of our readers. What about thin layers applied to coatings on paper? We closed this gap by using the ISIT, which gives us a measure of ink tack as a function of time after the ink is applied to the coating surface. We discussed our ideas fully last month in Part 1 of our second paper series, Offset Ink Tack and Rheology Correlation. This was done by comparing with a diluted ink and extrapolating the resulting viscosity function back to higher concentration. This month we report our findings in Part 2, correlated with the experiment in Part 1, and show that the mass fraction of the ink in initial contact with the coating surface increases linearly with time. These results are perhaps the first ever to capture offset ink solids content during tackification on coated paper.

Gane is vice president, research and development, and Schoelkopf is senior scientist at OMYA AG, Oftringen, Switzerland, and Matthews is a reader in applied physical chemistry with the Environmental and Fluids Modelling Group, Department of Environmental Sciences, University of Plymouth, Devon, UK. Email Schoelkopf at [e-mail].