Controlling the absorption dynamic of water-based ink into porous pigmented coating structures to enhance print performance

Previous work (Gane, P.A.C. and Ridgway, C.J., Advances in Printing Science and Technology Volume 27, 2001) has shown that the absorption of the fluid phase from a waterbased flexographic ink is affected by the polymer content in the fluid phase, filtercake formation within the contacting ink layer and the distribution of dissolved polymer between the filtercake and the coating structure. The study showed that enhanced setting rate could be obtained by controlling the surface area to porosity ratio of the printing surface. This paper now reports the effect on absorption dynamic and ink component holdout of introducing two different materials, each with a different form of porosity-creating structure, as constituents of consolidated tablets. These are (i) a high surface area highly porous precipitated silica, which under compression generates a structure with fine pores, and (ii) a novel relatively high surface area deformable calcium carbonate structure consisting of porous spherical aggregates covered with small plate-like crystals which consequently create a high permeation tortuosity. Both the materials had a median particle size of approximately 1 μm and were studied both in combination with standard dispersed ground calcium carbonate as well as on their own. A porous network model, PoreCor, was used to exemplify the important structural features of the porous structure controlling the inertial and viscous fluid uptake dynamics which in combination act to make broad pore size distribution structures containing fine pores absorb faster than more monosize distributions consisting predominantly of either fine or large pores. The centrifuged fluid phase of a flexographic ink was used as representative of a complex polymer-containing water-based dye ink which could be considered for standard flexography or as might be encountered in digital ink jet printing. Residual pigment in the centrifuged fluid phase was shown to be excluded from all the structures tested. The dissolved dyes readily enter the silica interstitial structures but not the novel carbonate structures, illustrating the effect of using high adsorptive surface area acting together with extended wetting front residence time (low permeability) to aid retention of dyestuff at the surface of a coating. This indicates that the novel carbonate can be used in paper coatings to give an enhanced print colour density along with a reduction in ink demand.