ON THE STABILIZATION OF CALCIUM CARBONATE SLURRIES FOR HIGH pH AND ION CONTENT COATING COLOURS, PaperCon 2014

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Achieving ink pigment holdout in digital inkjet printing is typically achieved by providing readily soluble multivalent cations already applied to or contained in the substrate surface and/or at raised pH. In the case of paper and board coatings, the coating colour is thus required to carry dissociated multivalent polymeric or inorganic salts/hydroxides, which are effectively primed to induce the ink pigment flocculation. A dispersed coating pigment slurry, based on the typical strongly surface charged dispersion stabilisation, be it anionic or cationic, cannot remain dispersed in the presence of such high pH and/or additional ionic content, but responds with a strong flocculation and rise in viscosity of the coating colour beyond the limits of flowability and coater runnability. We have investigated alternative stabilising systems, and found that by reducing or eliminating the charge density on the surface of coating dispersion components it is possible to move toward partial steric hindrance acting against salt-induced flocculation. By doing so, we have demonstrated the ability to produce pigment slurries able to withstand the extreme process conditions required today. The principle is exemplified by showing coating colour stability against the addition of CaCl2 retaining the previously good runnability and flowability, while still increasing both optical density and reducing the mottling of colour pigmented inkjet inks.