This work is aimed at characterising the chromatographic action of some typical paper coating pigments in relation to the separation and fractionation of offset printing ink components during absorption into the coating void structure, with particular emphasis given to the chemical and structural parameters of the pigments.
The separation phenomenon was studied experimentally using primarily large-scale model systems based on a modified thin-layer chromatography method. The separation of the ink constituents was detected directly from the absorption path using Fourier Transform Infra-Red (FTIR) microscopy. The coatings were characterised for their physical properties using mercury porosimetry, and the absorbing fluids for their viscosity and surface energy. The ink-coated paper contact on the realistic scale was evaluated using the concept of ink-on-paper tack development. Coating pigments used in the work included typical paper coating grade ground calcium carbonates, fine clays of different origins, as well as talc and precipitated calcium carbonate. The choice of pigments provided the possibility to investigate independently the chemical and physical aspects of pigment properties on the constituent fluid separation. Ink chemicals included representatives of the most typical offset ink components; namely mineral oil, linseed oil, ink resins and ink pigment. A blend of mineral and linseed oils was used as the main test fluid.
The differential interaction or adsorption-desorption on the pigments leads to separation of the mixture of mineral and vegetable (in this case linseed) oils as they are absorbed into the pigment coating structure, where the more polar linseed oil is preferentially retarded in the structure composed of the more polar pigments. The degree of the separation is directly proportional to the surface area of the pigment within a group of chemically and morphologically identical pigments. It is also affected by the surface chemistry of the pigment but more so in comparing dispersed and undispersed pigments, and polar versus non-polar rather than between the chemically different hydro-
philic dispersed pigments. Pigment morphology strongly influences the macroscopic flow behaviour due to changes in physical porous structure (pore size, porosity, tortuosity). The macroscopic flow behaviour, which is affected by all of the above mentioned structural and chemical parameters of both the solid and liquid phases, proved to be a universal parameter determining the separation efficiency of the oils by affecting the retention time. Added ink resins in the oil mix retards the overall absorption rate of the oils, but does not affect the separation tendency of the oils in most of the cases studied, except in cases where less polar pigments or dispersions are involved. Adding latex into the coating structure, on the other hand, creates an additional gradient for separation of the oils as oil diffusion into the latex provides further selectivity.