Interactions Between Flexographic Ink and Porous Coating Structures, PaperCon '09 Conference

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Previous work has shown that flexographic printing is largely controlled by the coating layer design, i.e. smoothness alone is not sufficient for good printability. The contact angle between ink and the coating surface, and the absorption speed of the liquid need to be controlled. In the work reported here, the absorption rate of flexographic ink fluids into porous pigmented structures has been measured in order to elucidate these previous application observations. Correlation is extended from the idealised homogeneity of well-characterised fluids to the removal of the fluid phase from a water-based flexo ink formulation, containing additives, such as surfactant polymers. The fluid phase is removed by the absorptive forces of the porous coating network acting against potential surface retardation structures forming at the interface between progressively concentrating ink and the porous medium. The absorption rate from the ink, however, is found to be faster than that of extracted fluid phase alone. This is interpreted as an obstructing effect by polymers, contained within the extracted fluid phase of the ink, blocking the initial high rate absorbing fine pores. Retention of these polymers in the concentrating ink filtercake on the other hand acts in contrast as an imbibition “pump” keeping the porous structure free from their blocking action. This effect has been incorporated into a filtercake model by modifying the proposal of Xiang and Bousfield. Counter to the case of offset inks, where a permeability decrease is predicted, the reduced polymer drag in flexo ink can be accounted for by an effective entrapment factor for the polymer within the ink, expressed in terms of a Darcy permeability increase. Analysis using mercury porosimetry on the ink filtercake structure provides information on the proportion of the immobilised ink pore volume fraction which contains the polymer. The solids volume fraction of the ink filtercake was found to match the sterically stabilised maximum volume fraction for immobilisation.