The rate of absorption of a flexographic ink fluid into a porous pigmented structure has been measured, based on the methods of Gane et al. (2000). Identification of a preferred network pathway for the absorption of low-density, low-viscosity fluids makes it possible to relate the short-term rate of absorption into a network structure of a coating layer to the observed rate of imbibition into compressed porous block samples of coating pigment. The work extends this correlation from the idealised homogeneity of well-characterised fluids to that of a fluid phase, consisting predominantly of water and additives such as surfactant polymers, derived from a commercial fiexographic ink. The ink fluid phase is removed by the absorptive forces of the porous coating network acting against potential surface retardation structures which form at the interface between progressively concentrating ink and the porous medium. The rate of absorption of fluid from the ink under supersource conditions is found to be faster than that of the extracted fluid phase alone. This is interpreted as indicating that polymers contained within the extracted fluid phase of the ink have an obstructing effect, blocking the initial highly absorbing fine pores. Retention of these polymers in the concentrating ink filtercake acts as an imbibition 'pump', keeping the porous structure free from their blocking action. This effect can be incorporated into a modified filtercake model (Xiang and Bousfield, 1998) so that, in contrast to the case of offset inks where a permeability decrease is predicted, the reduced polymer drag found in a flexographic ink can be accounted for by an effective entrapment factor for the polymer in the ink calculated in terms of a Darcy permeability increase. Mercury porosimetry studies of the ink filtercake structure provide information on the proportion of the immobilised ink pore volume fraction which contains the compressible polymer. The solids volume fraction of the filtercake was found to match the sterically stabilised maximum volume fraction for immobilisation.
展开▼
