The chromatographic separation of anionic dye in inkjet coating structures

During the ink setting process on coated inkjet paper, the porosity and pore size distribution of the coating structure determines how much, in which direction and at what speed the surface can absorb the ink. The capillary and permeation flow drives the liquid into the structure and controls the volume flow, respectively. The coating binder, quite often polyvinyl alcohol, has an effect on the inkjet ink imbibition by occupying a proportion of the available pore volume and absorbing via swelling of the polymer matrix. The aim of this work is to clarify the role of the porous structure and the coexistent swelling of binder during the liquid imbibition, with special attention paid to the fixation of dye and its distribution during the chromatographic separation process. The results confirm that water molecules diffuse into and within hydrophilic polyvinyl alcohol binder causing the binder swelling. The swelling affects the number of active small pores remaining available for capillarity, by reducing the diameter and volume of the remaining free pores, thus slowing the capillary flow, such that the permeation flow rapidly becomes the rate determining step rather than the desired fine capillary-driven liquid imbibition. On the other hand, water molecules diffusing into the binder structure open the polymer network so that the colorant molecules can also fit into it. This mechanism is reflected in the observation that, in the case of dye-based inks, there is always a clear (non-coloured) wetting front advancing before the colour front. The swelling binder, therefore, though reducing absorption dynamic, does act to provide sorbtion volume and surface for colorant, aiding an otherwise coating surface area limited function in respect to dye capture and fixation.