The role of ink component diffusion during absorption into inkjet coatings

Inkjet coatings usually contain specialty pigments with a high specific surface area and binders which provide high binding force. The final coating structure has a high pore volume and it contains a high frequency of fine diameter pores together with large pore volume capacity. During the inkjet ink absorption process, the capillary and permeation flow drives the liquid into the coating structure and controls volume flow, respectively. However, the role of diffusion within the timescale of the inkjet ink imbibition process has not been studied widely. The aim of this work is to clarify the interactive role of the porous structure and the coexistent swelling of binder during the liquid imbibition, with special attention to the fixation of dye and its distribution. The impact of polarity of the vehicle was studied using compressed pigment and formulation tablets, recording the liquid imbibition dynamic. Our results show that at the beginning of liquid uptake, at times less than 2 s, the small pores play the dominant role in respect to the ink vehicle imbibition. Simultaneously, water molecules on contact diffuse into and within hydrophilic binder, exemplified by polyvinyl alcohol, causing binder swelling. The swelling affects the number of active small pores and reduces the diameter of remaining pores, slowing the capillary flow as a function of time. At longer timescale, therefore, drainage by permeation flow wins over the capillary driven liquid imbibition. In the fixing of dye molecules, the electrostatic interactions are dominant. The type and modification of binder can affect the chemical groups to which the colorants can fix. On the other hand, the binder properties influence where the dye can physically go and thus where the colorants will finally locate. Polymer swelling acts to open the polymer network to colorant diffusion, such that the colorants can also transfer into the polymer network as a secondary process.