The comparative dynamics of bulk liquid flow and interpolymer diffusion during inkjet ink imbibition in porous coating structures: Dissertation

The focus of this thesis is to establish the timescale of interactions, physical and chemical, during dye-based inkjet ink imbibition into calcium carbonate (CaCO3) pigmented coatings. Comparison is made between conventional offset quality CaCO3, and special inkjet qualities in the form of either modified or precipitated CaCO3 combined with swelling diffusion driving or non-swelling diffusion-inert binder. The selection of pigment is based on the control of pore volume, pore size distribution and connectivity of the coating layer. Pigments with nano-size pores (intra-particle) are primarily exemplified. The final coating layers display discrete pore size bimodality in relation to the intra-particle and inter-particle pores. Polyvinyl alcohol (PVOH) is used as the diffusion sensitive binder and styrene acrylate latex (SA) as the bulk diffusion-inert binder. By changing the coating structures and using the contrasting binders, the roles of liquid diffusion, capillary pressure and permeation flow are clarified both in the short and long timescale imbibition. The wetting force within the finest capillaries drives the ink into the porous structure, whilst the viscous drag within the structure resists the movement. The nano-size capillaries initiate absorption of the ink vehicle, though typical impact pressure of an inkjet droplet is shown to provide forced wetting. During the flow, the hydrophilic binder swells, acting to close the smallest pores and reduce the remaining pore diameters. The total pore volume decrease competes with the initial capillarity. The diffusion is shown to have a marked effect on the polar liquid absorption rate into the PVOH-containing coatings over different timescales. The swelling opens the polymer matrix so that the colorant fits into the binder structure and can either hydrogen bond or become mechanically trapped. The diffusion coefficient of water in PVOH and on SA latex films is shown to be similar, despite the difference in geometry. Colorant fixing is enhanced mainly by the ionic interaction between the colorant and surface, and there is an optimal rate beyond which the colorant has insufficient time to translate under the Coulombic attraction toward the cationic adsorption sites or to respond to the binder matrix diffusion potential. The competing mechanisms of liquid flow and ab/adsorption are seen as crucial to developing a high quality print. High speed inkjet print density depends on the colorant location in the structure and the optical properties of the whole coated paper. Intercolour bleeding is also dependent on the coating absorption capacity at an adequately high rate in competition with colorant spreading.