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The comparative dynamics of bulk liquid flow and interpolymer diffusion during inkjet ink imbibition in porous coating structures
10 May 2012
Technical Research Centre of Finland (VTT)
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 (GCC), and special inkjet qualities in the form of either modified (MCC) or precipitated (PCC) 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 containing 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, respectively. 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 layer 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 coating layer capillaries drives the inkjet ink into the porous structure, whilst the viscous drag within the pore 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 subsequent 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, reducing absorption rate. 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 of the ink fits into the binder structure and can either hydrogen bond or become mechanically trapped there on drying. The diffusion coefficient of water in PVOH and on SA latex films is shown to be very similar, despite the difference between bulk diffusion and surface diffusion, respectively. The colorant fixing is enhanced mainly by the ionic interactions between the colorant and coating surface adsorption sites. The anionic colorant fixes under chromatographic separation to the cationic com-ponents of the coating layer. There is seen to be an optimal absorption rate be-yond which the colorant has insufficient time to translate under the Coulombic attraction potential toward the cationic adsorption sites or to respond to the binder interpolymer matrix diffusion potential. The final print density of a high-speed inkjet printed surface depends on the colorant location in the coating layer and the optical properties of the whole coated paper.
Therefore the competing mechanisms of liquid flow, ab- and adsorption are seen as crucial to developing a high quality print. The intercolour bleeding is additionally dependent on the coating layer capability to absorb enough ink at adequate high absorption rate in competition with colorant spreading on the surface. Spreading can be further curtailed using a cationic surface treatment.