In this work, stability of dispersions and foams containing CaCO3-based pigments and cellulose nanofibrils (CNF) was evaluated with the aim to reveal the mechanisms contributing to the overall stability of the selected systems. The utmost interest lies in the recently developed hydrocolloid hybrid CaCO3 pigments and their potential to form bionanocomposite structures when incorporated with CNF. These pigments possess a polyelectrolyte layer deposited on the surface of the particle which is expected to enhance the compatibility between inorganic and organic components. Stability assessment of both dispersions and foams was conducted using turbidity profile scanning. In dispersions, CNF provides stability due to its ability to form a firm percolation network. If surface-modified pigments are introduced, the favourable surface interactions between the pigments and CNF positively influence the stability behaviour and even large macro-size pigments do not interfere with the stability of either dispersions or foams. In foams, the stability can be enhanced due to the synergistic actions brought by CNF and particles with suitable size, shape and wetting characteristics resulting in a condition where the stability mechanism is defined by the formation of a continuous plateau border incorporating a CNF network which is able to trap the inorganic particles uniformly.

The porous network structure of a coating layer has a major effect on how quickly inkjet ink penetrates into the coated paper, and how large the pore volume is determines the capacity for ink volume uptake within the coating layer. If the penetration rate and/or pore volume are insufficient, the ink colors stay too long on the surface, resulting in undesirable mixing (intercolor bleeding) and trans-surface wicking (feathering). The aim of this work was to clarify whether it is possible to decrease the coating layer thickness of the specialty inkjet layer and still produce an inkjet printed surface using dye-based inks with low bleeding and, thereby, to define the reasons for limitations with respect to coat weight reduction. The online study of printed figures following printing nozzles on a high-speed inkjet printing press, by means of optical image capture, showed that the tendency for intercolor bleeding depends strongly on the coating layer thickness. As the printing speed increases, the pore network structure of the coating layer becomes increasingly important. The results show that, under the external pressure, aused by the surface tension and impact of the ink droplets themselves, the permeability of the coating layer dominates after at least 4 ms from the time of ink application. The coating pigment selection and the amount of poly(vinyl alcohol) binder did not influence this permeability onset time. Permeability allows the required ink volumes eventually to be absorbed, even if the total porosity of the coating is insufficient at low coat weight.

Flow properties of the calcite/poly(vinyl alcohol) (PVOH) system were studied and related to the microstructure of the suspension. Adsorption of PVOH on calcite was confirmed, and it results in a shift of the slipping plane out from the surface. The charge density at the surface is assumed to remain unchanged. Since the PVOH used is only partially hydrolyzed, the most likely adsorption conformation consists of residual acetate groups adsorbed to the surface and vinylalcohol groups extending outward from the surface as loops and tails. The microstructure and flow properties of the calcite/PVOH system was found to go through several different stages as a function of PVOH concentration. At low PVOH concentrations a gradual weakening of the initially formed floc network is observed as a function of PVOH concentration. Further addition of PVOH eventually leads to breakdown of the flocs which results in a sterically stabilized suspension with a very low viscosity. This state persists for a narrow concentration range of PVOH, and increasing the PVOH concentration over a certain limit leads to a second gradual increase in viscosity. The system is believed not to undergo reflocculation at high PVOH concentrations as judged from the nonelastic nature of the suspensions. Instead, the polymers form a viscous matrix in the solution while the particles remain well-dispersed. At high enough PVOH concentration, the free volume available for the particles is greatly reduced, and the viscosity increases sharply.

No abstract

No abstract