Novel methods to characterize ink - Coating interactions, coating structure and surface energy

Abstract

The main focus of this thesis is to analyze capillary adsorption of heterogeneous porous paper coatings. The approach is to develop a novel Gibbs energy model, analyze the structure and surface energy parameters of pilot coated paper and board samples, and compare results to ink tack kinetics. Recent studies have modeled and verified that when inertial force and kinetic energy are included, a smaller diameter capillary starts to fill faster. In addition, other retarding forces, such as local transient sticking of the ink vehicle at surface asperities or chemical inhomogeneities, capillary surface topography and connections between capillaries or physical inhomogeneities in coating, affect the total rate of imbibition. While these resistant factors are very important none of them changes the fact that the basic force driving spontaneous imbibition is the total liquid sorption energy in all various capillaries. This thesis suggests that the change in Gibbs free energy of a liquid associated with its imbibition into capillaries of coating can be used to describe such driving energy. It first derives an expression for this quantity then shows how specific Gibbs energy correlates with the time to reach maximum ink tack, tmax for carbonate-kaolin and latex based coatings. The application of the Gibbs energy model shows that for a relatively constant liquid-solid-vapor interface a larger capillary pore surface area strongly increases the rate of ink setting, as measured by the reduced tmax. In contrast, there is no correlation between ink tack development and pigment surface area or peak pore size of the coating which are currently in use. This new model analyzing Gibbs energy has advantages of combining porous structure variables (volume and diameter, Σ(Vi/Di)) and surface energy parameters (γcosθ) at the interface which often vary concurrently when coating components change, and being independent of many details of time-dependent variables. Surface topography affects liquid-solid-vapor/air interfaces. The second part of the thesis describes simultaneous characterization of surface topographies, using Near-field scanning optical microscopy. Finally, the thesis relates light scattering and absorption to the influence of fine kaolin and carbonates on coating structure and water soluble chemical such as fluorescence whitening agent distribution and efficiency in paper coating.