Optical reflectance as a function of liquid contact time and penetration depth distribution in coatings with mono and discretely bimodal pore size distributions, 2010 TAPPI Advanced Coating Fundamentals Symposium

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Refractive index (RI) gradients at the pigment-pore and the binder-pore interfaces occurring in the porous skeletal matrices of typical pigment coatings are being subject to modification during printing through the replacement of air in the pores by the liquid phase of the printing ink vehicle absorbed into the coating structure. This phenomenon can be utilised to obtain information about the location and amount of the liquid in the coating matrix as a function of contact time between the liquid and the coating exposed. We examined capillary absorption of mineral oil, used in offset printing inks, into model coatings compressed from dispersed calcium carbonate pigments with a range of pore structures, controlled by the pigment type, and the binder content. The results obtained with natural ground calcium carbonate (GCC) coatings with mono-modal pore size distributions were contrasted with rapidly absorbing coatings with high absorption capacity, displaying a discrete bimodal pore concept, based on a specifically designed, nano-featured, modified calcium carbonate (MCC) pigment. The intraparticle pores within the MCC pigment particles provide the absorption driving force for the preferred pathway wetting phenomenon emphasised at the short contact timescales of absorption, further controlled by the permeability of the coating structure through the interparticle voids arising from the relative arrangement of the pigment particles, and from the relative location, amount, and type of the surrounding binder particles. The results suggest analogous change in reflectance due to the absorption with both of the coating pore distribution types examined, and progressive absorption behaviour of the liquid, as monitored by the change in reflectance following a newly established relationship derived from the observational data. The findings support the concept of a preferred pathway flow for the wetting front, defined by differential pore size and connectivity, and a longer time saturation front flow lagging behind the wetting front, which, theoretically at the limit of infinite time, coincides with the wetting front, the time constant of the approach being related to the permeability of the porous network. Reflectance is most sensitive to filling of interparticle pores displaying size around 200 nm, which, in addition, are responsible for controlling the permeability of both coating types. The change in reflectance, therefore, demonstrates the filling of the connecting medium.