Impact of oil-binder interaction on pore structure in paper coatings as studied by internal reflectance. Part 2: Relation to structural expansion

In the case of ink setting on coated paper, significant decrease of accessible pore volume of the coating matrix can occur due to structural changes caused by swelling of latex binder particles when contacted with the vehicle of oil-based ink. The impact of latex swelling, reduced network connectivity and the subsequent effects on the dynamic of spatial liquid distribution characteristics within the expanded pore network were examined by a recently established optical reflectance measurement procedure based on the refractive index differences between the coating components and the surrounding medium present in the interparticle pores.

GCC model coatings with latex binders displaying differing swelling characteristics were exposed to liquid supersource imbibition without application of external pressure. Effects of both linseed oil and mineral oil on binder swelling and consequent structural expansion were studied. It is proposed that the linear reflectance behaviour in the saturated region is related to the mechanical restriction on expansion at the transition between the saturated and partially filling regions of the porous structure, and is consistent with an approximate catenary form in the macroscopic coating dimensional change as a function of depth. A conceptual structural change factor based on the change of reflectance in the saturated region is introduced, and is shown to correlate with the porosity loss values. The results suggest differences in the structural change factors in relation to the equilibrium oil uptake, coating porosity loss, and liquid propagation. The structural change factor also correlates the reflectance gradient within the coating thickness with regard to oil uptake with binder content as the binder level is increased.