Dynamic ink oil absorption in a pigmented porous coating medium studied by near-infrared diffuse reflectance spectroscopy

The rate of absorption (both long and short timescale) of typical heatset offset printing ink oils, namely mineral and linseed oil, has been studied on model ground calcium carbonate coating pigment tablets containing various amounts of either styrene–acrylic or styrene–butadiene binder. The pore structure characteristics of the tablets were determined using mercury intrusion porosimetry. The movement of the oils both on the surface of and within the porous structure of the pigment/binder tablets was studied under the influence of pressure-less capillary flow with subsequent diffusion through the connected void volume of the tablet. The wetting was analyzed by near-infrared (NIR) diffuse reflectance spectroscopy both as a single probe measurement and by hyperspectral imaging. The results showed that the rate of oil filling the structure was strongly dependent on the binder amount in the structure as well as the binder chemistry (oil- or non-absorbing binder), which supports previous findings. The liquid properties, and especially the viscosity of the liquid (oil), influenced the absorption rate. The gradients in absorbance indicated the presence of latex blocking access to some pores and reducing connectivity.