Novel techniques for investigating the permeation properties of environmentally-friendly paper coatings : the influence of structural anisotropy on fluid permeation in porous media
In this study, we have investigated the effects of structural anisotropy of porous media on the permeation of fluids. The motivation for the work was an increased understanding of the permeation of inks into paper coatings, which often contain platey or needle-like particles, which have been aligned during the coating process. However, the findings are also relevant to other systems, such as the sub-terranean migration of fluids, including pollutants, within shale that contains particles of high aspect ratio. Mineral pigments, comprising mainly of calcium carbonate or clay, are often are applied to the surface of paper to improve optical and printing properties. For a high quality image to be achieved, the coating should have sufficient capillarity to allow the ink film to set within the time-scale of a modern printing press. The permeation of fluids into a range of different coating formulations has been investigated, with its main focus on the following samples: Speswhite and Amazon90 SD, which belong to the Kaolin (day) mineral group, and OpacarbA40 and Albaglos, which belong to the Precipitated Calcium Carbonate (PCC) mineral group. The permeation was measured by five different techniques, including a novel use of the Ink Surface Interaction Tester. The results were modelled using a modified version of the software package ‘Pore-Cor’, which simulated both permeability and capillary absorption of a wetting liquid into porous media containing anisotropic voids, and allowed the effects of anisotropy to be isolated from other closely related pore properties. The model generated a simplified three-dimensional void network having pores with a rectangular cross-section and throats with an elliptic cross-section. From visual inspection of the modelled structures, the effect of anisotropy revealed advance wetting in the narrow features of Speswhite-CL and OpacarbA40-CL. Overall, to gain a clear understanding of the permeation of anisotropic structures both inertia and surface throat density is needed to be included in the Pore-Cor model. Once these factors were applied to the model, it was able to predict the permeation of fluids more successfully than those predicted by the Kozeny and aligned cylinders models. The insights gained from this study have allowed conclusions to be drawn about the nature of fluid permeation; they have therefore opened the way to more sophisticated modelling and the engineering of high performance coating structures.