Observing Fluid Transport Into Porous Coating Structures: Some Novel Findings, 1999 Coating Fundamentals Symposium Proceedings

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This paper discusses novel methods for observing the spreading and penetration behaviour of fluids into specially prepared consolidated blocks of paper coating pigments which allows for the equilibrium absorption of measurable droplet sizes without saturation of the pigment layer. The mechanisms studied are responsible for a range of printing phenomena. Variables such as particle size distribution, pigment surface treatment with dispersing agents and the applied pressure during consolidation of the structure give the possibility to create a wide range of porous structures with differing structure-property relationships. By bringing liquids of differing polarity and viscosity, stained and unstained, into contact with these porous blocks either as a super source or in droplet form the relevant phenomena of spreading, penetration and adsorption can be studied using image analysis techniques. Droplet absorption and pressureless fluid imbibition methods are contrasted to study the influence of surface free energy of the solid and liquid phases in combination with a given pore system and modelled geometry evaluated using mercury porosimetry and a pore structure modelling software (Pore-Cor). This software generates a three-dimensional pore network matching the experimental percolation characteristics and porosity, and models the liquid phase absorption into this network assuming a combination of inertial, viscous and capillary-force flow. Using this description of pore networks involving pores and access throats we have determined some expected fluid transport parameters by relating pore filling propensity to the size distribution of the interconnectivity. Results show that there is a discontinuity in the relationship between relative compaction, particle size, localised surface energy and the volume of liquid absorbed which, for non-polar fluids, can be significantly less than the volume measured by intrusion porosimetry. In practice these discontinuous absorption volume controlling mechanisms are seen as contributing factors to the different offset printing properties of gloss

and matt papers in relation to fountain solution absorption and ink tack development and decay, and, when inhomogeneously distributed on a coated surface, may be the single most common cause for mottle in modern coated papers.