Pigment Coating Permeability: Measurement and Correlation with Wetting Front Penetration, 2003 Advanced Coating Fundamentals Symposium Proceedings

While porosity may be the most important geometrical description of a porous structure, permeability is probably the most important structural characteristic. In paper science air permeation is the most widely used technique determine permeability and this is used mainly with uncoated papers. The focus of the work here is on the permeability of pigmented coatings alone and not as paper samples. The permeabilities of a range of porous samples are measured using a specially-constructed high pressure liquid permeation cell. Data of this kind have not been reported before in the literature. The samples consist of compacted fine isotropic mineral pigment - calcium carbonate - compressed over a range of compaction pressures resulting in a range of porosities. These samples have pore-throat diameters typically finer than 0.1 µm. The porosities and the pore size distributions of the samples have been determined by means of mercury porosimetry. The permeability is seen under certain conditions not to obey the well-known linearity of the Darcy relation as a function of applied liquid pressure differential. A considerable pressure-flux hysteresis is observed following saturation by imbibition. Furthermore, it is seen that there is no direct linear correlation between permeability and porosity, despite the use of a constant pigment particle size distribution and, hence, skeletal size distribution. The measured permeability displays a local maximum at a fractional porosity approximately 0.26 with a further distinct drop at around 0.27. Interestingly, this phenomenon is seen to correlate with data from water droplet equilibrium distribution within the same structures.

Taking into account the effects of earlier-proposed mechanisms of preferred pathway flow and film flow during imbibition, it is postulated that some pores remain unfilled during imbibition prior to the permeability study, such that there remains entrapped air or vapour phase in microscopic ganglia. This air does not dissolve in the aliphatic mineral oil used in the experiments. Structures with a delineated pore-throat structure will probably lead generally to observed non-linear phenomena within the range of dimensions studied here, which can have implications, amongst others, for microscopic filtration, catalysis and absorption phenomena.