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Moisture pickup in calcium carbonate coating structures: role of surface and pore structure geometry

Papermaking and coating require removal of liquid water to a final state of moisture content. During printing the coating surface is further exposed to water-based suspensions and solutions. The moisture adsorption on the pigment is influenced by, and in turn influences, the surface chemistry. The reaction of the porous coating structure to the uptake and retention of moisture can be critical in these processes. Forming compressed pigment tablets, it is observed that exposure to defined humidity under controlled temperature conditions leads to a measurable hygroscopy in pre-dried coating pigment structures, which is progressive with time. The initial rate of hygroscopic adsorption is shown to be related to the porous structure permeability and this indicates that the short time response is predominantly diffusion controlled. The longer term equilibrium hygroscopic uptake is more complex and depends on the humidity level involving mechanisms of surface adsorption as well as the potential for pore capillary condensation depending on local pore size and geometry when humidity level is high. Correlation at 75% relative humidity and room temperature is found between specific surface area and the equilibrium uptake, whereas at 100% relative humidity the structure relationship determined by pigment particle size, and hence pore size, and the porosity of the structure controls the equilibrium uptake. This latter effect has been conveniently parameterised via the geometrical structure coefficient expressed in the KozenyCarman equation and supported by pore network modelling.