Modified calcium carbonate coatings with rapid absorption and extensive liquid uptake capacity

Abstract

Special rapidly absorbing coatings with high absorption capacity are demonstrated based on new modified calcium carbonates which have been designed based on the absorption behaviour predicted by a combination of previous modelling using discrete pore and throat size distributions [Colloids Surf. A: Physicochem. Eng. Aspects 206 (2002) 217]. Firstly, the absorption driving force is determined by the proportion of fine pores present up to a size equal to a Bosanquet inertially-defined optimum within the timescale of absorption. Secondly, the flow of liquid to access the fine pores is controlled by the overall permeability of the assembled structures. Three new pigment morphologies, based on natural ground calcium carbonate (gcc), with special surface structure modifications, are contrasted with standard gcc by using consolidated tablet blocks made from a suspension of each pigment and chosen mixtures thereof. The blocks are characterised after drying by mercury porosimetry, and the absorption dynamic of a selected liquid is studied. It is clear that the new pigments develop a pore structure that absorbs at a much faster rate and have capacities for up to 10 times more fluid than conventional gcc pigment. These properties are advantageous for many types of digital printing including both oil-based and water-based inks. From scanning electron microscopy of the structures it can be seen that the combination of nano-surface features and pores (intraparticle voids), on an otherwise micro-particle, provides a combination of two discrete pore networks which allow the driving force associated with the nano-features and the permeability of the bulk (interparticle voids) to be separated. This feature is due to the fact that the structures assemble in a way analogous to a parallel electrical circuit, resulting in discrete fine pore regions accessed by a network of large pores. In such unique discrete network systems, we show qualitatively that the mercury intrusion curve provides a separable analysis of permeability and capillarity in respect to the inflection point of the cumulative intrusion curve. Correlation between the observed absorption rate and the two parts of the corresponding mercury intrusion curve are based on the features observed in SEM pictures free from any possible subjectivity. These properties can then be studied in relation to the absorption capacity expressed in terms of porosity.