Aggregation in concentrated kaolin suspensions stabilized by polyacrylate

• Aggregation in coating coluors is, in many cases, considered as a drawback since it may have negative consequences with regard to the runnability of the coluor. On the other hand, with a controlled aggregation, the structure of the colour may, in principle, be tailored in order to improve the properties of the coated paper. In both cases a better understanding of the aggregation mechanism is desired. Here the calcium ion induced aggregation of kaolin suspensions, dispersed with sodium polyacrylate (NaPAA), is characterized with rheological measurements and scanning electron microscopy. The aggregation behaviour of these systems is compared with that of kaolin suspensions (without dispersant) aggregated using pH changes. The results obtainded indicate the structure of the aggregates differ in these two systems. The calcium ion induced aggregation of the kaolin particles is extensively controlled by the interaction between the dispersant NaPAA and the cations. The aggregation is initiated at a critical ratio between the total concentration of calcium ions and the total concentration of polyacrylate monomers. It is suggested that the aggregation induced by the calcium ions can be understand in terms of Manning's counterion condensation theory as a result of delocalized binding of the calcium ions the PAA molecules reducing the charge of the PAA. It is, however, also possible that partly localized binding may influence the aggregation.

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