Effect of pH and Calcium Ion Concentration on the Flow Behaviour and Structure Formation of Clay/Calcium Carbonate Suspensions, 1995 Coating Fundamentals Symposium Proceedings

Colloid chemical phenomena play a major role in the paper coating process, from initial mixing to consolidation of the final coating layer. This study aims at a better understanding of the colloid chemistry of suspensions of kaolin clay and calcium carbonate, and the effect of pH and electrolyte (calcium ion) concentration on the rheological behaviour of suspensions. Controlled stress and capillary viscometers were used to characterize suspensions and coating colours in these respects. Measurements showed that in spite of a high salt concentration suspensions of clay and calcium carbonate can maintain good flow properties if the suspensions have a sufficiently high pH. At lower pH strong structure formation takes place if clay is present. This is partly due to the special chemical characteristics of the clay particle surfaces, but also to the interaction between adsorbed dispersing agent and calcium ion. Calcium carbonate did not show a pH-dependency to the same extent as clay. However, an increasing level of calcium chloride resulted in an increased degree of structure formation up to a certain plateau value after which further additions of the salt had no effect.