Runnability and flow-induced aggregation of paper coating suspensions

Besides high shear rheology and water retention, colloidal stability seems to be a key parameter controlling the runnability of paper coating colours. A clear correlation between the formation of stalagmites in pilot coater trials and the flow-induced aggregation of paper coating suspensions has been found. A ring-slit device has been developed capturing the essential features of the strongly converging flow field at the blade entrance of standard paper coating machines. The new device is mounted to a controlled flow-rate capillary rheometer. When aggregation takes place the slit is gradually clogged resulting in a strong pressure increase. Thus aggregates are simultaneously formed and "detected". This device allows an investigation of flow-induced aggregation phenomena even on a laboratory scale. The effect of pigment type, polymer dispersion, thickener and dispersing agent on the stability of paper coating colours has been studied systematically. The size and type of pigment as the main ingredient, the choice of the dispersant and the kind of colloidal stabilisation of the polymer dispersion play a crucial role for the stability of the complete formulation. Examples are presented where the stability of coating colours could be optimised by appropriate modifications of the binder composition.