P aper coating colors are concentrated aqueous suspensions that are applied to the surface of paper to improve optical performance, surface quality and printing properties. In the coating process, the coating color is applied on the paper sheet and excess is removed by a thin trailing blade, where the gap between the paper sheet and the blade is of the order of a few microns. Paper coating colors are colloidal suspensions made of mineral pigments such as kaolin and/or grinded calcium carbonate, binders such as latex or starch for better adhesive properties and water soluble polymers to control flow properties and water retention. The solids content ranges typically from 50% to 70% by weight. Mineral pigments represent 80% to 90% by weight of the solids content and their main function is to improve paper surface quality. The suspensions formulations vary upon the coating process employed and the final application of the paper. Coating colors are complex materials that depend strongly on the interactions between various components. These interactions may lead to the induction of structures that affect both the suspension properties during processing and the final coating quality. Brownian and other colloidal forces affect the rheological behavior of these suspensions.Properties of coated papers and their relation to the flow properties of coating suspensions have been investigated by Basta et al. (1998). In order to correlate flow properties of coating suspensions with the properties of coated paper, they proposed a polynomial model where regression constants and coefficients were obtained using multiple regression analysis. Most of the parameters showed clearly the effect of the formulation, rheology and additives on the improvement in the properties of the base paper. The viscoelastic properties of coating colors are believed to play a major role in the coating process. In particular, after the leveling, the rate of the recovery is controlled by both viscoelasticity and processing speed (Triantafillopoulos, 1996). Surface defects can also be correlated with viscoelasticity as shown by Grön and Dahvik (1997). Poor runnability due to dewatering of the color during the coating process has been related to viscoelasticity and wall slip (Willenbacher et al., 1997). The viscoelastic properties as well as the general rheological behavior of the coating colors depend strongly on the interactions between the different components.In this study, we have investigated coating colors based on clay. The pigment source was kaolin, a hexagonal plate-like shape pigment with negatively charged faces and charges on the edges that depend on the In this work we have investigated the effect of rheology modifiers on the rheological properties of concentrated (65 solids mass%) kaolin suspensions and six different coating colors. Measurements have been performed on kaolin-based suspensions, without rheology modifier and with either a classic cellulose thickener or associative polymers. It was noticed that suspensions containing a thickener...

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