Slip Rheology of Coating Colors Containing Calcium Carbonate Pigments with Narrow Particle Size Distributions, 2003 Advanced Coating Fundamentals Symposium Proceedings

The purpose of this experimental work was to improve the rheological performance of coating colors comprising 100 parts of calcium carbonate particles with narrow (steep) particle size distribution. We focused on minimizing water release of the coating color, while maintaining low high-shear viscosity. Coating colors with steep-particle size calcium carbonate lack water holding due to absence of fine pigment particles. Another consequence of steep calcium carbonate suspensions is their high viscosity, which is also ascribed to the absence of fine particles. The apparent slip velocity was determined to quantify lubrication capability. Presumably, high slip velocities of concentrated dispersions facilitate slipping through a blade nip, therefore improving coater runnability. While keeping the pigment and latex constant, we determined the type of calcium stearate additives and thickeners, which improve rheology at high shear rates. Although the desire exists to use 100% carbonate pigment of this type for improved end-use properties, coating rheology in blade and other coaters limits the practical limit to about 40-70% (of the total pigment level).

Both high-shear viscosity and apparent slip velocity are strongly dependent on solid content, as well as the type and amount of thickener. Addition of fine calcium carbonate particles was not sufficient to create slip, although highshear viscosity dropped significantly. Slip occurred, however, when we added a calcium stearate emulsion to the coating color containing sodium carboxymethylcellulose as a thickener. The apparent slip velocity decreased with increasing solid content and cobinder (thickener) concentration. This was attributed to the increased continous phase viscosity and potential migration and re-arrangement of the species from the calcium stearate emulsion next to the flow walls. It is not related to an increase in the free water of the coating. Observations from studies with two different calcium stearate emulsions further supported this assumption. The calcium stearate emulsion containing a protective hydrocolloid reduced the high-shear viscosity by 10-15% and substantially increased the apparent slip velocity in certain formulations. However, there is a limit of the effective addition level of lubricant to increase the slip; in our formulations it was 0.8 part %wt based on dry solids. The specific synergism between calcium stearate emulsions and thickener needs determination when forming a formulation strategy for coating colors containing 100 parts of calcium carbonate with narrow particle size distribution.