Impact of Different Coating Pigments and Molecular Weight of Carboxy Methyl Cellulose on Coating Structure, 1995 Coating Conference Proceedings

Johan Grön*

Åbo Akademi University

Stefan Kuni

Valmet Paper Machinery

The study was performed to obtain further knowledge about the formation of a coating color structure in the wet state and its transformation due to variations in the deformation force. The formation of a coating structure is mainly influenced by the packing degree of the pigment particles, and the rate of aggregation is primarily dependent on the cobinders in the coating color. A thorough characterization of the coating color structure in the wet state provides a solid basis for the understanding of the formation of the coating layer. This characterization is performed as a rheological and dewatering study of different coating colors. The study concerned coating colors containing two different pigments, clay and ground CaCO3 as well as a combination of these two pigments. The influence of different molecular weights and quantities of carboxy methyl cellulose is discussed due to their importance as additional parameters that influence coating color properties. The rheological study provides specific information about the flow properties under the impact of various shear forces. The dewatering study is performed as a detailed characterization of the parameters influencing coating color dewatering under static and dynamic conditions. The study shows that the pigment is the most important parameter in the formation of a coating layer and that the water soluble thickener is the main determinant of the rate of aggregation. An initial network structure is created in clay-based coating colors, when at rest, by bridge-bonding of adsorbed and unadsorbed CMC molecules. This network structure is, however, broken down to network fractions at an applied minor deformation force. These fractions of the network are further broken down, depending of their strength, at increased deformation or shearing of the coating color. When the network is broken down, the properties of the continuous phase become more important for the flow properties of the coating color. The coating colors with a blend of clay and ground calcium carbonate show the same tendency as the pure clay-based coating colors, however, at a reduced rate. The pure ground calcium carbonate coatings do not create a network structure. With these coating colors an addition of CMC only causes a thickening effect on the continuous phase, or in other words, a dispersion containing