The influence of thickeners on the rheological properties of PCC suspensions and on the surface structure of PCC-based layers

The relation between rheological properties of coating colors and surface structure was investigated using pigment suspensions based on precipitated calcium carbonate (PCC). The effects of three different thickeners - carboxymethyl cellulose, poly(vinyl alcohol), and an alkali-swellable emulsion - on the surface structure were studied. The aim was to investigate the relation between rheological properties of coating colors and surface structure and to evaluate methods for analyzing the particle order in the surface layers containing acicular pigments. A controlled shear stress rheometer was used to investigate the theological properties of the suspensions, while the surface structure of coatings was studied using Atomic Force Microscopy (AFM) and Interference Microscopy. Roughness parameters of surfaces were obtained from the AFM measurements and from Interference Microscopy measurements. Surface structure was interpreted in terms of the nematic structure of pigment particles and a relation between the structures of PCC suspensions and dry coatings could be seen. The storage modulus of the suspensions could be correlated to gloss of the coatings and to some extent also to surface roughness (determined from Interference Microscopy images). An increase in the storage modulus increased surface roughness and decreased gloss. The micro-roughness (determined from AFM images) of the coatings and the bulk porosity were better understood from the shape of the relaxation modulus curve than from the magnitude of the modulus itself. A correlation was found between longer relaxation times of the PCC suspensions and higher surface micro-roughness and surface porosity as well as higher bulk porosity.