Effect of polymeric thickeners on pigment coatings: Adsorption, rheological behaviour and surface structures

Polymeric thickeners play a vital role in controlling water retention in paper coating processes. In this work, the interactions between various polymer water retention agents (i.e., thickeners) and coating pigments were investigated, in an attempt to improve the understanding of the thickening mechanisms. Adsorption isotherms of the polymeric thickeners, up to four types of commercial thickeners based mainly on copolymers of acrylate and acrylic acid, onto mineral pigments (clay pigment and ground calcium carbonate, GCC) were determined. It was found that the adsorption behaviour was strongly affected by the surface structure of the pigment. All the selected thickeners showed greater affinity towards clay pigment than calcium carbonate. The influence of the selected thickeners on coating mixtures behaviour at low shear rate was also investigated. In coating mixture based on clay, it was found that HASE (hydrophobically modified alkali swellable emulsion) thickener that has the highest adsorption led to more effective thickening. All the selected thickeners showed moderate effect on the viscosity of the coating mixture based on GCC. The presence of thickeners also produced the coated paper with smooth and homogeneous surfaces, revealed by SEM observation.