The Relationship between Rheology, Application Method, and Final Coating Structure

Paper is coated to improve the printability and the visual properties. The factors that determine the coating structure are important because the structure determines the physical, optical, and printing properties. This work focuses on determining a relationship between the rheological properties of the coating, the application method, and the final coating structure. Coatings with different polymers and pigments were characterized in terms of steady shear and viscoelastic properties. These coatings were applied onto plastic film and paper with a laboratory rod coater, a spray coater, and a high-speed blade coater. The final coating structure after drying was characterized in terms of void fraction, water absorption rate, gloss, and light scattering coefficient. A relationship between the viscoelasticity of the suspensions and the final coating structure was found for the associative thickeners. However, the concentration and molecular weights of cellulosic thickeners did change the wet state rheology, but the final coating structure was not changed. Pigment shape, type, and size distribution are more important than the elastic modulus when predicting coating properties. For all coatings, a correlation is found between shear viscosity and gloss. Application method has an influence on the final structure: the light scattering coefficient changed slightly, but spray coating gave lower gloss, larger absorption rates, and larger void volume compared to the blade coating and rod coating methods. This result was due to poor leveling and spreading of drops at low coat weights