Troubleshooting Rheology Problems in Metered Size Press, 2000 Metered Size Press Forum
In this review paper we discuss runnability issues in size press operations, with emphasis on rheological requirements of coatings. First, we briefly review issues relevant to applications with traditional size press equipment and application of fluid continua, solutions of soluble polymers such as starch. Then the focus is on film coating with a metered size press (MSP), where the issues of film split, ribbing, orange peel pattern, and misting are discussed in details. The dynamics of the process when using both fluid continua and pigmented coatings are considered from the analytical and practical points of view. Consideration is given to operational, coating and substrate parameters that influence processability ad both low (up to 1,200-1,300 m/min) and high (above 1,300 m/min) speeds. Major portion of the discussion is on the dynamics and rheological requirements for pre-metering with smooth rods and transfer of films on to the substrate. With respect to solids and composition, coatings for MSP are similar to the ones used in blade coating but with some unique selection criteria for runnability. The optimum oepration involves pre-metering a uniform film without spitting at the rod nip and transferring most of the film on to the substrate to obtain the desired coat weight. Generally, the preference is for high-solids, more viscous-like pigmented coatings with low elasticity to avoid misting at the transfer nip. Low low-shear viscosity and quick relaxation assist leveling of ribs aftern high strains are applied onto the coating. While water retention is required at low speeds, quick immobilization is needed at very high speeds. The choice of pigments, latex, and soluble thickeners influences these rheological characteristics. The key is to minimize the thickness and viscosity of the non-immobilized layer of the coating at the transfer nip to the paper in order to avoid misting. Additionally, the sheet roughness of unfilled sheets and absorptivity need to be minimized to improve runnability. Highly filled sheets demonstrate misting at comparatively low speeds.