Troubleshooting Rheology and Runnability Problems in Metered Size Press, 2002 Metered Size Press Forum

In this review paper we discuss runnability issues in size press operations with emphasis on rheological requirements for pigmented 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 high-speed film coating with a metered size press (MSP), where runnability issues of film split, ribbing, orange peel pattern, and misting are important. The dynamics of the size press process using both fluid continua and pigmented coatings consider the analytical and practical points of view. Operational, coating and substrate parameters influence processability at both low (below 1,200-1,300 m/min) and high (above 1,300 m/min) speeds. Major portion of the discussion focuses on the dynamics and rheological requirements for pre-metering with smooth rods and transferring of films on to a web 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. Optimum operation 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. For pigmented coatings, 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 film ribs after high strains are applied onto the coating at the metering and transfer nips. While water retention is required at low speeds, particularly because the MSP is an on-line process where a hot web picks up the coating weight, quick immobilization is needed at very high speeds. Low solids and thickener amounts generate spitting or misting at low speeds. At high solids content and low speeds, the thickener selection and amount need to balance against the tendency of the coating to stick on to the transfer rolls. For high-speed coating, the desire is to minimize the thickness (i.e., by using higher solids) and viscosity of the non-immobilized layer of the coating at the transfer nip in order to avoid misting and minimize ribbing. The choice and amount of pigments, latex, and soluble thickeners therefore influences these bulk rheological characteristics. Additionally, the sheet roughness and absorptivity of unfilled paper need to decrease to improve MSP runnability. Highly filled papers demonstrate misting at comparatively low speeds.