Troubleshooting Rheology Problems in Metered Size Press, 1998 Metered Size Press Forum II Proceedings

This paper discusses runnability issues in size press operations, with emphasis on rheological requirements of coating films. First, we briefly review issues relevant to applications with traditional size press equipment 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 detail. 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 at both low (up to 1200- 1300 m/min) and high (above 1300 m/min) speeds. We are discussing the dynamics and rheological requirements for pre- metering with smooth rods and transfer 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 operation involves pm-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 coatings with low elasticity to avoid misting at the transfer nip. While water retention is needed a low speeds, quick immobilization is required at 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 in order to avoid misting. Additionally, sheet roughness and absorptivity need to be minim&d to improve runnability.