Mechanism to Explain Particle Size Segregation and Binder Depletion During Coating, 2000 Coating Conference Proceedings

R. E. Gagnon, T. D. Parish, D. W. Bousfield--Observations of coating processes frequently reveal increasing coating solids, decreasing binder content and increasing average pigment particle size in the recycle loop. Product quality can vary significantly due to these changes in coating composition. Observations made during production-scale experiments are presented. The experiments demonstrated time-dependant variations in paper properties and coating composition. A new mechanism is proposed to explain the time dependent size segregation of pigment particles, as well as the depletion of binder and other organic species. Forces experienced by the coating during application, and consolidation prior to metering result in acceleration perpendicular to the web of all coating components. A differential acceleration of pigment particles and colloidal material results from differences in particle diameter, density, and the effects of viscous drag. This differential acceleration produces a non-uniform z-directional displacement of components and non-homogeneity in the wet coating film. The wet coating layer metered and returned to the machine supply is different from both the bulk supply and the coating which has actually been applied to the sheet. For reasonable values of dewatering, viscosity, and particle size, the equations predict a separation of large and fine particles. The theoretical predictions are compared to production scale data.