Pulse Dewatering and the Mechanics of Blade Defects in High Speed Coating, 1994 Coating Conference Proceedings

The physical aspects of high speed blade coating depend on the time-scale of application as one dominating common factor, irrespective of specific coater type, basepaper etc. Unlike slower speed applications in which the flow regime is predominantly controlled by continuous shear phenomena, and largely predictable by continuum viscosity measurement, high speeds introduce deformation rates which, in certain parts of the process, exceed the relaxation rates of the constituent interactions operating between the components of the coating colour. Micro-interactions can occur over distances of the same order as the particle size of the components and can act collectively to create a macro-structure. Due to the wide variety of interactions operating on the thermodynamic time-scale, i.e. from hard sphere Brownian diffusion rates through to long-range depletion or exclusion flocculation, the response time characteristics of the coating colour to applied shear stress range from microseconds to greater than milliseconds. As the rate of the stress cycle increases with coating speed, for example under the pressure pulse dynamics of an application nip or at the blade, the coating colour appears to behave more akin to an elastic solid than a viscous fluid.