Process viscosity in reverse roll coating

The rheological behaviour of paper coating colours is investigated in the m etering section of a laboratory reverse roll coater. The objective is to co mbine measurements of pressure in the metering nip, and torque on the meter ing rod to calculate the process viscosity. Classical rheological tests and nip how numerical simulations are also required to complete the process vi scosity evaluation. This viscosity is compared with that measured from step growth experiments in a rheometer. The process viscosity was found to be 2 .5 to 6.3 times lower than the transient viscosity, depending on the coatin g colour formulation. The discrepancy observed between these two viscositie s is mainly attributed to the rheological properties of the coating fluids. Furthermore, structure breakdown at relatively low shear rates suggests th at steady state may not be reached in the metering nip. From numerical simu lations, the pressure-driven contribution to the flow in the centre of the metering nip has been found constant in the case of Newtonian fluids, depen dant on the shear-thinning index in the case of non-Newtonian fluids, and i ndependent of inertia effects.