Abstract
The rheological behaviour of paper coating colours is investigated in the metering section of a laboratory reverse roll coater. The objective is to combine measurements of pressure in the metering nip, and torque on the metering rod to calculate the process viscosity. Classical rheological tests and nip flow numerical simulations are also required to complete the process viscosity 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 coating colour formulation. The discrepancy observed between these two viscosities is mainly attributed to the rheological properties of the coating fluids. Furthermore, structure breakdown at relatively low shear rates suggests that steady state may not be reached in the metering nip. From numerical simulations, the pressure-driven contribution to the flow in the centre of the metering nip has been found constant in the case of Newtonian fluids, dependant on the shear-thinning index in the case of non-Newtonian fluids, and independent of inertia effects.
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