Rheological Profile
Aqueous coating fluids exhibit pseudoplastic behavior when applied through high speed curtain coaters, where molecular entanglement breaks down under mechanical force and viscosity drops rapidly. This fluid property allows formulators to pump heavy clay slurries through mill supply lines while maintaining high film retention on the moving paper web after deposition. Viscosity recovery must occur instantly once the fluid exits the coating head to prevent gravitational sag or orange peel texture formation on the dried sheet.
Blade coaters subject the same coating color to extreme velocity gradients, forcing polymer chains to align with flow directions and lowering hydrodynamic drag during high tonnage production runs.
Viscosity Control
Pumping systems handle high solid suspensions efficiently because apparent resistance decreases under mechanical stress, protecting positive displacement pumps from cavitation and overheating. Starch solutions and latex emulsions rely on this non Newtonian characteristic to pass through fine applicator nicks without blocking the metering rod. Flow curves plotted on logarithmic axes reveal how binder ratios alter the rate at which internal friction declines under rising rotational speeds.
Mill operators monitor temperature fluctuations carefully because thermal energy shifts the baseline viscosity before mechanical forces begin their work on the suspension.
Coating Application
Curtain coating heads demand precise fluid behavior to maintain a continuous film before it contacts the moving paper substrate at high production speeds. Excessively rapid breakdown causes curtain rupture and uncoated streaks across the width of the jumbo roll, while insufficient response leads to excessive coat weight variations. Formulators adjust coater geometry and solids content simultaneously to match the specific shear rate generated by the doctor blade or air knife.
High speed printing papers require this tailored fluid response to achieve uniform ink absorption and smooth surface gloss across every batch leaving the finishing department.
