High Shear Rheology Calibration Procedures for Aqueous Latex Anilox Evacuation
Calibrating capillary rheometers with Bagley and Mooney corrections ensures precise aqueous latex transfer from anilox cells, preventing spatters and excess coat weight.
Mooney Wall Slip
Rheological Phenomenon
Apparent velocity discrepancies occurring along channel boundaries modify fluid transport rates in capillary and slit flow channels. The mooney wall slip effect appears when complex fluids, such as highly filled coating colours or molten barrier polymers, form a thin, low-viscosity depletion layer immediately adjacent to rigid conduit walls. Because this depleted solvent or low-molecular-weight layer shears rapidly, bulk material slides through the conduit as an intact plug rather than following classical parabolic flow profiles.
The phenomenon produces misleadingly low apparent viscosity measurements unless capillary experiments employ multiple die diameters to mathematically extract true shear rates. Evaluating flow behavior during extrusion coating and high-solids blade coating requires correcting for this slip velocity.
Correction Method
Mathematical separation of slip velocity from true shear deformation relies on measuring total throughput across capillaries of varying diameters at identical wall shear stresses. Plotting apparent shear rate against the reciprocal of capillary radius allows the extraction of the slip velocity from the linear slope. In paper coating formulations, mooney wall slip occurs when pigment particles migrate away from smooth metallic surfaces under high shear gradients, creating a fluid boundary dominated by water and soluble starch.
Correcting experimental rheometer data ensures accurate pump sizing, pipe design and coat weight predictions for high-solids pigment dispersions running at mill scale.
Processing Consequence
Wall slippage alters coat weight distribution and blade pressure dynamics during high-speed paper coating operations. If slip velocity increases uncontrollably, metering blades suffer hydrodynamic lift, which causes uneven coating thickness and excessive blade bleeding. In polyethylene extrusion coating, mooney wall slip triggers surface distortions known as sharkskin when polymer melt exits extrusion dies at irregular velocities.
Formulators control slip magnitude through polymer polydispersity adjustments and rheology modifier additions. A verified mooney wall slip analysis isolates bulk viscoelastic properties from boundary lubrication effects.