Low-Shear and High-Shear Viscometry Calibration in Paper Coating Laboratories
Calibrating viscometers with certified oils across dual shear regimes prevents coater blade streaking and guarantees uniform paper coat weight application.
Viscous Dissipation
Thermal Transformation
Irreversible conversion processes transform mechanical work into internal thermal energy through friction within flowing, highly viscous fluids under sustained shear deformation. Coated packaging manufacturing encounters viscous dissipation in pigment slurries and high-solids formulations subjected to intense shear fields inside pumps, screening filters, and metering blade gaps. Velocity gradients cause fluid layers to slide against one another, dissipating kinetic energy as heat and warming the fluid matrix.
The physical mechanism loses dominance in low-viscosity aqueous applications, where convection and conductive heat transfer dissipate temperature variations faster than shear can generate thermal spikes.
Shear Heating
Mineral pigment suspensions deform at shear rates exceeding one million reciprocal seconds beneath paperboard metering blades. Viscous dissipation generates local heat in direct proportion to the product of suspension dynamic viscosity and the square of the shear rate. Narrow clearances and microsecond residence times beneath the blade restrict external heat dissipation, causing localized temperature surges inside the sheared fluid film.
Rheometers evaluating paper coatings at high rotational speeds register this effect through an unexpected drop in measured torque during testing. The internal heat generation creates temperature gradients across the measuring gap, confusing dilatant shear thickening with thermal thinning phenomena. Software models and experimental tests run short measurement cycles to isolate true suspension rheology from dissipation artifacts.
Runnability Hazards
Uncontrolled shear heating inside coating heads destabilizes paperboard converting processes by altering fluid properties at the point of application. Rising temperatures reduce fluid viscosity, thinning the applied liquid film and allowing metering blades to push deeper against the paper surface, which risks scratch marks and web tears. High temperatures drive water evaporation from aqueous coatings, triggering premature pigment consolidation and dried coating buildup along the blade heel.
Viscous dissipation inside transfer pumps degrades polymer thickeners and synthetic latex binders, causing agglomeration in recirculating slurry lines. Modern high-speed coating stations integrate heat exchangers on recirculation loops to strip dissipative heat before the formulation returns to the metering head. Viscous dissipation alters slurry rheology during high-speed paperboard coating runs unless formulation solids and cooling systems counteract thermal gains.