Non-Linear Rheological Transitions and Dilatancy Boundaries in Ultra-High-Solids Paper Coating Emulsions
Formulating paper coatings above seventy percent solids triggers dilatant shear jamming, inducing blade streaks that ruin printing runnability.
TAPPI T 648 Om-88
Testing Standard
Standardized laboratory test methods quantify the high-shear rheological behavior of mineral pigment slurries using high-speed rotational viscometers equipped with precise bob-and-cup geometries. Papermakers and coating formulators apply TAPPI T 648 om-88 to evaluate aqueous kaolin clay, calcium carbonate, and titanium dioxide slurries under deformation rates that approach commercial coating head conditions. The protocol measures rotational torque across ascending and descending spindle acceleration cycles, generating continuous rheograms that classify slurries as dilatant, thixotropic, or pseudoplastic.
The method stops applying to fully formulated coating mixes containing synthetic latexes, water-soluble binders, or dry matter contents outside specified mineral concentrations.
Rheological Procedure
Slurry samples condition to room temperature, undergo mixing to break settling structures, and enter the annular gap of an instrument meeting method requirements, such as a Hercules high-shear viscometer. A motor accelerates the cylindrical bob from zero to high rotational velocity, reaching specified rotational limits, and immediately decelerates back to zero. A recording drum or digital data recorder tracks torque resistance as an automated function of rotational speed, drawing complete up-and-down rheological loop traces.
Analysts calculate high-shear viscosity in centipoise at the maximum rotational speed from torque deflections, spring constants, and instrument bob factors. The emergence of a wide hysteresis loop reveals structural thixotropic breakdown, whereas outward deflection on the up-curve reveals dilatant shear thickening.
Process Governance
Pigment slurry contracts in paperboard manufacturing enforce viscosity ceilings determined in strict adherence to TAPPI T 648 om-88. Slurries exhibiting excessive dilatancy under this standardized shear program cause high blade pressures, streaking, and web ruptures during application on high-speed coater stations. Packaging board converters audit dry pigment shipments against the method to detect poor dispersant distribution or irregular particle size distributions before slurries enter storage silos.
Refining the chemical dosage of sodium polyacrylate dispersants relies on the protocol to identify the minimum viscosity plateau under demanding rotational shear. Data collected under the standard permit paper mills to predict how raw mineral components behave before costly formulation with polymeric binders. Adherence to TAPPI T 648 om-88 standardizes high-shear rheology reporting between pigment suppliers and coated paperboard mills globally.