Standard Bench Viscometry Methods for Aqueous Paper Coating Emulsions
Bench viscometry of paper coating emulsions requires standardized temperature control, verified spindle geometry, and multi-shear profiling to ensure runnability.
TAPPI T 648
Rheological Protocol
The official test method defines the procedure for determining the high shear viscosity of paper coating colors using a rotational viscometer. In mill laboratories, adherence to tappi t 648 allows technicians to evaluate the flow behavior of coating slurries under conditions that resemble high-speed blade coater operations. The procedure measures the torque required to rotate a bob inside a cup filled with the slurry at high rotational speeds.
This method is not intended for low shear measurements or for testing paints that have long drying times.
Practical Runnability
Converting paperboard at high speeds demands that the applied clay coatings do not clump or streak under the leveling blade. Applying the guidelines of tappi t 648 helps chemists identify the onset of dilatancy, a condition where the coating thickens rapidly under pressure. When this behavior is detected, formulators add specific dispersants or adjust the solids content to ensure the mix remains fluid.
The standardized test provides a reliable way to optimize these adjustments before starting a full scale production run.
High Shear Testing
Plotting the resulting torque against the rotational speed produces a rheogram that reveals the fluid’s flow characteristics. This curve indicates whether the coating color displays shear thinning or shear thickening properties during processing.