Optimum Dispersion in Blade Coating Operations, 2001 Coating Conference Proceedings

Dr. Rajan R. Iyer, Ray Hollingsworth, Dr. David R. Skuse, Dr. John C. Husband--Blade coater runnability is affected by many factors. In this paper, we have studied the steady state rheological properties of pigment slurries and coating colors, particularly in relation to their state of dispersion.

Industrial practice is to optimize dispersant levels based on low shear rate viscosity measurements, done using the Brookfield viscometer. It is reasonable to assume that optimizing dispersant dosage in the pigment slurry and coating color, based on low shear viscosity measurement would be inadequate for blade coating operations at high speeds.

In this work, we show that the dispersant dose for minimum pigment viscosity increases as the shear rate of measurement of the viscosity increases. A consequence of extra dispersant addition in pigment slurry is the presence of non-absorbed dispersant in the fluid phase. This increases ionic strength and causes depletion flocculation effects. Removal of non-absorbed dispersant through washing of the pigment slurry lowers the viscosity as well as improves viscosity stability over time. There is also some experimental evidence to suggest that order of dispersant addition when blending different pigments can influence final color rheology.

Pilot coating experiments were conducted using a freesheet base paper and increasing the level of the added dispersant. There is a significant reduction in rheological scratch counts observed on the pilot coater with increased level of dispersant in the coating color. The reduction in scratching showed the best correlation with color viscosity when a capillary viscometer modified with slit geometry was used for the viscosity measurement.