Apparent slip velocity of paper coatings in viscometric flows

Similar to other rheological properties, slip is an inherent characteristic of a pigmented coating suspension that depends on pigment crowdiness, the state of dispersion, and the flowability of the continuous phase. Slip mani fests how a coating behaves in a boundary flow. This is important because c oating processes involve boundary flows where traditional rheological measu rements of bulk provide only limited information. The principles of wall sl ip in polymer melt flows and coating suspensions are discussed. For concent rated suspensions, like coating colors, the local concentration of suspende d particles can be lower near a wall than in the bulk. Two methods are pres ented for calculating the apparent slip velocity at the wails in both capil lary and cylindrical viscometric flows. We demonstrate how to calculate the apparent slip velocity in a capillary flow, but there are still questions regarding the accuracy of apparent slip velocity calculations in a cylindri cal flow. Slip velocity as a function of shear stress decreases with increa sing solids and thickening of the continuous phase. Lubricants as coating a dditives were found to increase the slip velocity, particularly when using a calcium stearate. This beneficial effect was more pronounced at high (63% ) rather than at low (59%) solids. It is speculated that high slip velociti es, characteristic of low-solids coatings, may be associated with good coat er runnability.