Apparent Slip of Paper Coatings and the Influence of Coating Lubricants, 2001 Coating Conference Proceedings

Nick Triantafillopoulos, Annaleena Kokko, Tom Grankvist--Slip can be an important phenomenon in blade coating. Slip velocity under shear is a measure of slip and, like other rheological properties, it is an inherent characteristic of a pigmented coating dependent on pigment crowdiness, the state of dispersion, and the flowability of the continuous phase. It demonstrates how a coating behaves in a boundary flow. This is important because blade coating involve boundary flows, where bulk rheological properties provide only limited information. The physics of wall slip are analyzed and differences discussed between flows of polymer melts and highly concentrated dispersions. For dispersions, the local concentration of suspended particles in tube flow can be lower at the capillary wall than in the bulk, causing slippage. The apparent slip velocity is defined and calculated as a function of shear stress from measurements with a capillary viscometer. High slip velocities, characteristic of low-solids coatings, are associated with good coater runnability. It is found that, in a fully-developed capillary flow, slip velocity decreases with increasing coating solids and thickening of the dispersing phase of a coating. Lubricants as coating additives, on the other hand, increase the slip velocity at high solids (63%), particularly when using calcium stearate.