Operational limits of blade coating associated with high aspect ratio pigments: Part I-bench top blade coater

Operational issues, such as stalagmite formations, scratches, or spits, develop during blade coating of high solids coatings at high web speeds. Coatings that contain high aspect ratio pigments exhibit these difficulties at lower solids concentrations and slower machine speeds than coatings comprised of more spherical shaped pigments. There are a number of potential reasons behind this phenomenon, but a clear mechanism is not well established.

Using a bench top blade coater without a base sheet, the goal of this study is to understand the operational limits that occur due to the coating suspension interacting with the blade. Pigment suspensions are applied in excess in front of the blade. A camera mounted near the blade exit is used to detect defects and buildup on the backside of the blade. The machine speed at which deposits on the blade first appear was determined for three pigments having different shape factors and at varying weight fractions. A Newtonian fluid was also used in the coating device. A runnability window based on shape factor and solids content was developed. Small changes in the solids concentration displayed changes in the speed at which blade deposits begin to appear. No operational issues could be produced using the Newtonian fluid. Presence of the particles at solids concentration nearing their immobilization solids must be contributing to the development of runnability issues.