The effect of inorganic particles on slot die coating of poly(vinyl alcohol) solutions

Among various coating processes, slot die coating belongs to a class of pre-nietered coating, in which the coating film thickness can be predetermined. In the past, most research works oil slot die coating have focused mainly oil polymer solutions; very little information is available ssing suspensions as coating fluids. In this study, the effect of adding TiO2 and SiO2 in aqueous polyvinyl alcohol (PVA) solutions oil slot die coating is investigated. It was found that the stable coating window was enlarged with the addition of particles, and its size increased with solids concentration. This is due to the strong interaction between polymer and particles, resulting in a higher viscosity and surface tension. As a result, the upstream coating head is more stable and the maximum coating speed is extended to a high value, hence the coating window becomes larger. Although both viscosity and surface tension appear to contribute to the stability of coating flow, the effect of surface tension is more dominant. The surface tension of a suspension with porous particles was higher than one with hard solid particles. Consequently, the coating window obtained with the former was significantly larger than the latter. Flow visualisation revealed that under the same operating conditions, the upstream dynamic contact angle for the suspension was smaller than for the aqueous polymer solution. This observation could be related to the stability of the upstream coating head, and hence the coating window. The experimental flow fields were verified numerically with the aid of a numerical simulation package (Flow-3D). (c) 2005 Elsevier Inc. All rights reserved.