Non-Newtonian effects on pre-metered reverse roll coating

Abstract

Effects of non-Newtonian fluid properties on pre-metered reverse roll coating are studied experimentally. A variety of coating liquids exhibiting different rheological properties and surface tensions are formulated for the coating experiments. In the hard–hard roll coating system, liquid viscosity is the dominant factor in determining the thickness of the stripped film on the metering roll; whereas the thickness of the metered film on the applicator roll is dependent only on the liquid feed rate and the roller gap space. The stable operating window is bound between a lower and upper applicator roll speed, and between an initial and critical speed ratio of the two rollers. In the hard–deformable roll coating system, there exists a critical gap space below which the interaction between the deformable surface and liquid properties becomes important. At zero or negative gap, the upper limit of the wipe ratio – the metering roll speed divided by the applicator roll speed – for viscoelastic liquids is about 1.8 as compared to 1.2 for Newtonian liquids. Viscoelastic liquids also produce thicker films. The film surface stability depends strongly on the coating bead between the feeding die exit and the roller surface, and on the coating bead in the nip between the two rollers.