Visualization Study of Two-layer Reverse Roll Transfer

Reverse roll coating in which a thin single layer of liquid is applied onto a substrate has been used in industry for decades and has been extensively analyzed in the literature. Modern coatings, however, are often composed of more than one layer to improve product performance and to reduce manufacturing cost. Pre-metered methods such as slot, slide and curtain coating are typically used to produce such multilayer coatings. If the caliper of the substrate to be coated is not constant, the coating gap and consequently the final film thickness deposited on the web will also be non-uniform. In this study we focused on the use of reverse roll technique with slot die liquid delivery system to produce a uniform thin two-layer coating. The use of this coating technique to produce such a coating has not been previously explored. The liquid film surface as it is transferred from a rigid steel roll to a deformable urethane covered roll was visualized in order to find out how the uniformity of two layer coating is affected by the speed ratio between two rolls, layers wet thickness and liquid viscosities. The effect of these parameters on the ribbing frequency and amplitude was also investigated. The results show that in two layer coating, as in the single layer reverse transfer, there is a critical web speed above which ribbing occurs. The critical speed is determined by the bottom layer viscosity.