Fundamental Study of the Flow Stability in Reverse Roll Coating

Abstract

Reverse roll coating is widely used to coat a thin liquid layer onto a moving substrate. The metered liquid layer is created within the gap between a pair of co-rotating rolls. To avoid roll damage due to roll run-out, substrate caliper change or splice passage, when gap between rolls is small, one of the rolls will have a compliant polymer cover. The existence of a deformable cover in the gap between metal roll and rubber roll creates a elastrohydrodynamic flow field in that region. As the liquid passes through the coverging-diverging section within the gap it generates pressure, this pressure can deform the elastic roll surface, which in turn, alters the geometry of the gap and the flow. Therefore the uniformity of coating is effected differently than what is observed in the case, when only rigid rolls are used.
In this study, visualizations of the flow between a reverse deformable roll and solid stainless steel roll are done to determine how the uniformity of coating is effected by operating parameters: the speed ratio, roll diameter and liquid properties. The wavelength of ribbing is investigated to verify the effect of speed ratio, wet thickness and viscosity. The roll coating apparatus is used with 4 inch diameter rolls installed one above the other for these experiments. The ribbing wavelength is measured from images. The uniformity of coating and its dependence on capillary number is evaluated. Numerical simulation of the flow between 2rolls leading to prediction of ribbing wavelength is conducted.