Pergamon Chemical Enffineerin(t S'e 'e, Vo 52. No. 8, pp 1339 1347, 1997 I 1997 Elsevier Science Ltd..All rights reserved Printed in Great Britain Plh Forward roll coating of Newtonian fluids with deformable rolls: an experimental investigation Olivier Cohu and Albert Magnin* Laboratoire de Rh6ologie, B.P. 53 Domaine Universitaire, 38041 Grenoble Cedex 9, France and UMR CNRS 5520, Universitb Joseph Fourier Grenoble I, Institut National Polytechnique de Grenoble (Received 15 September 1995; accepted 9 April 19961 Abstract--Experimental investigations on forward roll coating of Newtonian fluids with deformable rolls were conducted, with the primary aim of predicting the coating thickness. First, it was shown that the elastic modulus of the rubber that covers the deformable rolls is often time dependent, and therefore must be determined according to the time-scale of the process. Having done this, a satisfactory agreement in terms of coating thickness was obtained with previous theoretical results. This agreement is limited to relatively thick rubber covers. It was shown, indeed, that decreasing the thickness of the cover below a critical value tends to decrease the coating thickness significantly. The time dependence of the mechanical properties of the rubbers involved in roll coating operations and the influence of the thickness of the cover may both explain why previous experimental correlations disagree significantly with available theoretical predictions. ~ 1997 Elsevier Science Ltd. All rights reserved Keywords: Coating; roll coating; deformable roll; Newtonian fluid. INTRODUCTION Forward roll coating is a very common way of deposi- ting a thin liquid film onto a moving substrate. Usu- ally, controlling the thickness of the coated film is of prime importance. This is done by having the fluid flow between two counterrotating cylinders, the surfa- ces of which move in the same direction at the nip. Most previous studies of forward roll coating fo- cused on the simple case of two rigid rolls separated by a narrow gap (see e.g. Pitts and Greiller, 1961; Savage, 1982; Coyle, 1984). In many cases, however, one of the rolls is rubber covered, and both rolls are pressed against each other by an external load W (Fig. 1). The coating thickness is thus governed by the operating parameters (speeds and loading), the mechanical properties of the solids, and the mechan- ical properties of the fluid. The general features of roll coating operations in- w)lving a deformable roll have been described by Coyle (1988a). He analysed the flow in the deformable nip by means of two dimensionless numbers, the *Correspondence address: Laboratoire de Rh6ologie, B.P. 53 Domaine Universitaire, 38041 Grenoble Cedex 9, France. elasticity number being the ratio of viscous to elastic forces, q V Es = (1~ ER and the load parameter being the ratio of the external load to the elastic forces, W Fw - . (2) ER Here r/is the viscosity of the fluid, which is assumed to be Newtonian. El and v being the Young modulus and the Poisson ratio of the rubber that covers the deformable roll, respectively, the effective elastic modulus of the solid materials involved is E = EI/(I- v2), assuming that the steel roll is much harder than the rubber covered one. R and V are, respectively, the radius and the speed of the rolls--the asymmetric case, where both rolls have different radii and speeds, is considered separately (Cohu, 1995). Johnson (1970) compiled the two dimensionless numbers Es and Fw into a single one .q3, defined as El4., q3) ~J3 ~ Es 1,2" 1339