Fundamentals of Coating Process

The deposition of a liquid layer onto a substrate is an essential part of the manufacturing process of  coated paper for printing, adhesive tapes and labels, pharmaceutical patches, packaging materials, photoresist films for microelectronics, electrolyte and electrode sheets for advanced batteries, separation membranes, magnetic tapes and disks, coil aluminum for beverage cans and coil steel for many applications, high performance sheet glass,  films for LCD displays, and many others.

A typical coating line involves different unit operations, from mixing, liquid flow, to drying and curing.  Coating is done in many different ways, leading to wide diversity of processes in different industries.  The ultimate goal of a successful coating operation is steady, uniform flow to produce a thin liquid layer of acceptable uniformity at as high a rate as possible.  However, the character of the liquid flow and therefore the quality of the deposited layer depend strongly on liquid properties, the speed of coating, and other operating parameters.  The search for better and less expensive products drives great technological needs for thinner coating layers for product performance or for less solvent to be dried; higher production speed; coating of more concentrated solutions and suspensions so that less solvent must be dried; shorter start-up, better control, and fewer defects, so that production yields are higher.

These needs are unlikely to be met without scientific understanding of the steps of the manufacturing process.  The most critical step of a coating process is the flow in the region where the liquid is deposited on the substrate.  This region is the coating bead.  Its steady states and their stability to disturbances and upsets are crucial.

The research projects are focused on fundamentally understanding the process by a detailed analysis of the flow and its stability to different types of disturbances and how the liquid properties affect the process.  The analysis is done both by theoretical analysis and benchtop experiments.  The group at PUC-Rio has a strong long term collaboration with the Coating Process Fundamentals Program at the University of Minnesota, funded by the late Prof. L.E. Scriven and directed by Profs. L. Francis and S. Kumar, where Prof. Marcio Carvalho serves as a Graduate Faculty.

Research on these aspects is challenging. The coating bead is only a fraction of a millimeter thick and at most a few millimeters long; it is up to 2 m wide and even 10 m wide in extreme cases; one or both of the surfaces that confine it move at speeds up to 3 m/s and even 30 m/s in extreme cases.  It is also bounded by liquid-air interfaces, i.e. free surfaces.  Because of all these factors, visualization and flow measurement experiments are extremely complex.  However, the investment pays off, visualizing the details of the flow in the coating bead reveals many important mechanisms that limits production.

 

Theoretical analysis is no less challenging because it demands computer-aided solutions (usually by the Galerkin method with finite element basis functions, Newton iteration and continuation) of the governing equations, which are made nonlinear by the capillarity in free surfaces, by inertia if it is appreciable, and by non-Newtonian behavior which is often present; analysis can be further complicated by elastohydrodynamic effects of compliant confining surfaces.

Stability of steady-state solutions to three-dimensional disturbances, viscoelastic aspects of elastohydrodynamic effects, and the consequences of non-Newtonian liquid behavior are three areas least well examined, much less well understood in scientific terms.

Recent and current research projects in this area include liquid bridge transfer in transient response to perturbations, gravure printing, slot coating of particle suspensions, patch coating, dynamic wetting, curtain coating stability.