Slot die coating of dilute suspensions

Abstract

An experimental study was carried out to investigate slot die coating of dilute suspensions. Polymethyl methacrylate (PMMA) particles of different sizes were added to glycerol solutions as test fluids. The coating flow experiment was performed on a lab coater. The operating window, which is a domain in which only a stable and uniform coating solution is possible, was determined experimentally. It was found that adding particles has little effect on the physical properties of glycerol solutions, as only the fluid viscosity increases slightly. Two coating defects are commonly observed; ribbing which is caused by flow instability refers to lateral waves on the coated surface, and air entrainment which implies that air bubbles are trapped in the coating solution. Adding particles has little effect on the operating window if the coating defect is ribbing. However, the operating window can be expanded substantially if the coating defect is air entrainment. Higher the particle concentration, or the smaller the particle size, more significant is the expansion of the operating window. The maximum coating speed may increase to 80% for high suspension particle density number. A previous study reported that there exists an optimal particle size which has the largest operating window for a given pure solution; this was not found in the present work. 

Curtain coating of dilute suspensions

“…Consequently, the contact line would be pushed back and re‐stabilized, causing the delay of air entrainment. The findings of Yamamura was further confirmed by Lin et al [21], who performed slot die coating experiments using the same PMMA particles in glycerol suspensions. However, the ribbing defect appeared not to be affected in the presence of particles.…”

“…However, in industrial applications, particles of different types are frequently added to coating solutions for functional requirements. For example, titanium oxides or silica particles are often added to solutions to produce backlit films for ink jet applications [19, 20]; poly(methyl methacrylate)(PMMA) particles are used to make diffusers for liquid crystal display panels [21]. Despite its important industrial applications, very few research articles dealing with suspension coating are available in open literature.…”

Coupled fluid‐particle modeling of a slot die coating system

“…In this simulation (with particle concentration of 4 vol %) the particles’ affect on liquid phase is minimal. At higher particle concentrations (>5%), particles may slightly increase the viscosity of coating fluid and expand the operating window . In the next section the effect of adhesion force between particles is studied.…”

“…Particle addition to coating solution (>5% volume fractions) can increase the surface tension and viscosity of the solution and as a result a more stable coating can be achieved . Addition of particles to the coating solution would also lead to particle agglomeration within the die.…”

The effect of shear‐thickening on the stability of slot‐die coating

“…Literature also exists on slot‐die coating of particle dispersions . Yamamura et al conducted coating tests on dilute suspensions of poly(methylmethacrylate) (PMMA) particles.…”