Abstract
In the past, most researchers have focused mainly on polymer solutions as coating fluids. Very few studies have examined the coating behavior of suspensions. In this study, two simplified systems that mimic the real industrial applications were chosen to analyze the coating flow. One is related to the light diffusing backlit film which uses TiO2 and/or SiO2 particles dispersed in polyvinyl alcohol(PVA) solutions and the other one is the diffuser film in flat display panels where PMMA particles are dispersed in glycerol solutions. Two methods, slot die and curtain coating were employed in the investigation. The main advantages of these two methods are their ability to pre-determine the product film thickness and to produce uniform film under prolonged operation. It was found that the stable coating windows were enlarged with the addition of particles in the (PVA) coating solutions, and the windows were increased with solid concentration. This is due to the strong interactions between polymer and particles, resulting in a higher viscosity and surface tension. The upstream coating bead is more stable with the addition of particles and the maximum coating speed is extended to a higher value, hence the coating window becomes larger. Although both viscosity and surface tension appear to contribute to the stability of coating flow, the effect of surface tension is more signifactant. Surface tension of a suspension consists of porous particles is higher than the one containing hard solid particles. Consequently, the coating window obtained with the former is substantially larger than the latter. Flow visualization revealed that under the same operating conditions, the upstream dynamic contact angle for the suspension was smaller than that for the aqueous polymer solution. This observation could be related to the stability of the upstream coating bead, and hence the coating window. The experimental flow fields were verified numerically with the aid of a numerical simulation package Flow 3D. Changing the pH of the suspension affects both the surface tension and the amount of PVA adsorbed on the particle surface. In the present study, the size of the coating window increases as pH increases for the TiO2 suspensions, but decreases for the SiO2 suspensions. This is because a greater number of PVA macromolecules are adsorbed on the TiO2 surfaces at basic environment, and the surface tension is slightly increased. The opposite effect was observed for the SiO2 suspension. Larger amount of PVA macromolecules are adsorbed on the SiO2 surfaces when pH is changed from basic to acidic, resulting in a higher surface tension for the SiO2 suspension at low pH. The larger the amount of PVA adsorbed, the stronger are the steric forces surrounding the particles. These forces are able to withstand higher stress fields in slot die coating, thus delaying the occurrence of air entrainment, resulting in an increase in the maximum coating velocity. In addition, the degree of hydrolysis and the molecular weight of PVA also affect the coating behavior. Higher degree of hydrolysis results in a larger surface tension and more adsorption of polymer onto the particle surfaces. As a result, the coating bead becomes more stable and the size of the coating window increases. The dimensionless film thickness increases with capillary number Ca which is the ratio of viscous force over surface tension and approaches a constant value for the high molecular weight PVA suspensions, but is only a weak function of Ca for the low molecular weight, low viscosity suspensions. At high Reynolds number Re, which is the ratio of fluid inertial to viscous force, the stability of the coating bead is controlled primarily by fluid inertia instead of surface tension. The effect of adding PMMA particles in glycerol/water solutions on curtain coating was also investigated. The aim is to study the relationship between the maximum coating speed with the coating solution viscosity, the height of the edge guide and the PMMA particle concentration in the suspension. It was found that the maximum coating speed decreased with increasing viscosity, as a result of either pure glycerol/water solution or by dispersing PMMA particles in the coating solution to form a suspension. This was due to the fact that coating solution with lower viscosity could sustain a higher speed at the dynamic wetting line. Consequently, a higher coating speed could be achieved for low viscosity fluids. Also, stable flow can be maintained at a much higher inertial force by increasing the curtain height, resulting in a postponement of the onset of air entrainment. Another positive effect is to increase the particle concentration in the coating solution. It appeared that higher particle number density could resist the air film from destroying the dynamic contact line, and hence, postponing the coating defects to occur at a higher coating speed.