The effects of pH, molecular weight and degree of hydrolysis of poly(vinyl alcohol) on slot die coating of PVA suspensions of TiO2 and SiO2

Slot die coating technology is currently the most widely used method for production of optical films. The main advantages of this coating method are its ability to pre-determine the product film thickness and to produce uniform film under prolonged operation. In the present study, a laboratory-scale slot die coating equipment was used to investigate the effects of pH, molecular weight and degree of hydrolysis of polyvinyl alcohol (PVA) on the coating window of PVA suspensions of TiO2 and SiO2. Measurements of steady shear viscosity, surface tension, and amount of PVA adsorption on particle surfaces of different suspensions, together with observations of coating bead by means of a flow visualization technique, were used to analyze the coating flow behavior. Changing the pH of the suspension affects both the surface tension and the amount of PVA adsorbed on the particle surface. While surface tension and degree of PVA adsorption were found to increase with increasing pH for the TiO2 suspensions, opposite effects were observed for the SiO2 suspensions. The coating bead was found to be more stable for suspensions with higher values of surface tension and PVA adsorption, thus delaying the occurrence of air entrainment. Four different PVA with varying molecular weight and degree of hydrolysis were used in the suspensions. Higher degrees of hydrolysis resulted in an increase in polymer adsorption, and hence larger surface tension. By contrast, higher molecular weight PVA resulted in a higher viscosity and a smaller surface tension, thus causing air entrainment to occur at a much lower maximum coating speed.