The Effects of pH, Molecular Weight and Degree of Hydrolysis of Poly(Vinyl Alcohol) Suspensions on Slot Die Coating

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 TiO2 in polyvinyl alcohol (PVA) suspensions on the coating window. 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 pH affected the suspension rheology and its stability. Increasing the amount of PVA adsorbed on the TiO2 surface resulted in an increase in surface tension, which was responsible for the expansion of the coating window or an increase of the maximum coating speed at which flow defects occurred. 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.