Effects of the Molecular Weight and Concentration of Poly(vinyl alcohol) on Slot Die Coating

An experimental study was carried out to investigate the effects of the molecular weight and concentration of aqueous poly(vinyl alcohol) (PVA) solutions on the stable operating window of slot die coating. Various coating defects were observed outside the operating window. The window was found to expand with the PVA concentration and molecular weight increasing at low concentrations and to reach a maximum size at the gel point concentration, which corresponded to a critical Deborah number. Beyond this point, the effect of fluid elasticity became dominant, and the window began to shrink. This phenomenon was in contrast to that observed for low-molecular-weight glycerol solutions, in which the operating window was found to contract with the concentration or viscosity increasing. This anomalous behavior was attributed to the stretching or extension of long-chain PVA molecules, which effectively stabilized the fluid motion in the coating bead region. The maximum coating speed could be correlated with the PVA concentration and molecular weight in terms of the capillary number as a function of the dimensionless concentration. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 654--662, 2010