Coating window for double layer extrusion slot coating of poly (vinyl‐alcohol) solutions

The expansion of coating windows for low-viscosity poly(vinyl-alcohol) (PVA) solutions is achieved via utilization of a higher viscosity PVA solution as the stabilizing carrier layer. It is found that PVA solutions of higher viscosities possess a higher degree of processability than those of lower viscosities, and can serve as a stabilizing carrier layer when coating lower viscosity PVA solutions. The stabilizing effect of the carrier layer enhances with increasing viscosity of the carrier layer, but reaches a saturation state at a high enough viscosity. It is further found that the coating defects occurring at lower maximum coating speeds are dominated by the lower, carrier layer, while those at higher maximum coating speeds are controlled by the upper, carried layer. Consequently, there exists an optimal layer flow rate ratio at which the coating window is the largest. A new type of coating defect, called spreading failure, is observed for coating situations involving a thin and much less viscous upper layer.