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 v iscosities, and can serve as a stabilizing carrier layer when coating lower viscosity PVA solutions. The stabilizing effect of the carrier layer enhan ces with increasing viscosity of the car-her layer, but reaches a saturatio n state at a high enough viscosity. It is further found that the coating de fects 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 f low rate ratio at which the coating window is the largest. A new type of co ating defect, called spreading failure, is observed for coating situations involving a thin and much less viscous upper layer.