Abstract
This research is to investigate the coating window for the double-layer coextrusion coating of PVA solutions. From the past literatures, we know that the viscousity of PVA solution increases with concentration, and the coating window also enlarges with it. It is because PVA solutions have shear-thinning effect and extended long polymer chains under high shear rates, and both effects are helpful for the stability of the resulting liquid film. Therefore we use PVA solutions of higher viscousity as the carrier-layer to enlarge the coating window for PVA solutions’ of lower viscousity. I also analyze the effect of various operating factors. I found that highly viscous PVA solution can effectively serve as the carrier layer. The coating windows enlarge with the viscousity of carrier layer and the effect becomes saturated gradually. The coating defects under low coating speeds mostly are air entrainment which are dominated by the carrier layer, but under high coating speeds the defects become ribbing, which are dominated by the upper layer. Additionally I found there exists an optimal floe rate ratio for the upper/lower layer. Operating under the optimal flow rate ratio, I can get the largest coating window, and if our flow rate ratio is greater than the optimal one, under high coating speeds the upper layer would become unstable and produce defects like ribbing and break line, etc. that make the coating window smaller. If the flow rate ratio is less than the optimal one, there would occur a kind of defect called “spreading failure” making the coating window shrink.