本研究為利用一外部接觸雙層共擠壓型模具針對聚乙烯醇(polyvinyl alcohol , PVA)水溶液進行塗佈實驗。過去文獻對於PVA/水溶液研究指出，PVA/水溶液其黏度隨濃度之增加，其塗佈視窗也隨之變大，此乃導因於在高剪切率下呈現剪切稀化的效應以及高分子長鏈有助於液膜的穩定性所致。故實驗以高黏度之PVA/水溶液作為底層，藉其最大塗佈速率較高之特性，來提高低黏度PVA/水溶液的塗佈範圍。同時並探討流體物性及操作參數對其塗佈視窗所造成之影響。
研究主要是以俞文正(1994)對甘油/水溶液進行外部接觸型雙層共擠壓塗佈模式作為基礎架構，對PVA/水溶液進行研究，且進一步以塗佈視窗來對塗佈特性進行分析，經實驗結果發現，以高黏度的PVA/水溶液作為攜帶層的確能提高上層操作流體之塗佈範圍，且隨攜帶層黏度增加使塗佈視窗變大；而後漸趨於緩和，在低塗佈速率下出現之缺陷為下層溶液主導之空氣滲入現象；而在高塗速下則多為上層低黏度溶液所支配的橫向波現象。另外在改變不同上下層流量比的研究中發現，我們可在一特定黏度之雙層共擠壓塗佈中，找到一最適流量比，在此流量比下操作，可得到最大的塗佈視窗，而在流量比過大時，因上層溶液在高塗速下之不穩定性，產生橫向波及破裂線等現象；而在流量比低時易因上層溶液膜厚過低而出現spreading failure現象。

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.

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