含有機/無機粒子塗液於精密模具塗佈流動之分析

Analysis of suspensions with organic/inorganic particles on precision die coating flow

朱文彬(Wen-Bing Chu)

指導教授 ： 劉大佼

國立清華大學/工學院/化學工程學系/博士(2007年)

摘要

塗佈研究以往集中在對高分子溶液系統進行探討，而目前工業應用上則有極多塗液為懸浮液體，因此本研究選擇兩種具代表性的塗佈懸浮液，進行其塗佈行為之分析，其系統分別為(1)TiO2及SiO2粒子添加在聚乙烯醇(polyvinyl alcohol, PVA)溶液，此為噴墨燈箱片之簡化系統；(2) PMMA粒子添加在甘油/水溶液，此為面板關鍵零組件擴散片之簡化系統。本研究使用狹縫式塗佈(slot die coating)及淋幕式塗佈(curtain coating)為製程載具，這兩種塗佈方式皆屬於預調式塗佈技術，其最大特點在於可預先計量且在長時間操作下具有相當高的塗膜均勻性及穩定性。
添加無機粒子到聚乙烯醇溶液中經實驗發現，添加粒子後懸浮液塗佈視窗較單純PVA溶液大，且隨著粒子濃度的增加有明顯變大的趨勢。主要原因是高分子與粒子間的強吸附導致黏度及表面張力增加，使得塗佈液珠上游彎月面更加穩定並延後缺陷發生的時機，因此使得塗佈視窗變大。雖然黏度及表面張力對塗佈液珠的穩定有不同之影響，但發現表面張力的效應遠大於黏度，且表面具有孔洞結構的粒子會較實心粒子有較大的表面張力及塗佈視窗。
我們亦建立一套即時顯微觀測技術以觀察塗佈液珠形狀，發現在相同的塗佈操作條件下，含有粒子的懸浮液其上游動態接觸角較高分子溶液來的小，這個現象也顯示出塗佈液珠的上游彎月面會較穩定，因此得到較大塗佈視窗。同時我們也利用理論工具(Flow 3D)進行塗佈液珠流場之行為研究，可以了解流場內速度與壓力之分佈。
具TiO2及SiO2之懸浮液系統，pH值改變將影響懸浮液體的表面張力及PVA的吸附量，在本研究中我們發現TiO2系統的塗佈視窗會隨著pH值增加而變大，而SiO2系統的塗佈視窗會隨著pH值增加而變小，這是因為TiO2懸浮液在鹼性環境下有較多的PVA分子被吸附在TiO2表面，且此時的表面張力略微增加；相反的，當SiO2懸浮液從鹼性到酸性的環境下會有較多之PVA高分子吸附在粒子表面且表面張力較大，當粒子表面吸附較多的PVA分子會使得粒子間的立體障礙排斥力增強，此時內聚力的增加將可承受較大的剪切應力，因此有助於塗佈液珠的穩定，而使得最大塗佈速度時的缺陷延後發生。
此外，PVA的分子量及鹼化度也會影響塗佈行為，研究結果發現鹼化度愈高的懸浮液系統擁有較大的表面張力及吸附量，有助於穩定塗佈液珠行為，因而得到更大的塗佈視窗。對於高分子量的PVA懸浮液，其無因次濕膜厚度隨著capillary number有先增加而後趨於定值的趨勢，然而對於低分子量的PVA懸浮液其無因次濕膜厚度與capillary number關係不大，反而是此時有較高的Reynolds number，代表慣性力已取代表面張力主導整個塗佈液珠的穩定行為。
本研究亦分析含不同粒徑PMMA粒子之甘油/水溶液，在淋幕式塗佈時空氣滲入速度延後發生與流體黏度、邊桿高度及粒子濃度間的關係。結果發現，不論是單純的甘油/水溶液或是添加PMMA粒子於甘油/水溶液後的懸浮液，其最大塗佈速度皆隨黏度的增加而下降，這是因為低黏度的流體在動態潤濕線處可承受較高的速度變化才會破壞液膜的穩定，因此可得到較高的塗佈速度。隨著邊桿高度的增加，液膜從高處落下產生的慣性力有助於延後空氣的滲入時機而得到較高的塗佈速度。當粒子數量密度增加時，亦有助於阻擋空氣破壞動態潤濕線，因此可有效延後空氣滲入，而得到更高的穩定塗佈速度。

關鍵字

精密塗佈 ； 聚乙烯醇 ； 粒子 ； 淋幕式塗佈 ； 狹縫式塗佈

並列摘要

In the past, most researchers have focused mainly on polymer solutions as coating fluids. Very few studies have examined the coating behavior of suspensions. In this study, two simplified systems that mimic the real industrial applications were chosen to analyze the coating flow. One is related to the light diffusing backlit film which uses TiO2 and/or SiO2 particles dispersed in polyvinyl alcohol(PVA) solutions and the other one is the diffuser film in flat display panels where PMMA particles are dispersed in glycerol solutions. Two methods, slot die and curtain coating were employed in the investigation. The main advantages of these two methods are their ability to pre-determine the product film thickness and to produce uniform film under prolonged operation.

It was found that the stable coating windows were enlarged with the addition of particles in the (PVA) coating solutions, and the windows were increased with solid concentration. This is due to the strong interactions between polymer and particles, resulting in a higher viscosity and surface tension. The upstream coating bead is more stable with the addition of particles and the maximum coating speed is extended to a higher value, hence the coating window becomes larger. Although both viscosity and surface tension appear to contribute to the stability of coating flow, the effect of surface tension is more signifactant. Surface tension of a suspension consists of porous particles is higher than the one containing hard solid particles. Consequently, the coating window obtained with the former is substantially larger than the latter.

Flow visualization revealed that under the same operating conditions, the upstream dynamic contact angle for the suspension was smaller than that for the aqueous polymer solution. This observation could be related to the stability of the upstream coating bead, and hence the coating window. The experimental flow fields were verified numerically with the aid of a numerical simulation package Flow 3D.

Changing the pH of the suspension affects both the surface tension and the amount of PVA adsorbed on the particle surface. In the present study, the size of the coating window increases as pH increases for the TiO2 suspensions, but decreases for the SiO2 suspensions. This is because a greater number of PVA macromolecules are adsorbed on the TiO2 surfaces at basic environment, and the surface tension is slightly increased. The opposite effect was observed for the SiO2 suspension. Larger amount of PVA macromolecules are adsorbed on the SiO2 surfaces when pH is changed from basic to acidic, resulting in a higher surface tension for the SiO2 suspension at low pH. The larger the amount of PVA adsorbed, the stronger are the steric forces surrounding the particles. These forces are able to withstand higher stress fields in slot die coating, thus delaying the occurrence of air entrainment, resulting in an increase in the maximum coating velocity.

In addition, the degree of hydrolysis and the molecular weight of PVA also affect the coating behavior. Higher degree of hydrolysis results in a larger surface tension and more adsorption of polymer onto the particle surfaces. As a result, the coating bead becomes more stable and the size of the coating window increases. The dimensionless film thickness increases with capillary number Ca which is the ratio of viscous force over surface tension and approaches a constant value for the high molecular weight PVA suspensions, but is only a weak function of Ca for the low molecular weight, low viscosity suspensions. At high Reynolds number Re, which is the ratio of fluid inertial to viscous force, the stability of the coating bead is controlled primarily by fluid inertia instead of surface tension.

The effect of adding PMMA particles in glycerol/water solutions on curtain coating was also investigated. The aim is to study the relationship between the maximum coating speed with the coating solution viscosity, the height of the edge guide and the PMMA particle concentration in the suspension. It was found that the maximum coating speed decreased with increasing viscosity, as a result of either pure glycerol/water solution or by dispersing PMMA particles in the coating solution to form a suspension. This was due to the fact that coating solution with lower viscosity could sustain a higher speed at the dynamic wetting line. Consequently, a higher coating speed could be achieved for low viscosity fluids. Also, stable flow can be maintained at a much higher inertial force by increasing the curtain height, resulting in a postponement of the onset of air entrainment. Another positive effect is to increase the particle concentration in the coating solution. It appeared that higher particle number density could resist the air film from destroying the dynamic contact line, and hence, postponing the coating defects to occur at a higher coating speed.

並列關鍵字

precision coating ； polyvinyl alcohol ； particles ； curtain coating ； slot die coating

參考文獻

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