Preliminary Study on the Mold Flow and Structure in the Co-extrusion of EVOH/PP Multi-layer Films
本研究採用EVOH 與PP二種材質做為研究材料，研究中將使用自行設計之分流模具、擴張模具，搭配實驗室先前架設之共擠出設備，將原為兩層之高分子膜以二的倍數反覆交疊，逐漸增加層數並減低各層厚度，製作EVOH/PP超多層膜，並以熱差掃描熱量分析儀(Differential Scanning Calorimeter, DSC)、光學偏光顯微鏡、掃描式電子顯微鏡觀測各薄膜之結晶度及結構變化。
Because EVOH and PP exhibit superior barrier capabilities against oxygen and water vapor, the combination of these dissimilar materials in a multilayer film can synergistically improve the overall barrier performance and reduce costs. Numerous studies have indicated that the improvement of barrier capabilities in multilayer films is a consequence of crystalline deformation or dispersion. In this study, we investigated a multilayer film whose overall thickness remains unchanged but whose internal layers are significantly multiplied, resulting in a substantial decrease in the thickness of each layer. The transformation affects the internal microstructure and polymer crystallinity of the film. EVOH and PP were used as research materials. We developed a new mold design that can separate and expand co-extruded polymer films using a co-extrusion system. The mold enables repeated overlay of two polymer layers to reduce the layer thickness and increase the number of layers. The specimens were subsequently compared by optical microscopy, scanning electron microscopy, and differential scanning calorimetry to determine the relationships among layer thickness and crystallinity. The results indicate that, as the space between each layer decreases under compression, the crystallinity also begins to decrease. When the thickness is less than 30 μm, the PP crystallinity begins to decrease. The crystallinity of the PP core layer significantly decreases from 36.94% in the case of 45μm layer thickness to 15.8% in the case of 15μm layer thickness. The size of the spherulites visible in the thicker layers is substantially reduced in the thinner layers. Furthermore, the microstructure is becoming more homogeneous and more dispersed. The crystallinity results and the polarized optical microscope observations indicate the same conclusion, i.e., space is a very important factor for crystal growth. The crystallinity of a polymer will sharply decrease when the layer thickness is smaller than the spherulites’ size scale. Hopefully, the results of this research will pave the way for further research in multilayer films.