| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 103 === 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.
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