電漿改質多壁奈米碳管強化聚乳酸射出成型後的結晶性與熱穩定性之探討

Abstract

聚乳酸( polylactic acid, PLA)是以玉米澱粉提煉的乳酸為單體，是目前最具有發展前途的生物可分解材料，由於聚乳酸缺乏耐衝擊強度和熱穩定性，應用範圍被受限制。若要提升聚乳酸的機械性能和熱穩定性，可透過具有許多特殊性質的奈米碳管作為填充物，然而無機奈米碳管和有機高分子的不相容合性，導致奈米碳管無法在高分子中有效的分散，仍然無法改善聚乳酸的缺點。 本研究目的為探討電漿改質多壁奈米碳管添加於聚乳酸內對射出成型後材料的結晶性、熱穩定性與機械性能所產生的變化，以提供相關產業發展之依循。 研究第一部分以電漿改質技術，通入氧氣反應氣體，經過三組不同的反應時間後，使無機碳管表面帶有親水性官能基。透過化學分析光譜儀(ESCA)觀察改質後的碳管表面官能基，再將各組碳管與聚乳酸熔融混煉 (Melt-blending) 、造粒，並透過射出成型(Injection-Molding)獲得奈米複合材料。第二部分則將所射出之PLA/CNT 試片，藉由熱重分析儀(TGA）及熱量分析儀(DSC)檢測添加原始碳管、各組改質碳管對提升聚乳酸(PLA)熱穩定性效果如何。透過拉伸試驗機測量其機械性能，再利用掃描式電子顯微鏡(SEM)觀測試片拉斷面之超結構。 研究結果顯示，添加原始碳管無法改善聚乳酸射出成型後的機械性質與熱性質，而經電漿改質15min後的射出成型試片，結晶度有明顯的提升，玻璃轉換溫度(Tg)、熔融溫度(Tm)、拉伸強度分別可增至63.63℃、166.6℃、58.5MPa。 由一系列的分析可得，透過電漿改質方式，可解決昔日欲增強有機高分子材料而添加無機材料進行射出成型，卻又因無機與有機的不相容而產生反效果之窘境，此法可作為日後相關產業之參考。

PLA is a lactate monomer extracted from corn starch, which is the most promising biodegradable material. However, PLA’s application is limited due to its lack of resistance of impact strength and thermal stability. It is possible to use carbon nanotubes as fillers to improve PLA’s mechanical properties and thermal stability. Nevertheless, due to the incompatibility between inorganic carbon nanotubes and organic polymers, carbon nanotubes are unable to distribute effectively in polymers. Thus the disadvantages of PLA remain. This study is to investigate when adding the plasma-modified multiwall carbon nanotubes into PLA, the changes of material crystalline, thermal stability and mechanical properties after injection molding and to provide some guidance for future industry development. The first part of this study is to introduce hydrophilic functional groups onto the surface of nanotubes via plasma modification, using oxygen as reaction gas and undergoing three sets of different reaction time. To obtain the nano composite materials, using ESCA to observe the functional groups on carbon nanotubes’ surface after modification, then mixing different carbon nanotubes with PLA, pelletizing and injection molding. The second part is to analyze the injected PLA/CNT specimen by TGA and DSC, to evaluate the effectiveness of improved thermal stability with respect to original carbon nanotubes and each set of modified carbon nanotubes. The mechanical properties are measured by Material Testing System and the superstructures of specimen’s broken sections are observed by SEM. Results show that adding original carbon nanotubes is not able to improve the mechanical properties and thermal stability of PLA after injection molding. In contrast, the injected specimen with plasma modification for 15 min shows significant increase of crystalline, and the glass transition temperature, melting temperature and tensile strength increase to 63°C, 166°C and 58MPa respectively. A series of analysis indicate that via plasma modification, it is possible to solve the dilemma of adding inorganic materials for injection molding to strengthen the organic materials and the adverse effect caused by incompatibility between inorganic and organic materials. This method may be used as a reference for the related industries in the future.