PLA結晶度與其延伸特性之關聯性之探討

Abstract

聚乳酸(Polylactic Acid, PLA)是以玉米澱粉提煉的乳酸為單體，為目前最具有發展性的生物可分解材料，但礙於聚乳酸的機械性質，使得其應用層面受到限制。若能有效的改善其機械性質，便能增加其應用範圍，而透過不同的加工方式使得聚乳酸分子內部產生高次結構，可改變聚乳酸的機械性質，因此若能有效的控制高次結構的產生，便能依不同的需求產生不同機械性質的聚乳酸成品。 本研究的目的是在探討PLA的晶度對其延伸加工之影響。研究中先利用射出成形製作PLA薄片，再將其進行熱處理，並藉熱處理時間的控制以獲得不同結晶度的PLA薄片供延伸加工使用。結晶度的量測係採用熱差掃描熱量分析儀(Differential Scanning Calorimeter，DSC)。本研究調查了熱處理時間對PLA結晶度的影響，並探討了PLA薄片結晶度對其最大延伸倍率的影響，以及延伸方向與延伸後的PLA薄片的結晶度的關係。 研究結果顯示：不同熱處理時間產生不同的結晶度，結晶度越高，試片的延性較差，最大延伸倍率較低。垂直於結晶方向的延伸製程所造成的結晶度下降成度相較於平行於結晶方向的延伸製程要來得多；而垂直結晶方向的延伸倍率相較於順著結晶方向的延伸也較低。 整體而言，若要製作PLA薄膜，則必須控制PLA於低結晶度的狀態下做延伸，且延伸方向要順著結晶方向，俟延伸過後再依產品需求藉由熱處理產生所需的高次結構。

Poly lactic acid (PLA), a lactate monomer extracted from corn starch, is the most promising biodegradable material. Although its application is limited due to its poor mechanical properties, these mechanical properties can be improved by super-structure generation. This study investigates the relationship between the crystalline and stretch characteristics of PLA. First, a PLA sheet is produced by injection molding, then the sheet is annealed. To obtain different degrees of crystallinity of a PLA sheet for stretch processing, annealing time must be controlled. Crystallinity is measured by Differential Scanning Calorimeter (DSC). This study also investigates the effect of annealing time on the degree of crystallinity of PLA, the effect of crystallinity on the maximum stretch ratio, and the relationship between stretch direction and crystallinity. Experimental results show that different annealing times create different degrees of crystallinity. As the degree of crystallinity increases, the ductility and the maximum stretch ratio decrease. The crystallinity of the stretch direction perpendicular to the crystal decreases more than in the stretch direction parallel to the crystal. The stretch ratio in the stretch direction perpendicular to the crystal is less than that in the stretch direction parallel to the crystal. Overall, it must be controlled in the low crystallinity when producing PLA thin film. And the stretch direction must along the direction of crystal orientation, then generate the super-structure by annealing after the stretch.