逐次雙軸延伸之製程參數對於聚丙烯薄膜內部結構之影響

Abstract

為增進高分子薄膜之附加價值，提升現有產品之性能與創造新的應用方式，解明高分子薄膜的結構之形成機制具有相當的必要性。本研究以單一聚丙烯(PP)材料射出成形製成薄板作為延伸基材，透過分析不同製程參數下所形成的薄膜成品間之內部結構差異，歸納出逐次雙軸延伸製程中的各項延伸參數，對於薄膜內部結構的影響。延伸後的薄膜成品透過光學顯微鏡進行內部結構觀測，並結合透濕性測試結果，分析在平面逐次雙軸延伸製程中，薄膜的結構、性能與延伸參數間的關係。 研究結果指出，在逐次雙軸延伸製程中，先沿機械方向(MD)延伸，促進薄膜產生配向性，再以較低溫度進行垂直方向(TD)之預延伸促使其內部結構分離，會使薄膜在之後的垂直方向主延伸後，容易於內部形成網狀結構。而在薄膜沿機械方向延伸期間，將MD延伸溫度在結晶溫度區間內提高，會使薄膜具有較多的網孔結構；另一方面，MD延伸倍率的增加，雖然會降低薄膜的厚度，使得粒子穿透路徑縮短，但在內部結構上會產生網孔結構縮小的現象，抑制了透濕度的增加幅度。在薄膜進行垂直方向之預延伸時，TD預延伸溫度越高，則網孔結構越難以形成；而TD預延伸倍率的增加，會使網孔結構的尺寸差異增大，薄膜厚度增加且透濕度下降。在進行最終沿垂直方向主延伸時，TD延伸溫度之增加會使材料變得更易均勻變形，形成的網孔尺寸較為均勻，厚度變得較薄，透濕度也大幅提升，但TD延伸溫度過高時使網孔結構比例與平均尺寸大幅下降，透濕度也明顯減少；而TD延伸倍率之增加，會使薄膜厚度減少，網孔結構比例以及網孔尺寸也明顯上升，透濕度亦隨之增加，適用於調整網孔尺寸。 本研究對於聚丙烯薄膜結構形成機制，以及各階段延伸過程之延伸參數對於薄膜結構與性能的調節之解析，能夠改善現有薄膜產品之品質，以及作為產品設計時調整薄膜內部結構、厚度與透濕性之依據，進而節省產品開發時間與生產成本。

To improve the qualities of polymer membrane products and to expand their potential uses, the mechanisms involved in formation of the interior membrane structure must be analyzed. This study investigated the interior structures of sequential biaxially stretched membranes to determine how stretching parameters affect the interior membrane structure. The polypropylene sheets for stretching were made by injection molding. The effects of stretching parameters to the interior structures and characteristics of membranes were analyzed by optical microscopy investigation and moisture permeability test. Experimental results indicate that, in the sequential biaxial stretching process, increasing the orientation of the structure of a membrane produced through machine direction (MD) stretching and subsequently separating the interior structure of the membrane by pre-stretching it in the transverse direction (TD) at a low temperature can yield a net-like interior structure after TD stretching. During MD stretching, additional net-like structures can be generated in the membrane by increasing the MD stretching temperature to a temperature approximately equal to crystallization temperature. Although increasing the MD drawing ratio can reduce the thickness of the membrane and shorten the path of moisture transmission, it can also reduce the sizes of the net-like structures and limit the increase in membrane moisture permeability. Increasing temperature during TD pre-stretching markedly reduces the number of net-like structures. Increasing the drawing ratio of TD pre-stretching causes inconsistencies among the sizes of the net-like structures, increases the thickness of the membrane, and reduces the moisture permeability of the membrane. Increasing TD stretching temperature can cause the membranes to become more rubbery, producing net-like structures with consistent sizes, reducing the thickness of the membrane, and increasing the moisture permeability of the membrane. However, when the TD stretching temperature is excessively high, the quantity and the average size of the net-like structures tends to decrease, causing a reduction in membrane moisture permeability. Expanding the drawing ratio in TD tends to increase the quantity and sizes of the net-like structures. The thickness of the membrane decreases and the moisture permeability of the membrane improves with an increase in the TD drawing ratio. The result suggests that control of the TD drawing ratio would be suitable to control the sizes of the net-like structures. This study analyzed the interior structure forming mechanism of membranes. The effects of stretching parameters to the interior structures and characteristics of membranes are identified. The experimental results suggested several ways to improve membrane quality, and guides for parameter adjustment during membrane design to make product development more efficient and effective.