多層化結構高分子薄膜之製備與特性分析之研究

Abstract

多層化結構高分子薄膜是一種單一結構的薄膜產品，包含了兩種或兩種以上的高分子，彼 此之間形成多層化且平行的結構。高分子薄膜通常用於包裝用途，例如食品包裝、葯品包 裝及化妝品包裝，這些用途的包裝膜特性需求通常不只一種，而單一材料所製備的高分子 薄膜又無法提供多樣化的性質。因此將兩種或兩種以上高分子結合成多層化結構的薄膜產 品就應運而生了。多層化結構高分子薄膜的製備方式通常有兩種，一種為共擠押製備，另 一種為混鍊製程，本研究將對這兩種製程所製備的薄膜特性做分析研究。 由共擠押製程所製備具有多重性質的多層化結構高分子薄膜，其整體性質的表現則是來自 於各層性質的貢獻。本研究以HDPE/tie/PA-6為例，來驗證性質預估的方法，由各層的性質 來預估任何不同厚度組成三層薄膜的拉伸性質及氣體穿透速率，若這預估的方法有效，對 於在薄膜組成的設計與搭配上，將會是一個非常經濟及有效率的方法。用series model來 預估氣體穿透率（包含氮氣、氧氣、二氧化碳及水蒸氣），與實驗值有非常好的吻合性。 在拉伸性質方面，首先找到適當的構成方程式來描述薄膜的拉伸行為，而構成方程式中用 來描述三層薄膜拉伸行為的各個參數，可經由各層參數及additive rule來進行預估，從實 驗值與預估值的比較結果來看，在低速的拉伸測試時，有很好的吻性，但在高速拉伸時， 由於黏滯熱（viscous dissipation）的產生，導致了實驗值與預估值之間的不一致。 在一般的共擠押製程中，由於黏著層的層數佔將近一半的層數，如五層薄膜LDPE/tie/ EVOH/tie/LDPE，而黏著層的主要功能為界面黏著，對整體的性質表現無實值的貢獻，因此 在本研究中，將黏著層的塑料混鍊至LDPE塑料中而當成混鍊層（blend layer），如此不僅 可增加LDPE與EVOH的接著性，並藉此將原有的五層減少為三層薄膜，而形成blend/EVOH/ blend的三層高分子薄膜，如此也可降低共擠押模具的設計成本，簡化製程上的操作。在固 定混鍊層與EVOH的厚度比例下，界面黏著力，隨著黏著塑料在LDPE中含量的增加，黏著力 愈強，由FTIR的分析圖來看，這是由於黏著劑的馬來酸酐與EVOH的氫氧基反應形成化學鍵- 酯基（ester band）所造成的結果。而在拉伸強度的表現上，黏著塑料含量的改變並沒有 造成明顯的變化，在撕裂強度，卻隨著黏著劑含量的增加，而明顯的下降。在氧氣穿透率 方面，在固定黏著劑的含量下，改變混鍊層與EVOH的厚度比例，氧氣穿透率有很明顯的下 降。而在水蒸氣穿透率方面，則是呈現持平，而並沒有如預期般的上升，這是由於在EVOH 中氫鍵的產生，由FTIR中可知，隨著EVOH的增加，氫鍵的逐漸的增強，因此造成水分子不 易穿透，因此呈現持平的結果。 另外在本研究中，欲利用另一種製程，製備具有多層化形態的高分子薄膜，使這單一薄膜 同時具有各成份的特性，此種製程為混鍊製程。欲混鍊的材料為高阻氧性的材料-EVOH、高 阻水氣材料-LDPE以及相容劑-LDPE-g-MAH，經由單螺桿塑化後進入吹袋模具，而製備成薄 膜。由SEM圖來看，隨著相容劑含量的增加，分散相EVOH的長度逐漸的變短，但層化的數目 也愈多。另外由OM圖來看，分散相EVOH的形狀則是逐漸由類似長條纖維狀而變成小顆粒的 圓球狀。由DSC的分析圖得知，在第一次升溫掃描時所求得的熔化熱比第二次高了許多，這 是由於在製備薄膜的程中造成了順向結晶（stress-induced crystallization）所造成。 在氧氣阻隔性方面並不如預期般的有明顯的下降，這是由於在製備薄膜的過程中，因為高 度的雙延伸，造成分散相與連續相之間產生微微孔洞（microvoid）所導致的結果。在拉伸 性質方面，縱向（MD）的拉伸強度在相容劑為1phr時，有一極大值，這是由於此時較為剛 性的EVOH的形狀呈現長條狀，因此具有纖繀強化（fiber-reinforcement）的作用，隨著相 容劑的增加，EVOH的形狀呈現圓球會，此作用便消失，再加上兩相的界面產生了微孔洞， 因此強度便下降。在TD（縱向）的拉伸強度，應會隨著相容劑的增加而逐漸的增加，但是 在高含量的時候，卻呈現持平的趨勢，這是由於微小孔洞將強度有所抵消所致。

Laminar polymeric films are usually fabricated by coextrusion and blending processes. In a coextrusion process, a laminar polymeric film is formed into multilayer and parallel structures. In a blending process, on the other hand, the laminar polymeric film formed has laminar morphology of dispersed phases in its blend film. Because of the wide range of applications of laminar polymeric films as packaging materials, studies of the processes for forming, and the characteristics of, laminar polymeric films have become increasingly important. In this study, the aims are to predict the properties of the multilayer film and investigate the effect of adhesive on the laminar polymeric film. In Chapters 2 and 3, we successfully fabricated three-layer (A/B/C) films, comprising high-density polyethylene (HDPE), tie layer [high-density polyethylene-grafted maleic anhydride (HDPE-g-MAH]], and polyamide-6 (PA-6), by a coextrusion blown-film process. The tensile behavior of the three-layer film can also be predicted from its component layers by using an additive rule and an empirical constitutive equation — , where σT, εT, and are the true stress, the true strain, and the true strain rate, respectively, K and γ ε are constants, and m is the strain rate sensitivity — and a simplified constitutive equation — , where εT, σ0, and γ are the true strain, true yield stress and the strain hardening parameter, respectively — over the range of plastic deformation. There exists a good agreement between the experimental and predicted data at low crosshead speeds, but there is a relatively large discrepancy between them at high speeds, for both constitutive equations, because of the heat generated during deformation. The valid strain range for the latter is smaller, however, than that for the former. On the other hand, the series model was examined to predict permeability of HDPE/tie/ PA-6 three-layer film; there exists a good agreement between experimental data and this model for predicting both gas and water vapor permeabilities of three- layer films containing various volume fractions of PA-6. Conventionally, one or more tie layers are used in coextrusion processes, e. g., in the preparation of HDPE/tie/PA-6 mentioned above, but having additional tie layers in a coextruded film makes the fabrication process more complex and expensive. To eliminate the need for tie layer(s) and to reduce the number of layers, we have also successfully fabricated three-layer (A/B/A) films, comprising a varying content of ethylene–vinyl alcohol copolymer (EVOH) as the internal layer and blends of low-density polyethylene (LDPE) and adhesive [ low-density ethylene grafted with maleic anhydride (LDPE-g-MAH]] as the external layers, by a coextrusion blown-film process. In Chapter 4 ,we describe our investigation of the mechanical properties and compare their oxygen and water vapor permeabilities to a series model reflecting the content of adhesive. The peel strength increased sharply at LDPE-g-MAH content > 12.5 wt%; we associate this increase with a promotion of adhesion between layers that arises from the formation of ester bonds, as determined by FTIR spectroscopy, between EVOH and LDPE-g-MAH. The tensile strength did not change significantly upon increasing the LDPE-g-MAH content, but it had a small effect on elongation and modulus in both the machine and transverse directions. Tear strength decreased continuously, in both directions, upon increasing the LDPE-g-MAH content. The oxygen permeabilities of the three-layer films remained almost constant upon varying the amount of LDPE-g-MAH and all followed the series model. The water vapor permeabilities of the three-layer films, however, were affected by the degree of hydrogen bonding, which we analyzed by FTIR spectroscopy, to result in a discrepancy between the experimental findings and the series model, especially when the EVOH content was increased. An alternative process to fabricate laminar polymeric film is the blending process. In Chapter 5, we investigated the morphological, thermal, barrier, and mechanical properties of low-density polyethylene/ethylene–vinyl alcohol blend (LDPE/EVOH; 85/15 wt%) in highly and biaxially oriented blown films. We used linear low-density polyethylene-grafted maleic anhydride (LDPE-g-MAH) in various amounts as the compatibilizer for this immiscible system. Thermal analyses of the blend films indicated that their melting temperatures, crystallization temperatures, and heats of fusion remain almost constant upon varying the amount of compatibilizer. The addition of the compatibilizer did not adversely affect the inherent properties of the blends, especially their barrier properties, through constraint effects of the grafted EVOH (EVOH-g- LD). The heat of fusion of EVOH obtained during the first heating was much higher than that of the second as a result of the stress-induced crystallization that occurs during the blown-film process. Oxygen permeation measurements demonstated that the oxygen barrier properties of both highly and biaxially oriented blown films decrease upon increasing the amount of compatibilizer, although morphological analysis indicated that the blends exhibit better laminar dispersion of the EVOH phase in the LDPE. The increase in oxygen permeability results from the formation of microvoids at the interface between the two phases during the blown-film process. Mechanical measurements indicated that there exists an optimal amount of LDPE-g-MAH at which both the tensile and tear properties are maximized in both the machine and transverse directions.