Dissertation summary Organic chemistry: Study on the manufacturing and properties of gas barrier multilayers films and applications

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Objectives of the dissertation: Researching and fabricating gas barrier multilayer films based on the polyme blends and exploring the applicability of packaging to preserve dry agricultural products.

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MINISTERY OF EDUCATION VIETNAM ACADEMY OF AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ------------- NGUYEN TUAN NAM STUDY ON THE MANUFACTURING AND PROPERTIES OF GAS BARRIER MULTILAYERS FILMS AND APPLICATIONS Scientific Field: Organic Chemistry Classification Code: 62 44 01 14 DISSERTATION SUMMARY HA NOI - 2020
The dissertation was completed at: Institute of Chemistry Vietnam Academy of Science and Technology Scientific Supervisors: 1. Dr. Nguyen Tien Dung The Faculty of Chemistry – Hanoi National University of Education 2. Dr. Nguyen Thanh Tung Institute of Chemistry - Vietnam Academy of Science and Technology 1st Reviewer: ........................................................................... ................................................................................. ................................................................................. 2nd Reviewer: .......................................................................... ................................................................................. ................................................................................. 3rd Reviewer: ........................................................................... ................................................................................. ................................................................................. The dissertation will be defended at Graduate University of Science And Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay District, Ha Noi City. At ….. hour….. date….. month …..2020. The dissertation can be found in National Library of Vietnam and the library of Graduate University of Science And Technology, Vietnam Academy of Science and Technology.
1 A. INTRODUCTION 1. Background Packaging plays an important role in the food supply chain. They are not only used to store, preserve and transport products but also used as a marketing tool to bring added value to the product. Packaging protects food from environmental influences such as oxygen, moisture, light, dust, volatile compounds and microorganisms [1], it acts as a barrier between the atmosphere around food and the outside environment. Oxygen and steam are the two main reasons leading to the deterioration of food quality. Therefore, the development of the gas barrier packaging products with low air and water vapor permeability is a research direction that has been interested recently. According to Smithers Pira, in 2015, the world consumed about 1.4 million tons of gas barrier packaging films, in 2016 it was 1.86 million tons, with the growth rate of 4.7%/year. The regions using the most gas barrier packaging films were Asia-Pacific with 30.9%, followed by Western Europe (27.6%) and North America (26.8%). Among the materials used for packaging, plastic in the form of thin films has many outstanding advantages compared to other packaging types such as: light, durable, elastic, transparent, resistant to air and vapor. High water, and at the same time have good heat sealing and sealing ability. Polyethylene (PE) is the most commonly used plastic for packaging thanks to its good moisture barrier, low cost, but poor resistance to O 2, aromas and essential oils. Like PE, PA6 has good moisture barrier but poor O2 and CO2 barrier. Therefore, the recent trend that scientists are most interested in developing is to combine these polymers with another polymer with high gas barrier ability in the form of polymer blends or multilayer films. The most widely used and highly gas barrier polymer is EVOH [2]. Combining EVOH with PE or PA6 can create a material that has both highly mechanical and good gas, solvent, and moisture resistance, suitable for applications requiring high gas barrier such as food packaging or dry agricultural packaging ... Agriculture plays an important role in Vietnam's economy with great developments. However, post-harvest losses of agricultural products are still high, from 15-20% for foods, the main reason is that the research, application and deployment of post-harvest technologies have not timely responded and are full of requirements. In the country, there are also studies on the use of gas barrier packaging to preserve dry food and agricultural products, but the gas barrier packaging products on the market today are all imported products with expensive prices. Stemming from the above problems, the thesis focuses on: "Study on the manufacturing and properties of gas barrier multilayer films and applications".
2 2. Objectives of the dissertation Researching and fabricating gas barrier multilayer films based on the polyme blends and exploring the applicability of packaging to preserve dry agricultural products. 3. Main contents of the thesis - Study on fabrication and properties of some polymer blends based on EVOH (PE/EVOH and PA6/EVOH polymer blends). - Fabrication and study on properties of gas barrier multilayer polymeric films based on polymer blends of EVOH. - Study on application of gas barrier multilayer packaging to preserve some dry agricultural products (maize, soybeans). 4. Cấu trúc của luận án The dissertation has 111 pages, including the Preface, Chapter 1: Overview, Chapter 2: Experiment, Chapter 3: Results and discussions, Chapter 4: Conclusions, Pubblications, with 29 images, 21 tables and 90 references. DISSERTATION CONTENTS CHAPTER 1. LITERATURE REVIEW In Vietnam, although the share of the agricultural sector in the country's GDP has decreased in recent years, its contribution to growth has remained stable at 16-18%. The agricultural still plays an important role when it creates more than 40% of total employment for the whole country. According to the World Food and Agriculture Organization (FAO), postharvest losses in developing countries amount to 20-30%. In our country, the statistics show that the loss of yield during and after harvest for rice of 11-13%, maize of 13- 15%, concentrated in the stages of harvesting, drying, preserving and milling. The main cause is due to harvesting, packaging, transport and storage. On the basis of synthesizing domestic and foreign research documents, it can be seen that gas barrier storage is an advanced preservation method, widely applied in the world today. Packaging materials capable barrier of gas, moisture and solvents are increasingly popular, especially in the field of food preservation and dry food. However, non-polar hydrocarbon solvents and their mixtures (such as xylene, toluene, white oil, etc.) can easily seep through the conventional polymer packaging (PE and PP) to degrade and cause health and food safety issues. Therefore, combining polymers with specific features into new polymer materials that have all the preeminent features of the component polymers, meeting the technical requirements of gas barrier packaging products are essential. .
3 Studies have shown that EVOH has good hydrocarbon barrier properties and is considered an oxygen barrier in food packaging applications where oxygen is essential due to its high oil resistance and gas barrier properties. However, its moisture-absorbing properties, being relatively brittle and expensive, diminish its advantages. In order to reduce costs while maintaining good gas barrier properties, studies on polymer blends of EVOH have attracted a lot of attention in recent years, especially polymer blends of EVOH with PE and PA6. In the gas barrier packaging technology, the multi-layer extrusion technology proved to be effective in improving the poor gas barrier properties of polyolefin packaging. This technique allows the desired properties of polymers to be combined into one structure with enhanced performance. In this way it is possible to take advantage of the inherent barrier properties and protect the plastic layer from high relatively humidity. In addition, this plastic layer is also protected from corrosion, avoiding miscibility problems and improving transparency of film. In Vietnam, there are no research projects on manufacturing gas-barrier multilayer polymeric packaging for agricultural preservation applications, especially dry agricultural products. Therefore, this dissertation chooses the topic "Study on the manufacturing and properties of gas barrier multilayer films and applications". CHAPTER 2. EXPERIMENTS 2.1. Materials and equipments 2.1.1. Materials Linear low density polyethylene (LLDPE), ethylene-vinyl alcohol copolymer (EVOH), low density polyethylene graft maleic anhydride (PE-g- MAH, MAH content of 0,1%), polyamide 6 (PA6), corn seed HQ 2000, soybean seed DT96. 2.1.2. Equipments Supermix high-speed mixer, Brabender Plastograph®EC device (Germany), hydraulic press, LyssyL-100-5000 water vapor permeability tester, N500 gas permeability tester, plastic SD-35 single screw extruder, 3SJ-G2000 multilayer film blowing device, BP-1068 mechanical measuring device, Thermo Nicolet Nexus 670 Fourier Transform Infrared Spectroscopy, differential scanning calorimeter (DSC 204 F1 Phoenix) and a thermogravimetry analysis system (TGA 209 F1 Libra), SM-6510LV and JEOL 6490 scanning electron microscope, BZQ 500 vacuum device, Mitutoyo IP67 electronic film thickness measuring device, scientech scales, readability 0,001 (g), oven and laboratory equipments.
4 2.2. Study methods 2.2.1. Preparation of polymer blends based on EVOH 2.2.1.1. Preparation of PE/EVOH polymer blends LLDPE and EVOH resins, PE-g-MAH compatibilizer were melt-blended in an internal mixer model Plastograph® EC (Germany) at 190 oC, rotor speed of 45 rpm for 5 min. The PE/EVOH polymer blends were compression moulded into 1 mm thick plates on a GoTech hot press at 190 oC at a pressure of 20MPa for 5 min. 2.2.1.2. Preparation of PA6/EVOH polymer blends PA6 and EVOH resins were melt-blended in an internal mixer model Plastograph® EC (Germany) at 215 oC, rotor speed of 30 rpm for 5 min. The PA6/EVOH polymer blends were compression moulded into 1 mm thick plates on a GoTech hot press at 190 oC at a pressure of 20MPa for 5 min. 2.2.2. Preparation of gas barrier multilayer films based on EVOH 2.2.2.1. Preparation of PE/PE-EVOH/PE gas barrier multilayer films The film blowing process was performed on a multi-layer film blowing device Model 3SJ-G2000, with the following layer structure: LLDPE/PE-g- MAH polymer blend (layer 1)/PE/EVOH polymer blend (layer 2)/LLDPE/ PE-g-MAH polymer blend (grade 3). The parameters of the 3 screws in the film blowing device are as follows: screws 1 and 3 with a diameter of 65mm, the ratio of L/D = 30, the temperature of the heating zones and die heads are 180, 190, 200, 210, 210oC, screw speed 20 rpm; screw 2 has a diameter of 70mm, the ratio L/D = 30, the temperature of the heating zones and the die head is 210, 210, 220, 220, 220oC, screw speed 15 rpm. 2.2.2.2. Preparation of PE/PA-EVOH/PE gas barrier multilayer films The film blowing process was performed on a multi-layer film blowing device Model 3SJ-G2000, with the following layer structure: LLDPE/PE-g- MAH polymer blend (layer 1)/ PA6/EVOH polymer blend (layer 2)/LLDPE/ PE-g-MAH polymer blend (grade 3). The parameters of the 3 screws in the film blowing device are as follows: screws 1 and 3 with a diameter of 65mm, the ratio of L/D = 30, the temperature of the heating zones and die heads are 180, 190, 200, 210, 210oC, screw speed 20 rpm; screw 2 has a diameter of 70mm, the ratio L/D = 30, the temperature of the heating zones and the die head is 230, 230, 240, 240, 240oC, screw speed 15 rpm. 2.2.3. Application of gas barrier multilayer films in the preservation of some dry agricultural products 2.3.3.1. Application of gas barrier multilayer films in the preservation of maize grain Corn was selected according to standard 10TCN 513: 2002, is packed into bags with the size of 25x35cm, and thickness of 80 µm, weight 2 kg/bag, then vacuumed on the BZQ 500 machine (vacuum pressure -0.08MPa). The packed samples are stored in the laboratory. Samples were analyzed quality
5 indicators such as: moisture, protein, starch, lipid content and yeast-mold infection degree every month. 2.3.3.2. Application of gas barrier multilayer films in the preservation of soybean Soybean was selected according to standard TCVN 4849:1989, is packed into bags with the size of 25x35cm, and thickness of 80 µm, weight 2 kg/bag, then vacuumed on the BZQ 500 machine (vacuum pressure - 0.08MPa). The packed samples are stored in the laboratory. Samples were analyzed quality indicators such as: moisture, protein, lipid content, acidity in extract oil and yeast-mold infection degree every month. CHAPTER 3. RESULTS AND DISCUSSIONS 3.1. Study on preparation of polymer blends based on EVOH The dissertation has investigated two factors affecting the properties of the material: compatibilizer content and the ratio of LLDPE/EVOH. - The effect of the ratio of LLDPE / EVOH: content of LLDPE-g-MAH compatibilizer 4%, weight ratio of PE/EVOH in polymer blends 90/10, 80/20, 70/30, 60/40, 50/50. - The effect of the LLDPE-g-MAH compatibilizer content: the ratio of LLDPE/EVOH 70/30, compatibilizer content of 0-10%. 3.1.1. Study on preparation of PE/EVOH polymer blends 3.1.1.1. Viscosity of PE/EVOH polymer blends * Effect of the ratio PE/EVOH on the viscosity PE/EVOH polymer blends Curves of mixing torque versus time for PE/EVOH polymer blends containing 4% LLDPE-g-MAH compatibilizer at different PE/EVOH ratios were shown in fig. 3.1. Fig. 3.1. Curves of mixing torque versus time for PE/EVOH polymer blends The results showed that the torque in the molten equilibrium of the PE/EVOH polymer blend increased as the EVOH content increased.
6 Specifically, torque of PE/EVOH polymer blend in molten equilibrium with the ratios of 90/10, 80/20, 70/30, 60/40 and 50/50 are 15.7; 17.8; 18.7; 19.4 and 19.9 N.m, respectively. * The effect of the PE-g-MAH content on the viscosity of the PE/EVOH polymer blend was shown in fig. 3.2. Fig. 3.2. Curves of mixing torque versus time for PE/EVOH 70/30 polymer blend with different PE-g-MAH content Fig. 3.2 showed that when the PE-g-MAH compatibilizer was present, torque in the molten state of PE/EVOH polymer blends increased as compared to when not using the compatibilizer even though PE-g-MAH has low viscosity. The results also showed that when increasing the content of PE-g-MAH compatibilizer, the torque of the polymer blend increased. 3.1.1.3. Mechanical properties of PE/EVOH polyme blends Effects of PE/EVOH ratio on mechanical properties of PE/EVOH polymer blends with and without PE-g-MAH compatibilizer were presented in Table 3.1. Table 3.1. Effect of the composition ratio on the mechanical properties of PE/EVOH polymer blend PE/EVOH PE-g-MAH Tensile Elongation at ratio compatibilizer (%) strength (MPa) break (%) 100/0 29.5 1005.4 90/10 19.6 765.7 80/20 15.8 420.3 70/30 0 12.4 256.2 60/40 10.1 148.5 50/50 7.6 89.6 0/100 25.2 17.1 90/10 24.5 359.2 80/20 26.4 389.3 70/30 4 28.3 404.9 60/40 27.1 367.5 50/50 25.5 315.8
7 In samples without PE-g-MAH compatibilizer, both tensile strength and elongation at break of the polyme blend samples decreased with increasing EVOH content. This is because PE and EVOH have differences in nature, chemical structure, polarity, surface interaction energy ... so the low adhesion between PE and EVOH leads to agglomeration of a large EVOH amount in the PE matrix. In the presence of a compatibilizer, the mechanical properties of the polymer blends were significantly improved. Tensile strength and elongation at break increased because PE-g-MAH acted as an effective compatibilizer between EVOH dispersion phase and PE matrix. Adding PE-g-MAH increased the dispersion of EVOH and increases the adhesion between phases to enhance the mechanical properties of polymer blends. The results also showed that in the presence of 4% compatibilizer, when the EVOH content increased from 10-30%, the tensile strength and elongation at break increased. Tensile strength was significantly improved when increasing EVOH content in polymer blends possibly due to high tensile strength of EVOH. However, when the EVOH content was > 30%, the mechanical properties of the polymer blends decrease because the tensile strength was strongly influenced by the phase interaction between PE and EVOH. This showed that when the EVOH content increased, the binding capacity of PE and EVOH decreased. The effect of the PE-g-MAH content on the mechanical properties of the PE/EVOH polymer blends was presented in Table 3.2. Table 3.2. Effect of PE-g-MAH content on mechanical properties of PE/EVOH 70/30 polymer blend PE-g-MAH content Tensile strength Elongation at break (%) (MPa) (%) 0 12.4 256.2 2 25.6 379.8 4 28.3 404.9 6 26.5 416.3 8 24.1 420.4 10 23.9 418.5 The results showed that the tensile strength of the polymer blends increased when the content of compatibilizer increased from 0-4%. However, when the PE-g-MAH content continued to increase to > 4%, the tensile strength of the polymer blend decreased and then stabilized. Elongation at break increased when the PE-g-MAH content increased to 6%, then almost unchanged. However, when the PE-g-MAH content continued to increase, the mechanical properties of the polymer blend samples decreased.
8 3.1.1.4. Surface morphology of PE/EVOH polymer blends SEM images of fracture surface of PE/EVOH polymer blends without and with PE-g-MAH compatibilizer wre shown in Figure 3.6 and Figure 3.7. a) PE/EVOH 90/10 b) PE/EVOH 80/20 c) PE/EVOH 70/30 d) PE/EVOH 60/40 e) PE/EVOH 50/50 Fig. 3.4. SEM images of fracture surface of PE/EVOH polymer blends without PE-g-MAH compatibilizer a) PE/EVOH 90/10 b) PE/EVOH 80/20 c) PE/EVOH 70/30 d) PE/EVOH 60/40 e) PE/EVOH 50/50 Fig. 3.5. SEM images of fracture surface of PE/EVOH polymer blends with 4% PE-g-MAH compatibilizer SEM images of fractured surface of the polymer blends showed that in the polyme blends without compatibilizer (Fig. 3.4), the phases dispersed unevenly, forming distinct phase separation regions, the dispersion of EVOH in PE matrix was quite large and coarse. With polymer blends containing compatibilizer, when the EVOH content increased from 10-30%, the component polymers had good dispersion and compatibility with each other.
9 However, when increasing EVOH content in polymer blend from 40-50%, the compatibility between EVOH and LLDPE phases becomes worse. 3.1.1.5. Differential scanning calorimetry (DSC) of PE/EVOH polymer blends DSC curves of PE/EVOH polymer blends with 4% PE-g-MAH compatibilizer at different PE/EVOH ratios were summarized in Table 3.3. Table 3.3. Effect of the LLDPE/EVOH ratio on thermal properties of polymer blends LLDPE/EVOH Tg (oC) Tm (oC) Tc (oC) ratio Polyme blend LLDPE EVOH LLDPE EVOH 100/0 -20.7 121.8 - 104.8 - 90/10 -14.5 121.3 182.1 104.7 161.9 80/20 -2.4 120.4 182.7 104.6 161.3 70/30 6.32 122.0 183.0 102.5 159.2 60/40 6.5 và 38.5 120.7 184.0 103.8 161.1 50/50 6.0 và 38.8 122.3 184.2 104.3 160.9 0/100 40 - 184.4 - 162.3 The results showed that at 4% PE-g-MAH compatibilizer, when increasing the EVOH content from 0-30%, a Tg in the range Tg of EVOH (Tg = 40oC) and Tg of LLDPE (Tg = -20.7oC). This proves that in the presence of a PE-g-MAH compatibilizer, at 10-30% EVOH, the two polymers had good compatibility with each other. When the EVOH content continued to increase from 40-50%, two Tg values appeared in the range of Tg of the two component polymers. However, there was a shift in Tg of LLDPE to Tg of EVOH. This proves that at the EVOH content of 40–50%, in the presence of a PE-g-MAH compatibilizer, there was partial compatibility between the two polymers. The results also showed that the melting temperature Tm of LLDPE in polymer blends did not change much when the EVOH content increased. However, when increasing the EVOH content, the Tm value of EVOH in the polymer blends increased slightly but was smaller than the Tm value of EVOH resin. Crystallization temperature (Tc) of LLDPE in blends is almost unchanged. Meanwhile, when increasing EVOH content, Tc value of EVOH in polymer blend decreased slightly compared with Tc of EVOH resin. 3.1.2. Study on preparation of PA6/EVOH polyme blends To study preparation of polymer blend PA6/EVOH, polymer blend samples were prepared at different ratios of PA6/EVOH 100/0, 90/10, 80/20, 75/25, 50/50 and evaluate the properties of the polymer blend samples. 3.1.2.1. Viscosity of PA6/EVOH polyme blends The effect of the components rate on the torque of PA6/EVOH polymer blends were presented in Fig. 3.10.
10 Fig. 3.10. Curves of mixing torque versus time for PA6, EVOH, PA6/EVOH polyme blends The results showed that the torque in the molten equilibrium of PA6/EVOH polymer blends was lower than EVOH and higher than PA6. In addition, when increasing the EVOH content in the polymer blend, the torque in the molten equilibrium of the polymer blends increased. This may be due to the interaction between the organizational groups in the polymer blend, namely the amino groups of PA6 and the hydroxyl groups of EVOH. When EVOH increased, the number of hydrogen bonds between hydroxyl groups and amine groups increased, increased the intermolecular and intermolecular bonds leading to an increase in torque. 3.1.2.2. Mechanical properties of PA6/EVOH polymer blends Effects of the PA6/EVOH ratio on the mechanical properties of the polymer blends wre presented in Table 3.5. Table 3.5. Mechanical properties of PA6/EVOH polymer blends PA6/EVOH ratio Tensile strength Elongation at break (%) (MPa) (%) 100/0 60.4 29.5 90/10 58.7 42.5 80/20 52.6 58.6 75/25 48.6 74.2 50/50 32.4 68.4 0/100 25.1 17.2 The results showed that the tensile strength of PA6/EVOH polymer blends decreased when the EVOH content increased. However, the elongation at break increased when the EVOH content increased from 0-25%, when the EVOH content increased to higher than 25%, the elongation at break decreased. 3.1.2.3. Surface morphology of PA6/EVOH polymer blends Morphology of fracture surface of PA6/EVOH polymer blends was
11 shown in figure 3.11. PA6/EVOH 90/10 PA6/EVOH 80/20 PA6/EVOH 75/25 Fig. 3.11. SEM images of fracture surface of PA6/EVOH polymer blend Observing the fracture surface SEM images of the PA6/EVOH polymer blends, it was found that the fracture surface was relatively smooth, difficult to distinguish the morphology of the two phases PA6 and EVOH after mixing. This demonstrates a good dispersion of the two phases together. Surface morphology of the polymer blends after soaking in dioxane solution was shown in Figure 3.12. PA6/EVOH 90/10 PA6/EVOH 80/20 PA6/EVOH 75/25 Fig.3.12. SEM images of PA6/EVOH polyme blends after soaking in dioxane solution The results showed that with samples containing low EVOH content (Fig. 3.12a), there was no EVOH dispersion zone in the PA6 substrate. When the EVOH content increased (Fig.3.12 (b), (c) and (d)), the corrosive EVOH regions appear and the number of these eroded regions increases and the size of the regions were larger as EVOH the content in polymer blends increased.. 3.1.2.4. Differential scanning calorimetry (DSC) of PA6/EVOH polymer blends Effect of PA6/EVOH ratio on thermal properties (crystallization temperature Tc, melting temperature Tm and glass transition temperature Tg) of PA6/EVOH polymer blends was presented in Table 3.6. The results showed that the melting temperature of PA6 in the PA6/EVOH polymer blends decreased from 226.3oC to 207.1oC when the EVOH content increased from 0-50%, and the molten heat absorption process of EVOH was not observed. Crystallization temperature of PA6 decreased from 193oC to 170oC when the EVOH content increased to 50%. This indicated the formation of
12 intermolecular and intramolecular hydrogen bonds and chemical interactions between two polymers that form block copolymers (consisting of EVOH blocks and PA6 blocks). Table 3.6. DSC analysis results of polymer PA6, EVOH and PA6/EVOH polymer blends PA6/EVOH Thermal properties of polyme blends ratio Tc (oC) Tm (oC) 100/0 193.0 226.3 90/10 189.2 218.1 80/20 185.0 214.8 75/25 181.1 213.3 50/50 170.4 207.1 0/100 162.3 184.4 3.2. Study on preparation of gas barrier multilayer films based on polyme blends of EVOH and evaluate the lifetime of films 3.2.1. Study on preparation of PE/PE-EVOH/PE gas barrier multilayer films 3.2.1.1. Effect of PE-g-MAH compatibilizer content on properties of PE/PE- EVOH/PE gas barrier multilayer films To evaluate the effect of PE-g-MAH compatibilizer content in layer 1 and layer 3 on the properties of the PE/PE-EVOH/PE multilayer film, PE/PE- EVOH/PE multilayer film with layer 2 (PE/EVOH polymer blend) accounted for 15% in volume, layer 1 and layer 3 (LLDPE/PE-g-MAH polymer blend) accounted for 85% in volume were prepared. The PE-g-MAH compatibilizer contents in layer 1 and layer 3 were 0-8%. 3.2.1.1.1. Effect of PE-g-MAH compatibilizer content on surface morphology of PE/PE-EVOH/PE gas barrier multilayer films SEM images of the fracture surface of the PE/PE-EVOH/PE 3-layer films with the PE/EVOH polymer blend content of 15% with and without the compatibilizer was shown in Figure 3.16. (a) (b) (c) Fig.3.16. SEM images of the fracture surface of the PE/PE-EVOH/PE films a) without PE-g-MAH; b) with 2% PE-g-MAH; c) with 4% PE-g-MAH SEM image of the fracture surface of the PE/PE-EVOH/PE films containing the PE-g-MAH compatibilizer showed that with 2% of the compatibilizer, the phase separation between layers was still relatively clear,
13 but when the content of compatibilizer increased to 4%, it was difficult to observe the division between layers, showing good adhesion between the layers. It can be explained that the EVOH layer requires sufficient amount of PE-g-MAH to develop covalent bond through the reaction between the anhydride group of PE-g-MAH and hydroxyl groups of EVOH on the interphase surface between LLDPE and EVOH. 3.2.1.1.2. Effect of PE-g-MAH compatibilizer content on mechanical properties of PE/PE-EVOH/PE multuilayer films Effect of PE-g-MAH content on the mechanical properties of PE/PE- EVOH/PE multilayer films was shown in fig. 3.17. 35 650 Elongation at break (%) Tensile strength (MPa) 30 600 25 550 20 500 15 Độ bền stength Tensile kéo đứt 450 Độ dãn dài at Elongation khibreak đứt 10 400 0 2 4 6 8 PE-g-MAH content Fig.3.17. Effect of PE-g-MAH content on the mechanical properties of PE/PE-EVOH/PE films The results showed that when adding 2% PE-g-MAH, the tensile strength of the multilayer films decreased slightly, then the tensile strength increased slightly when the PE-g-MAH content reached 4% and was almost unchanged if the content of PE-g-MAH continues to increase. Thus, it can be seen that PE-g-MAH had unsinificantly affect on the mechanical properties of the PE/PE-EVOH/PE multilayer films. 3.2.1.1.3. Effect of PE-g-MAH compatibilizer content on oxygen transmission rate (OTR), water vapor transmission rate (WVTR) of PE/PE- EVOH/PE multilayer films The effect of the PE-g-MAH content on the permeability of the PE / PE-EVOH / PE multilayer film is summarized in Table 3.8. Table 3.8. The oxygen and water vapor tranmisison rate of the PE/PE- EVOH/PE multilayer films with different PE-g-MAH contents (15% of PE/EVOH) The content of PE-g-MAH (%) Transmission rate 0 2 4 6 8 Oxygen transmission rate 3.01 3.22 3.26 4.58 5.64 (ml/m2.ngày) Water vapor transmission 6.87 6.85 6.89 6.86 7.01 rate (g/m2.ngày)
14 The results showed that when the PE-g-MAH content increased from 0 - 4%, the oxygen transmission rate of the PE/PE-EVOH/PE film was almost unchanged. However, at the PE-g-MAH content> 6%, the oxygen transmission rate increased. The results also showed that the water vapor transmission rate of the films was not affected by the PE-g-MAH content. 3.2.1.2. Effect of PE/EVOH polyme blend (layer 2) content on properties of PE/PE-EVOH/PE multilayer films To evaluate the effect of PE/EVOH polyme blend layer content on the properties of the PE/PE-EVOH/PE multilayer film, PE/PE-EVOH/PE multilayer film with PE-g-MAH compatibilizer content layer 1 and layer 3 of 4%, PE/EVOH polymer blend content of 5 – 20% were prepared. 3.2.1.2.1. Effect of PE/EVOH polyme blend content on mechanical properties of PE/PE-EVOH/PE multuilayer films Effect of the content of PE/EVOH polyme blend on the mechanical properties of PE/PE-EVOH/PE multilayer films was summarized in table 3.9. Table 3.9. Mechanical properties of PE/PE-EVOH/PE multilayer films The content of Tensile strength Elongation at break PE/EVOH layer (%) (MPa) (%) 5 32.3 680.3 10 31.4 661.7 15 30.5 624.8 20 29.8 605.5 Mechanical properties measurement results showed that tensile strength and elongation at break of multilayer films decreased slightly when increasing the content of PE/EVOH polymer blend. This was due to the PE/EVOH polymer blend layer had much lower elongation at break than LLDPE resin. Especially, EVOH is a copolymers with high crystallinity (58- 70%), so it is quite brittle and low elongation, so increasing their content lead to reduce elongation at break of the multilayer film. 3.2.1.2.2. Effect of PE/EVOH polyme blend content on oxygen transmission rate (OTR), water vapor transmission rate (WVTR) of PE/PE-EVOH/PE multilayer films Results of measuring oxygen and water vapor transmission rate of multilayer films with different PE/EVOH polymer blend contents were presented in Table 3.10. The results showed that when the concent of PE/EVOH polymer blend layer increased, the oxygen transmission rate of the multilayer films decreased significantly. When the content of PE/EVOH polymer blend layer from 5 - 15%, oxygen transmission rate decreased sharply and then decreased insignificantly when the content of PE/EVOH polymer blend layer increased to 20%. The water vapor transmission rate of PE/PE-EVOH/PE films
15 increased with increasing content of PE/EVOH blend layer. This is explained by EVOH having -OH groups leading to its hydrophilic properties. Increasing concentration of blend layer PE/EVOH means an increase in EVOH content leads to an increase in the number of OH groups, making them more hydrophilic. Table 3.10. The oxygen and water vapor transmission of the PE/PE- EVOH/PE multilayer films (4% PE-g-MAH) The content of PE/EVOH polymer blend (%) Transmission rate 5 10 15 20 Oxygen transmission rate 20.60 12.13 3.26 2.34 (ml/m2.ngày) Water vapor transmission 4.78 5.34 6.89 9.58 rate (g/m2.ngày) 3.2.2. Study on preparation of PE/PA-EVOH/PE gas barrier multilayer films 3.2.2.1. Effect of PE-g-MAH compatibilizer content on properties of PE/PA- EVOH/PE gas barrier multilayer films To evaluate the effect of PE-g-MAH compatibilizer content in layer 1 and layer 3 on the properties of the PE/PA-EVOH/PE multilayer film, PE/PA-EVOH/PE multilayer film with layer 2 (PA6/EVOH polymer blend) accounted for 10% in volume, layer 1 and layer 3 (LLDPE/PE-g-MAH polymer blend) accounted for 90% in volume were prepared. The PE-g-MAH compatibilizer contents in layer 1 and layer 3 were 0-10%. 3.2.2.1.1. Effect of PE-g-MAH compatibilizer content on surface morphology of PE/PA-EVOH/PE gas barrier multilayer films SEM image of fracture surface of the PE/PA-EVOH/PE three layer films with the PA6/EVOH polymer blend content of 15% with compatibilizer was shown in Figure 3.19. (a) Fracture surface of (b) Fracture surface of (c) Fracture surface of film LLDPE/PE-g-MAH PA6/EVOH polymer polymer blend layer blend layer Fig.3.19. SEM image of fracture surface of the PE/PA-EVOH/PE film with 4% PE-g-MAH compatibilizer Observing the fracture surface of film (Figure 3.19a), it can be seen that the interaction at the interface between the two phases is relatively good. This is because in the molten extrusion state, on the surface between the
16 membranes there is a covalent bond between the carbonyl group of MAH in PE-g-MAH and the amino group in PA6 and the hydroxyl group in EVOH. This strong interaction lead to increased adhesion between LLDPE/PE-g- MAH polymer blend layer and PA6/EVOH polymer blend layer. 3.2.2.1.2. Effect of PE-g-MAH compatibilizer content on mechanical properties of PE/PA-EVOH/PE multuilayer films Effect of the PE-g-MAH content on the mechanical properties of PE/PA-EVOH/PE mulitayler films was shown in figure 3.20. 35 700 Elongation at break 30 650 Tensil strength (MPa) 600 25 550 (%) 20 500 15 Độ bền stength Tensile kéo đứt 450 Độ dãn dài at Elongation khibreak đứt 10 400 0 2,5 5 7,5 10 PE-g-MAH content Fig.3.20. Effect of the PE-g-MAH content on the mechanical properties of mulitayler films The results showed that PE-g-MAH unsignificantly affected to the tensile strength of the PE/PA-EVOH/PE multilayer films. The results also showed that the elongation at break of the PE/PA-EVOH/PE multilayer films increased slightly with the addition of PE-g-MAH and the elongation at break increased when the PE-g-MAH content in film increased. 3.2.2.1.3. Effect of PE-g-MAH compatibilizer content on oxygen transmission rate (OTR), water vapor transmission rate (WVTR) of PE/PA- EVOH/PE multilayer films The influence of the PE-g-MAH content on the permeability of the PE/PA-EVOH/PE multilayer films is summarized in Table 3.11. Table 3.11. The oxygen and water vapor tranmisison rate of the PE/PA- EVOH/PE multilayer films with different PE-g-MAH content (10% of PA6/EVOH) The content of PE-g-MAH (%) Transmission rate 0 2.5 5 7.5 10 Oxygen transmission rate 1.86 2.52 2.43 4.21 5.39 (ml/m2.ngày) Water vapor transmission 6.70 6.69 6.72 6.79 6.75 rate (g/m2.ngày) The results showed that the presence of PE-g-MAH increased oxygen tranmisison rate. However, when the PE-g-MAH content increased from 2.5 to 5%, the oxygen tranmisison rate was almost unchanged. When the PE-g- MAH content was > 5%, the oxygen tranmisison rate increased. The results
17 also showed a similar trend for the PE/PE-EVOH/PE multilayer films, PE-g- MAH did not affect the water vapor tranmisison rate of the PE/PA-EVOH/PE multilayer films. 3.2.2.2. Effect of PA6/EVOH polyme blend (layer 2) content on properties of PE/PE-EVOH/PE multilayer films To evaluate the effect of PA6/EVOH polyme blend layer content on the properties of the PE/PA-EVOH/PE multilayer film, PE/PA-EVOH/PE multilayer film with PE-g-MAH compatibilizer content layer 1 and layer 3 of 5%, PA6/EVOH polymer blend content of 5 – 20% were prepared. 3.2.2.2.1. Effect of PA6/EVOH polyme blend content on mechanical properties of PE/PE-EVOH/PE multuilayer films Effect of the content of PA6/EVOH polymer blend layer on the mechanical properties of mulayer films was presented in table 3.11. Table 3.11. The mechanical properties of multilayer films with different PA6/EVOH polymer blend layer contents The content of Tensile strength Elongation at break PA6/EVOH layer (%) (MPa) (%) 5 30.7 560.4 10 34.1 520.8 15 35.2 512.7 20 35.7 508.6 The results in table 3.11 showed that the tensile strength of films increased slightly when the polymer blend layer content increased from 10- 15%. When continuing to increase the content of polymer blend layer to 20%, the tensile strength is almost unchanged. 3.2.2.2.2. Effect of PA6/EVOH polyme blend content on oxygen transmission rate (OTR), water vapor transmission rate (WVTR) of PE/PA-EVOH/PE multilayer films Results of measuring oxygen and water vapor transmission rate of multilayer films with different PA6/EVOH polymer blend contents were presented in table 3.13. Table 3.13 The oxygen and water vapor transmission of the PE/PA- EVOH/PE multilayer films (5% PE-g-MAH) The content of PA6/EVOH blend layer (%) Transmission rate 5 10 15 20 Oxygen transmission rate 4.54 2.43 1.69 1.32 (ml/m2.ngày) Water vapor transmission 5.6 6.72 8.1 11.1 rate (g/m2.ngày) The results showed that, when the content of blend layer (middle layer thickness) increased, the O2 gas transmission rate decreased significantly. In terms of water vapor transmission rate (WVTR), when the content of blend
18 layer increased, the WVTR increased slightly. Compared with the PE/PE-EVOH/PE multilayer membrane with the same middle layer content, it was found that the oxygen barrier ability of the PE/PA-EVOH/PE films was better, or the oxygen transmission rate was lower. This can be explained by the fact that PA6 has a much higher barrier to oxygen than PE but less than EVOH. In contrast, the water vapor barrier ability of PE/PA-EVOH/PE multi-layer films was less than that of PE/PE- EVOH/PE films with the same middle layer content, because PA6 contains polar amide groups, which are hydrophilic so its water vapor transmission rate was higher. General comment: it can be seen that with the same content of the middle layer, the PE/PE-EVOH/PE multi-layer film has lower WVTR but higher OTR as compared with PE/PA-EVOH/PE multilayer film. So, depending on the purpose of use, choose the right film. According to some studies, to make the gas barrier film, the OTR of the film must be ≤ 5 ml/m2.day and the WVTR of the film must be ≤ 8 g/m2.day. The results showed that the PE/PE-EVOH/PE multilayer film with the PE/EVOH polymer blend content of 15% and the PE/PA-EVOH/PE multilayer film with the PA- EVOH polymer blend content of 5, 10 and 15% (signed PAEV-5, PAEV-10, and PAEV-15, respectively) were unsatisfactory as gas barrier film. 3.3. Study on application of gas barrier multilayer films in the preservation of some dry agricultural products 3.3.1. Study on application of gas barrier multilayer films in the preservation of maize grain Corn at 10.98% moisture content is packed into PAEV-10 bags, then vacuumed on BZQ 500 machine (Maize-CK) (vacuum pressure -0.08MPa). The control samples (without vacuum, Maize-T) were similarly conducted. 3.3.1.1. Effect of packaging conditions on maize quality The changes in quality of maize grain under packaging conditions were summarized in Table 3.15. Table 3.15. The quality of maize grain under packaging condition during storage Quality Time (months) Samples indicators 0 2 4 6 8 Moisture Maize-CK 10.98 10.96 11.04 11.09 11.18 content (%) Maize -T 10.98 11.30 11.64 11.95 12.05 Starch content Maize -CK 74.58 74.49 74.32 74.26 73.02 (%) Maize -T 74.58 74.33 73.06 71.93 70.14 Protein content Maize -CK 9.07 9.06 8.97 8.81 8.60 (%) Maize -T 9.07 9.04 8.89 8.12 6.93 Lipid content Maize -CK 4.20 4.18 4.16 4.08 3.89 (%) Maize -T 4.20 4.15 3.87 3.56 3.04