Multilayer Barrier Film of Biaxially Oriented PA6/ EVOH by Double Bubble Tubular Film Process

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1 INVITED PAPERS M. Takashige 1 *, T. Kanai 2 1 Idemitsu Unitech Co, Hyogo, Japan 2 Idemitsu Kosan Co, Chiba, Japan Multilayer Barrier Film of Biaxially Oriented PA6/ EVOH by Double Bubble Tubular Film Process In the market, a film adapted to environmental and barrier problems is desired. The previous report discussed easy tear and gas barrier PA6 film. This report discusses the multilayer PA6 film having high barrier EVOH layer. A biaxially oriented PA6 film produced by double bubble tubular film process has high strength, but does not have enough gas barrier property compared with K-coated PA6. In the following a multilayer film with gas barrier layer is studied. A high gas barrier layer of EVOH is added to the center layer in the outer layers of PA6 films. The gas barrier ability increases with increasing the content of EVOH and decreasing ethylene content of EVOH, but at the same time the mechanical properties and processability get worse. From this point of view, it is found that the optimum content of EVOH is between 20 % and 43 %. In a biaxially oriented multilayer film composed of PA6 layers and EVOH one keeps high strength and high gas barrier. The material and production technology of a PA6 film can solve environmental and barrier problems. 1 Introduction As there have been strong needs of long life food packaging in the market, a high oxygen barrier film is desired. An ethylenevinylalcohol copolymer (EVOH) which has high gas barrier property was developed by Kuraray Co., Ltd. in 1972 and it has been applied for food packaging and gas barrier end-usage [1 to 4]. In recent years problems like the de-chlorination and the waste reduction, have brought concerns in the packaging industry. Especially the waste reduction has been a serious problem. In order to achieve this waste reduction, there is a rapid increasing shift from a bottle to a standing pouch for repackaging use in an effort to utilize resources effectively. For the provision of this standing repackage pouch, thin and strong biaxially oriented PA6 film is indispensable. PA6 film has inferior oxygen gas barrier performance. K coated film, which has a surface of biaxial stretching PA6 film with polyvinylidene chloride in order to raise the oxygen gas * Mail address: M. Takashige, Idemitsu Unitech Co., Ltd., Kou, Shirahama-cho, Himeji-City, Hyogo, Japan masao.takashige@iut.idemitsu.co.jp barrier performance, is used. When this film is incinerated, it generates toxic gas such as chlorine gas and dioxin. So the development of oxygen gas barrier film without K coated layer is desired. A previous paper discussed an oxygen gas barrier PA6/MXD6 blend film which satisfies formability and physical property using blending technology [5]. As a gas barrier resin, poly (m-xylene adipamide) (MXD6) was selected. But the high oxygen gas barrier film which is higher gas barrier than K coated film is sometimes required. The patent describes EVOH film produced by tentering process [6] and double bubble tubular film process [7]. Biaxially oriented process elevates gas barrier property and mechanical properties. But EVOH single layer film limits the level of strength. If both gas barrier and high strength are required, lamination of PA6 and EVOH film is needed. But film cost is expensive. The development of the film with compatible performance of toughness and high barrier is the goal. A polymer blend of PA6 and EVOH has been discussed by Tae [8]. It is known that this blend is reactive from his research. There are several reports issued on double bubble tubular stretching technology [9 to 23] of formability and structure analysis for resin such as PET, PBT, PPS, PA6-12 and PA12 that are reported by White, Kang, Song, Rhee et al. [11 to 20]. However, there are no reports on double bubble tubular technology of the multilayer film which may produce the film with compatible performance of toughness and high barrier. The analyses of deformation behavior of PA6 was reported previously according to stretching stress analyses [23]. A previous paper discussed easy tear PA6 film [24]. This study was carried out to clarify the relationship among multilayer composition, gas barrier property and physical properties of stretched film. It should aim at the development of a high oxygen barrier film keeping the toughness of biaxial stretching PA6 film by using multilayer film including EVOH layer. 2 Experimental 2.1 Equipment Apparatus used for the double bubble tubular film process is shown in Fig. 1. By using three extruders namely an u 40 mm extruder (L/D = 24), an u 40 mm extruder (L/D = 25) as outer layers PA6 and an u 40 mm extruder (L/D = 25) as a center 86 Hanser Publishers, Munich Intern. Polymer Processing XXI (2006) 1

2 Fig. 1. Schematic view of double bubble tubular process (multi layer process) a: extruder (EX1, EX2, EX3), b: die (3 layers), c: cooling bath, d: take up roll, e: air ring, f: heating furnace, g: annealing, h: winding layer EVOH with a three layer circular die of the diameter of 75 mm and the lip clearance of 1 mm and with a water-cooling ring having the diameter of 90 mm. While passing through for second blowing, which is composed of two pairs of pinch rolls and a heating furnace (a far infrared radiation heater is self-contained), this raw film was stretched simultaneously in the machine and transverse directions by using internal bubble air. The stretched film was heatset using a heat treatment device. 2.2 Material The material was Ube Nylon 1023FD (PA 6) with mean molecular weight of and the relative viscosity of g r = 3.5 in 98 % sulfuric acid as a solvent. Five different EVOH grades were used, as shown in Table 1. EVOH is produced by Kuraray and EVOH ethylene contents were 27 to 47 mol% with different MI. Resin properties listed in Table Experimental Method The melt process conditions of the non-stretched film were 270 C for resin temperature at the die exit, 1.2 for blow up ratio, and 6.0 for draw down ratio respectively. Three layer thickness ratio is changed 2/1/2, 2/2/2 and 2/3/2 for outer layer/center layer/outer layer. EVOH grade in the center layer was changed. PA6 was used for outer layers from these compositions. The extrusion rate was 17.6 kg/h and the take up velocity was 7.0 m/min. Film was quenched in water at 20 C to suppress crystallization. The stretching device consists of a heating/stretching furnace and an air ring. The air ring, which injects air downward at an angle of 45, was installed at the upper part of the heating furnace. The standard condition for stretching process was set at 310 C for process temperature (temperature of heating furnace) and MD (Machine Direction)/TD (Transverse Direction) = 3.0 /3.2 for stretching ratio respectively. The stretched film was heat treated to prevent shrinkage, using a heat treatment device of the tenter process. The stretching ratio of both MD and TD are determined by the inside bubble pressure and different roll speeds between top rolls and bottom rolls. In this manner, the second bubble is simultaneously stretched in both machine direction and transverse direction. Fig. 2 shows the measurement method of stretching stress. Stretching stress were calculated with help of Eq. 1. r TD =(DP R)/H L, (1) Properties Grades L101 F101 H101 E105 E156 Ethylene content mol% Density g/cm Melting point C Glass transition point C Crystallization point C Melt index 190 C g/10 min Melt index 210 C g/10 min Table 1. EVOH grades (ethylene vinyl alcohol copolymer) Intern. Polymer Processing XXI (2006) 1 87

3 Fig. 2. Measurement method of stretching stress (the internal bubble pressure was measured using the digital manometer) where DP, R, H L are inside bubble pressure, final bubble radius and final film thickness respectively. The bubble internal pressure in the double bubble tubular process was obtained using the digital manometer (Yokogawa-Hokushin). The maximum stress at the end point of stretching may be obtained. 2.4 Mechanical Properties The evaluation of gas barrier and toughness was also carried out. Oxygen gas permeability was carried out under 23 C 60 %RH condition by using Mocon Oxtran. The toughness was evaluated by using the film impact strength equipment with 1/2 inch ball produced by Toyo Seiki Seisaku-Sho, Ltd. 3 Results and Discussion The influence of EVOH content in multilayer film and ethylene content of EVOH on stretchability, stretching stress and physical properties of biaxially oriented film including gas permeability is studied by using the double bubble tubular multilayer film machine. Outer layers are composed of PA6 and center layer is EVOH as a gas barrier layer shown in Fig. 3. A global picture of the whole experimental procedure is shown in Table Multilayer Structure and Processability of Biaxially Orientation The influence of EVOH ratio in the multilayer film on stretching stress is shown in Fig. 4. The stretching stress decreases with increasing EVOH ratio. Fig. 5 shows EVOH is higher Fig. 3. Multi layer structure (PA6/EVOH 3 layers) < Process condition > < Evaluation item > 1 Layer ratio (1 conditions) 1 Stretching stress 2 EVOH ethylene contents 2 Film impact strength (5 conditions) 3 Process temperature 3 Gas permeability 4 Cooling condition 4 Crystallization 5 Adhesive 5 Bubble instability 6 Multilayer vs. Blend 6 Thickness uniformity 7 Interface adhesive strength 8 Characteristic in hot water Table 2. Experimental procedure (PA6/EVOH/PA6) 88 Intern. Polymer Processing XXI (2006) 1

4 Fig. 4. Relationship between EVOH ratio and stretching stress (ethylene content 32 mol% EVOH) Fig. 5. Relationship between film temperature and tensile modulus for non stretched film (ethylene content 32 mol% EVOH) temperature dependence of tensile modulus than PA6, so EVOH has large stretching stress and is difficult to be stretched under the low temperature. The stretching process window of EVOH is narrower than PA Dependence of Stretchability from Temperature The results of stretching stress obtained by changing the set temperature of heating furnace are shown in Fig. 6. The stretching stress decreases with increasing stretching temperature. It is found that multilayer film shows higher temperature dependence of stretching stress than PA6. As a result, stretching process window of EVOH is narrower than one of PA Influence of EVOH Ethylene Content The influence of EVOH ethylene content on stretching stress of multilayer film is shown in Fig. 7. At the same layer thickness 2/1/2, 5 different ethylene content EVOH (L101, F101, H101, E105, E156) were prepared. The stretching stress increases with decreasing ethylene content of EVOH, which means films Fig. 6. Relationship between process temperature and stretching stress (ethylene content 32 mol% EVOH outside/center/inside = 1/1/1) Fig. 7. Relationship between ethylene contents of EVOH and stretching stress (layer ratio = 1/1/1) Fig. 8. Relationship between temperature of cooling water and crystallinity (ethylene content 32 mol% EVOH) Intern. Polymer Processing XXI (2006) 1 89

5 Ethylene content mol% Degree of Polymerization with low ethylene EVOH contents is difficult to be stretched. As ethylene content of EVOH increases, hydrogen bond formation of EVOH decreases and as result the stretching stress decreases. Hydrogen bond is due to be suppressed by ethylene. Fig. 8 shows the influence of cooling water temperature on raw film crystallinity. This result shows that EVOH has faster crystallization speed than PA6. The cooling condition is very important for EVOH to control the crystallinity and it influences the stretchability. The crystallization rate of EVOH under the isothermal temperature condition is shown in Table 3, which was reported by Ikari [26]. The short t 1/2 which is half crystallization time means fast crystallization. The crystallization speed of EVOH is influenced by ethylene content and molecular weight. Fig. 9 shows the relationship between cooling condition of raw film and stretching stress. As the temperature of cooling water for producing the raw film increases, the stretching stress increases and it makes difficult stretching and it sometimes occurs bubble break. It is considered that the high crystallinity is main reason. The key point is to reduce the crystallinity by fast cooling when the unstretched film is produced. t 1/2 s Table 3. EVOH crystallization rate (ethylene vinyl alcohol copolymer) Fig. 9. Relationship between temperature of cooling water and stretching stress (ethylene content 32 mol% EVOH, layer ratio = 2/1/2) 3.4 Physical Properties Oxygen gas permeability of multilayer film as a function of EVOH/PA6 content ratio is shown in Fig. 10. It is found that oxygen gas permeability drastically decreases with increasing until 20 % EVOH content, then becomes constant. Film impact strength of multilayer film decrease with increasing EVOH content is shown in Fig. 11. It is obviously because PA6 has the strongest physical properties in the film. Fig. 12 shows the relationship between oxygen gas permeability of multilayer film and ethylene content of EVOH. In spite of the same EVOH layer thickness, oxygen gas permeability decreases with decreasing EVOH ethylene content. The impact strength as a function of process temperature is shown in Fig. 13. The impact strength increases with decreasing process temperature, apparently because PA6/EVOH film can have high impact strength by achieving high stretching stress. 3.5 Film Thickness Distribution The influence of stretching temperature on stretched film width is shown in Fig. 14. As the stretching temperature increases, the film width fluctuation becomes clear because of bubble instability during the stretching process. It is due to the bubble tension. Fig. 10. Relationship between EVOH contents and oxygen gas permeability (PA6/EVOH/PA6) with ethylene content 32 mol% EVOH Fig. 11. Relationship between EVOH layer ratio and film impact strength (PA 6/EVOH/PA 6) with ethylene content 32 mol% EVOH) 90 Intern. Polymer Processing XXI (2006) 1

6 Fig. 12. Relationship between ethylene content of EVOH and oxygen gas permeability (layer ratio = 1/1/1) Fig. 13. Relationship between process temperature and film impact strength (ethylene content 32 mol% EVOH, layer ratio = 1/1/1) Fig. 14. Relationship between process temperature and bubble diameter uniformity (ethylene content 32 mol% EVOH, layer ratio = 1/1/1) The bubble stability of multilayer film including EVOH layer is a little worse than PA6. Fig. 15 shows that film thickness uniformity produced at high stretching temperature is bad. As Fig. 4 shows the tensile modulus of EVOH as a function of temperature, EVOH modulus is larger than PA6 one and the temperature sensitivity of EVOH is high. So film thickness uniformity is very much influenced by stretching temperature. To elevate stretching temperature makes low stretching stress and easier stretching, but the film thickness uniformity is getting worse. The influence of EVOH ratio and EVOH ethylene content on stretchability and film physical properties is clarified. From the above results, it is concluded that the grades F and H which have ethylene content 32 mol% and 38 mol% respectively are optimum grades having wide process windows. Properties Multi layer film Blend film Structure Outer layer PA6 100 % PA6 80 % + Center layer EVOH 100 % EVOH 20 % Oxygen gas permeability cm 3 /m 2 day 23 C 0 %RH Inner layer PA6 100 % 2 50 Film impact strength J/m Gelation Good Bad (Gelation) Processability Good Good Laminate strength Good Hot water resistance 90 C Good (Clear) 100 C Bad (White) Table 4. Physical properties of biaxially oriented PA6/EVOH film, multilayer film vs. blend film (outside/center/inside = 2/1/2) Intern. Polymer Processing XXI (2006) 1 91

7 Fig. 15. Relationship between process temperature and thickness uniformity (ethylene content 32 mol% EVOH, layer ratio = 1/1/1) 3.6 Interface Adhesive Strength of Multilayer Film The interface adhesive strength of the multilayer film was evaluated. As PA6 has an affinity to EVOH and high adhesive strength is obtained between PA6 and EVOH, there is no need to use adhesive as the middle layer. For this reason, this multilayer film has a big advantage. 3.7 Characteristics in Hot Water As the PA6 films sometimes require heat treatment in hot water, the multilayer film composed of PA6 and EVOH were evaluated in hot boiling water. The result is shown in Table 4. Below 90 C surface appearance changes are not observed. In boiling water (100 C) the film color turns white. Apparently it is because EVOH absorbs water at high temperature and ethylene-vinylalcohol copolymer crystallization proceeds. 3.8 Blend Material of PA6 and EVOH A blend material of PA6/EVOH 80/20 was stretched by using double bubble tubular film machine. Three layer multilayer film PA6/EVOH/PA6 2/1/2 was also stretched in order to compare with blend material including 20 % EVOH. Oxygen gas permeability of multilayer film and blend one are shown in Table 3. In spite of same EVOH content, Oxygen gas permeability of multilayer film is superior to the blend film. The blend of EVOH and PA6 is possible to stretch, but it forms a gel. EVOH and PA6 are reactive materials and material recycle is hard. Because both PA6 and EVOH have polar groups such as OH functional group and amide one, they are reactive and produces crosslinking structure. 4 Conclusion A multilayer film composed of PA6 layer and EVOH layer shows high strength and high barrier and could solve some environmental and barrier problems. PA6 in the outer layers and EVOH in the center layer are used. It is stretched biaxially by the double bubble tubular film machine. The content of EVOH 20 to 43 % is optimum and the biaxially oriented film is produced successfully. Low ethylene content of EVOH gives high barrier film, but it is more difficult to be stretched. High EVOH content gives high gas barrier property, but leads to decreases the mechanical properties. It is concluded that the grades F and H which have ethylene content 32 mol% and 38 mol% respectively are optimum grades having wide process windows. The PA6/EVOH multilayer film has high interface adhesion without any adhesive. The surface color of multilayer film turns white in hot boiling water, so its application in hot boiling water over 100 C is restricted. As a high strength film using non-halogen material can be produced, the waste and environmental problems are cut down. References 1 Motoishi, Y., Harita, S.: Paper and Plastics 12, p. 65 (1984) 2 Motoishi, Y., Harita, S.: Gosei-Jyushi Kogyo 2, p. 79 (1989) 3 Ikari, K.: J.P.I Journal 23, p. 291 (1985) 4 Ikari, K.: Trigger 4, p. 26 (1985) 5 Takashige, M., Kanai, T., Yamada, T.: Int. Polym. Process. 19, p. 147 (2004) 6 Japan Patent (1980) Kodera, Y., Ikari, K., Miake, S. 7 Japan Patent (1984) Kondo, K., Ishiguro, S. 8 Tae, O. A., Chang, K. K., Han, M. J.: Polym. Eng. Sci. 30, p. 341 (1990) 9 Kanai, T., Takashige, M.: Seni-gakkaishi 41, p. 272 (1985) 10 Takashige, M.: Film Processing, Kanai, T., Campbell, G. (Eds.), Progress in Polymer Processing Series. Hanser, Munich (1999) 11 Kang, H. J., White, J. L.: Polym. Eng. Sci. 30, p (1990) 12 Kang, H. J., White, J. L., Cakmak, M.: Int. Polym. Process. 1, p. 62 (1990) 13 Kang, H. J., White, J. L.: Int. Polym. Process. 5, p. 38 (1990) 14 Rhee, S., White, J. L.: Int. Polym. Process. 16, p. 272 (2001) 15 Rhee, S., White, J. L.: Polym. Eng. Sci. 39, p (1999) 16 Song, K., White, J. L.: Polym. Eng. Sci. 40, p. 902 (2000) 17 Song, K., White, J. L.: Int. Polym. Process. 15, p. 157 (2000) 18 Song, K., White, J. L.: Polym. Eng. Sci. 40, p (2000) 19 Rhee, S., White, J. L.: SPE ANTEC. Tech. Papers. 59, p (2001) 20 Rhee, S., White, J. L.: SPE. ANTEC Tech. Papers. 59, p (2001) 21 Takashige, M., Kanai, T., Yamada, T.: Int. Polym. Process. 18, p. 368 (2003) 22 Takashige, M., Kanai, T., Yamada, T.: Int. Polym. Process. 19, p. 47 (2004) 23 Takashige, M., Kanai, T., Yamada, T.: Int. Polym. Process. 19, p. 56 (2004) 24 Takashige, M., Kanai, T.: Int. Polym. Process. 5, p. 287 (1990) 25 Takashige, M., Kanai, T.: Int. Polym. Process. 20, p. 100 (2005) 26 Okaya, T., Ikari, K.: Polyvinyl Alcohol Development. Finch, C. A. (Ed.), John Wiley & Sons, London (1992) Date received: August 23, 2005 Date accepted: January 26, 2006 You will find the article and additional material by entering the document number IPP0110 on our website at 92 Intern. Polymer Processing XXI (2006) 1