Thickness Uniformity of Double Bubble Tubular Film Process for Producing Biaxially Oriented PA 6 Film

Transcription

1 FILM M. Takashige 1 *, T. Kanai 2, T. Yamada 3 1 Idemitsu Unitech Co., Ltd., Hyogo, Japan 2 Idemitsu Petrochemical Co., Ltd., Chiba, Japan 3 Kanazawa University, Ishikawa, Japan Thickness Uniformity of Double Bubble Tubular Film Process for Producing Biaxially Oriented PA 6 Film It is known that while the double bubble tubular film process gives better impact strength and more uniform shrinkage balance than the tenter process, yet it is relatively poor in film thickness uniformity. The film thickness uniformity of the PA 6 produced by the double bubble tubular film process was investigated. The optimum stretching stress during the double bubble tubular film process exists. The bubble breaks over stress 130 MPa and bubble is unstable below stress 60 MPa. In the optimum condition, which is process temperature 310 C and stretching ratio 3.2, stretched film uniformity was twice worse than nonstretched film one. The thickness uniformity of the film stretched in the optimum condition was better than that of the film stretched at higher process temperature and at lower stretching ratio. In the observations of the bubble sample through the polarizing plates, deformation pattern showed the equality of deformation. Thickness uniformity of non-stretched film significantly affects the thickness uniformity of biaxially oriented film. Local stretching ratio, which varies along the film width, influences tensile modulus and tensile strength at break. In order to obtain the uniform physical properties of film, it is important to produce the film with uniform thickness. Film thickness uniformity can be improved when stretching stress is high and bubble stability is good. These factors are influenced by stretching temperature, stretching ratio and air velocity from air ring. 1 Introduction * Mail address: M. Takashige, Idemitsu Unitech Co., Ltd Kou, Shirahama-cho, Himeji-city, Hyogo, Japan masao.takashige@iut.ipc.idemitsu.co.jp In recent years, environmental problems have come into question in the packaging industry. As these problems, the de-chlorination and the waste reduction have been closed up in this industry. Especially this waste reduction has been a serious problem, so it is desired that this problem will be solved. 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 PA 6 film is indispensable. The demand for the biaxially oriented PA 6 film has become popular. Several kinds of manufacturing processes have been developed to produce this biaxially oriented PA 6 film featured by high strength. Among them the double bubble tubular process producing biaxially stretched PA 6 film is the best one in terms of impact strength. It is valued highly for the purpose of safe distribution of products. The biaxially stretched film is required to pass the fabricating process in which there are severe requirements such as multicolor printing. For this reason, it becomes a vital technical importance to improve thickness uniformity. It is known that while the double bubble tubular film process gives better impact strength and more uniform shrinkage balance than the tenter process, yet it is relatively poor in film thickness uniformity [1]. Nylon is also featured by the strong hydrogen bonding owing to the molecular structure having polyamide bonding intrinsic to the resin, and therefore it is better to conduct simultaneous biaxial stretching than sequential one. The film thickness uniformity improvement by means of flow analysis of circular dies [2] as well as the film uniformity improvement through cooling control of blown film, automatic bolt control, and the like [3] were reported. There are several reports issued on tubular stretching technology [4 to 15]. The analyses of deformation behavior of PA 6 were reported previously according to stretching stress analyses [16]. The previous paper discussed three topics. They are (1) the relationship between process condition and film stretching stress, (2) deformation rate during double bubble process and birefringence, and (3) scale-up rule for double bubble tubular film process. In this paper, the experiments under the various process conditions were carried out to clarify the factors governing this film thickness uniformity and improve it. The technical results obtained from this research were used in the production process to produce uniform thickness film. The detail studies on the process conditions governing the film thickness uniformity in double bubble tubular PA 6 film process are described below. Intern. Polymer Processing XIX (2004) 1 Hanser Publishers, Munich 47

2 Fig. 1. Schematic view of double bubble tubular film process, a: extruder, b: die, c: cooling bath, d: take up roll, e: air ring, f: heating furnace, g: annealing, h: winding 2 Experimental 2.1 Equipment Apparatus used for the double bubble tubular film process is shown in Fig. 1. By using a 40 1 mm extruder (L/D = 24) with a 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, non-stretched film was produced. While passing through the part of apparatus 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 heat-set using a heat treatment device. 2.2 Material The material was Ube Nylon 1024FD15 (PA 6) with the mean molecular weight of and the relative viscosity of g r = 3.75 in 98 % sulfuric acid as a solvent. 2.3 Experimental Method The effect of process temperature for the second blowing (temperature of heating furnace), stretching ratio and air velocity on the film thickness uniformity was investigated. Non-stretched film having a variety of local thickness distribution was obtained by de-centering the clearance of the die lip. The process condition of non-stretched film was 265 C for resin temperature at the die exit, 1.2 for blow up ratio, and 6.0 for draw down ratio respectively. The extrusion rate used was 17.6 kg/h and the take up velocity used was 7.0 m/min. Film was quenched in water at 18 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. Fig. 2. Measurement method of stretching stress (the bubble internal pressure was measured using a digital manometer) 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 thickness of non-stretched film was 130 lm, and it became 13.5 lm after stretching. The stretched film was heat treated to prevent shrinkage, using a heat treatment device of the third bubble tubular process. The thickness uniformity of non-stretched film and heattreated stretched film was measured. The thickness uniformity was normalized by the mean thickness as: Thickness uniformity (%) = (maximum thickness minimum thickness)/ (2 mean thickness) 100. (1) The thickness variation at each location was normalized by the mean thickness as: Thickness variation at each location (%) = (thickness at each position mean thickness)/ mean thickness 100. (2) Fig. 2 shows the measurement method of stretching stress. Stretching stress was calculated with the help of the theoretical equation reported by the authors et al. [16]. The maximum stress at the end point of stretching may be obtained by the following equation. r = DP R/H L, (3) where r: stretching stress in the TD, DP: internal bubble pressure, R: bubble radius at the end point of stretching, H L : film thickness at the end point of stretching. The bubble internal pressure was obtained using the digital manometer (Yokogawa Hokushin). 2.4 Observation of Deformation Behavior In order to analyze the behavior of deformation pattern, the extruder and take-up device were suddenly stopped and bubble sample was collected. 48 Intern. Polymer Processing XIX (2004) 1

3 Tubular film samples obtained under various conditions during the stretching process were taken and each of them was inserted between two polarizing plates crossed with each other to examine the deformation state of stretching bubbles by applying incident light from below. The optical anisotropy of the film from the start point to the end point of stretching was observed. Each sample was photographed using a high sensitivity film. 2.5 Evaluation of Local Physical Properties The relationship between actual stretching ratio and tensile properties of stretched film in the circumferential direction (Transverse Direction) such as tensile modulus and tensile strength was evaluated. For this evaluation the film stretched from non-stretched samples with three different thickness uniformity (± 4%, ± 10 %, ± 23 %) were used. 3 Results and Discussion The important factors of the double bubble tubular film process are bubble stability and thickness uniformity. There were many process conditions which influence bubble stability and thickness uniformity, for example process temperature (temperature of heating furnace), stretching ratio, air velocity, deformation rate, cooling condition, thickness uniformity of non-stretching film and so on. In these factors, four kinds of condition, which are process temperature (temperature of heating furnace), stretching ratio, air velocity and thickness uniformity of nonstretching film were chosen for detailed investigation. 3.1 Process Temperature The relationship between stretching stress for the blowing up ratio of 3.2 and process temperature (temperature of heating furnace) is shown in Fig. 3. The stretching stress in TD decreases with increasing process temperature. The bubble break- Fig. 3. Relationship between process temperature and stretching stress Fig. 4. Influence of process temperature on film thickness uniformity Fig. 5. Relationship between stretching ratio and stretching force Fig. 6. Relationship between temperature and tensile modulus Intern. Polymer Processing XIX (2004) 1 49

4 A) B) C) D) age occurs over the stress 130 MPa and bubble instability occurs below the stress 60 MPa. The thickness uniformity of non-stretched and stretched film vs. process temperature is plotted in Fig. 4. The thickness uniformity of stretched film becomes worse with increasing process temperature. As the stretching stress rises, the thickness uniformity improves. The temperature dependence of the stretching force versus strain curve, which is obtained by tensile testing of nonstretched film in the constant temperature oven, is plotted in Fig. 5. When the process temperature increases, the strain hardening is suppressed. This may be the reason for lower thickness uniformity. Fig. 8. Relationship between stretching ratio and stretching stress Fig. 7. Observation of the second bubble sample for different process temperatures using polarizing plates, A: 280 C, B: 310 C, C: 330 C and D: 350 C The relationship between film temperature and tensile modulus is plotted in Fig. 6. Tensile modulus is influenced by film temperature. The temperature dependence of tensile modulus is high, and it becomes easy to be stretched by increasing temperature. Polarizing plate observation photographs of the stopped samples, which were obtained at the different process temperatures, are shown in Fig. 7. In general, observation of captured film between crossed polarizing plates showed the following characteristics. In the region before the blowing up, there was no transmission of light because of low optical anisotropy of the film in this region. In the vicinity of the starting point of blowing up, strong interfer- Fig. 9. Influence of stretching ratio in TD on film thickness uniformity 50 Intern. Polymer Processing XIX (2004) 1

5 A) B) C) D) ence color was observed for all the samples. This result suggests the development of significant optical anisotropy in this region. The molecular orientation was speculated to be in the MD because the optical anisotropy started to appear before the one-set point of blowing up in some samples. In the vicinity of the end point of blowing up, optical anisotropy disappeared. Tendency of molecular orientation toward the equal biaxial orientation because of stretching in TD and smaller thickness of the film in this region are attributable for the disappearance of the optical anisotropy. At the low blowing up temperatures where films with higher uniformity was obtained, blowing deformation was steep and the region of film exhibiting high optical anisotropy concentrated in the vicinity of the starting point of blowing up, and also variation of optical anisotropy along circumference of the bubble was limited. Therefore through the observation using two polarizing plates it was conclude that it is important to set the stretching temperature low enough to keep bubble stable, but not too low for the continuous production of films without bubble breakage. 3.2 Stretching Ratio Fig. 8 shows the relationship between stretching ratio and stretching stress at the process temperature of 310 C (temperature of heating furnace) and stretching ratio in MD of 3.0. The stretching stress in TD increases with increasing stretching ratio. The bubble breakage occurs over the stress 130 MPa and bubble instability occurs below the stress 60 MPa. The thickness uniformity of non-stretched and stretched film with stretching ratio in TD is plotted in Fig. 9. The film thickness uniformity tends to improve when the stretching ratio in TD is increased and the stretching stress rises. When the stretching ratio in TD increases, the thickness uniformity is improved because of the enhanced strain hardening (Fig. 5). Understandably, it is important to set the high stretching ratio in TD in order to keep bubble stable, while the stretching ratio needs to be low enough to keep stretching stress permissible for continuous production of film without bubble breakage. The stopped samples with various stretching ratio in TD observed using two polarizing plates are shown in Fig. 10. While film with good thickness uniformity passes through the limited region of strong anisotropy, which starts from the stretching start point, the one with poor thickness uniformity is likely to be very unstable since it comes out clearly with uneven deformation in TD. This result is the same as the one obtained under different temperatures shown in Fig Air Velocity Fig. 10. Observation of the second bubble sample for different stretching ratios using polarizing plates, stretching ratio in TD 2.8(A), 3(B), 3.2(C) and 3.4(D); stretching ratio in MD = 3 The thickness uniformity of non-stretched and stretched film produced by changing air velocity is plotted in Fig. 11. It is understood that air blowing by air ring from the top of the stretching area helps preventing the process temperature becoming too high as well as shortening the deformation range during the stretching, thus contributing thickness uniformity very well. It was confirmed that the faster the air velocity is, the better the film thickness uniformity becomes. Intern. Polymer Processing XIX (2004) 1 51

6 3.4 Relationship between Non-Stretched Film Thickness Uniformity and Stretched Film One The relationship between thickness uniformity of nonstretched and stretched films is plotted in Fig. 12. From the results of studying the influence of the thickness uniformity of non-stretched film by changing ± 4%, ± 10 %, and ± 23 % on thickness uniformity of the stretched film, it was clear that the poorer the thickness uniformity of non-stretched film is, the worse the thickness uniformity of stretched film becomes markedly. Fig. 11. Influence of air velocity on film thickness uniformity Fig. 12. Influence of thickness uniformity of non-stretched film on film thickness uniformity A) B) Fig. 13. Comparison of the film thickness variation for non-stretched film with thickness uniformity ± 4% (A) and stretched film (B) Thickness variation along TD of non-stretched film (± 4%) is shown in Fig. 13. The relationship between film position and thickness variation of non-stretched film and stretched film are plotted in Fig. 13A and Fig. 13B, respectively. In order to analyze the correlation of thickness variation between non-stretched and stretched samples, thickness of corresponding location for films with thickness uniformity of ± 4% were plotted as shown in Fig. 14. It was found that the local thickness of non-stretched and stretched films has linear relation. The least square analysis was carried out using the regression equation Y = CX + D. Because analyzed D in the regression equation was small enough, the regression coefficient C, which was analyzed to be 2.0, can be regarded as the amplification coefficient. The thickness variation of non-stretched and stretched samples for the thickness uniformity of non-stretched films of ± 10 % and ± 23 % were plotted as shown in Figs. 15 and 17, respectively. The correlation of the thickness of corresponding location for non-stretched and stretched films with thickness uniformity of ± 10 % and ± 23 % were plotted as shown in Figs.16 and 18, respectively. The amplification factors of these two samples were analyzed to be 1.8. Polarizing plate observation photograph of the stopped sample, with various thickness uniformity of non-stretched film, is shown in Fig. 19. While the film with good thickness uniformity passes through the range of strong anisotropy in short time from the stretching start point, the one with poor thickness uniformity is likely to be very unstable since it comes out clearly with uneven deformation in the transverse direction. It is understood that it is important to set the good thickness uniformity of non-stretched film. The regression coefficient is defined as amplification factor. The relationship between non-stretched film thickness uniformity and amplification factor is plotted in Fig. 20. The amplification factor was approximately 2.0 (1.8 ~ 2.0) in the stable stretching range. Fig. 14. Plot of the relation between local thickness variation of nonstretched film and that of stretched film. Amplifying factor was evaluated from the slope of this figure 52 Intern. Polymer Processing XIX (2004) 1

7 Fig. 15. Comparison of the film thickness variation for non-stretched film with thickness uniformity ± 10% (A) and stretched film (B) Fig. 16. Plot of the relation between local thickness variation of nonstretched film and that of stretched film. Amplifying factor was evaluated from the slope of this figure Therefore it was confirmed that at the condition of stable stretching process, the stretching film thickness uniformity is about twice as bad as non-stretched film thickness uniformity. It the thickness turned out that the thickness uniformity of non-stretched film is vital for the improvement of uniformity of stretched film. The stretching stress in the width direction was calculated by measuring the internal bubble pressure. If unevenness in thickness is present in the transverse direction, it results in a local change in stretching stress. Fig. 21 shows the relationship between film thickness and stretching stress. From the above result it turns out that if there occurs difference in stretching stress due to thickness unevenness in the transverse direction, it leads to a local difference in stretching ratio. In other words, it is understood that the larger the difference in non-stretched film thickness is, the worse the thickness uniformity of stretched film is. A local thick portion causes a local drop in stretching stress and a decrease in bubble temperature. As a result, the thicker portions are less stretched; on the contrary thinner portions are more stretched. As a consequence uniformity of the stretch film was twice as bad as that of the non-stretched film. It became known that thickness uniformity of stretched film might be improved even in tubular stretching process by combining the optimum stretching condition and the improved thickness uniformity of non-stretched film. Fig. 17. Comparison of the film thickness variation for non-stretched film with thickness uniformity ± 23% (A) and stretched film (B) Fig. 18. Plot of the relation between local thickness variation of nonstretched film and that of stretched film. Amplifying factor was evaluated from the slope of this figure Intern. Polymer Processing XIX (2004) 1 53

8 A) B) C) Fig. 19. Observation using polarizing plate of the second bubble sample prepared from non-stretched films of various thickness uniformity A: ± 4%,B:± 10 %, C: ± 23 % 3.5 Correlation between Actual Stretching Ratio and Film Physical Properties The amplification factor is about 2.0, and it means that thicker portions are more difficult to be stretched, and thinner portions are easier to be stretched. In other words, the actual stretching ratio is liable to induce difference in physical properties locally as compared with the setting ratio. The relationship between local thickness uniformity and tensile modulus is plotted in Fig. 22. It is clear from the experiment that in view of the physical properties of film, the portion with small actual stretching ratio in the TD has low tensile modulus in the MD. In PA 6 film, its molecular structure leads to a reverse tendency between the direction of stretching ratio and the change in tensile modulus. This is because the hydrogen bonding is formed in the direction at right angles to the configuration of molecular chains. Fig. 20. Relationship between thickness uniformity of non-stretched film and amplifying factor Fig. 21. Relationship between film thickness and stretching stress The relationship between local thickness uniformity and tensile strength is plotted in Fig. 23. In tensile strength, the correlation was confirmed with actual stretching ratio. Tensile strength is related to a change in the stretching direction. Actual stretching ratio, which is the local stretching ratio along the film width, influences tensile modulus and tensile strength. It was found that it is important to produce the uniform film in order to obtain the uniform physical properties of film. 3.6 Relationship Between Process Conditions and Stretching Stress, Process Stability and Thickness Uniformity The relationship between process conditions and stretching stress, process stability, and film thickness uniformity was summarized in Table 1. It was concluded that film thickness uniformity is improved when the stretching stress is higher and the bubble stability is better. This means that deformation region in the stretching process needs to be short, stretching temperature needs to be low and air blowing is required. The completion of smooth stretching deformation in a short time holds the key. 54 Intern. Polymer Processing XIX (2004) 1

9 Processing conditions Stretching stress Bubble stability Thickness uniformity Fig. 22. Relationship between local thickness variation and tensile modulus Fig. 23. Relationship between local thickness variation and tensile strength 4 Conclusion The thickness uniformity of the PA 6 film obtained by a double bubble tubular film process was investigated. The optimum stretching stress during the double bubble tubular film process exists. The bubble breaks over stress of 130 MPa and bubble is unstable below stress of 60 MPa. In the optimum conditions, which is process temperature 310 C (temperature of heating furnace) and stretching ratio in TD 3.2, stretched film uniformity was twice worse than nonstretched film one. The thickness uniformity of the film stretched in optimum conditions was better than that of the film stretched at higher process temperature and at lower stretching ratio in TD. In the observations of the bubble sample through the polarizing plates, pattern of optical anisotropy showed the homogeneous deformation. Thickness uniformity of nonstretched film significantly affects the thickness uniformity of biaxially oriented film. Process temperature Stretching ratio Air velocity Deformation rate Actual stretching ratio, the local stretching ratio along the film width rather than the average stretching ratio, influences tensile modulus and tensile strength. In order to obtain the uniform physical properties of film, it is important to produce the film with uniform thickness. If there exists partially thick portion, the rate of its increasing temperature delays during passing through the heating furnace. As a result, the thicker portions are less stretched. On the contrary, thinner portions are more stretched. Film thickness uniformity can be improved when stretching stress is high and bubble stability is good. These factors are influenced by stretching temperature, stretching ratio and air velocity from air ring. References 1 Yamada, T.: Production Technologies of Films and Membranes of New Film and Membrane. Kagaku Kougyo Nippo, Tokyo (1997) 2 Pendikoulious, J., in: Film Processing, Kanai, T., Campbell, G. (Eds.), Progress in Polymer Processing Series. Hanser, Munich (1999) 3 Predöhl, W.: Film Processing, in: Kanai, T., Campbell, G. (Eds.), Progress in Polymer Processing Series. Hanser, Munich (1999) 4 Kanai, T., Takashige, M.: Seni-gakkaishi, 41, p. 272 (1985) 5 Takashige, M., in: Film Processing, Kanai, T., Campbell, G. (Eds.), Progress in Polymer Processing Series. Hanser, Munich (1999) 6 Kang, H. J., White, J. L.: Polym. Eng. Sci. 30, p (1990) 7 Kang, H. J., White, J. L., Cakmak, M.: Int. Polym. Process. 1, p. 62 (1990) 8 Kang, H. J., White, J. L.: Int. Polym. Process. 7, p. 38 (1992) 9 Rhee, S., White, J. L.: Int. Polym. Process. 16, p. 272 (2001) 10 Rhee, S., White, J. L.: Polym. Eng. Sci. 39, p (1999) 11 Song, K.,White, J. L.: Polym. Eng. Sci. 40, p. 902 (2000) 12 Song, K., White, J. L.: Int. Polym. Process. 15, p. 157 (2000) 13 Song, K., White, J. L.: Polym. Eng. Sci. 40, p (2000) 14 Rhee, S., White, J. L.: SPE Antec Tech. Papers. 59, p (2001) 15 Rhee, S., White, J. L.: SPE Antec Tech. Papers. 59, p (2001) 16 Takashige, M., Kanai, T.: Int. Polym. Process. 5, p. 287 (1990) Date received: July 31, 2002 Date accepted: November 21, 2003 Table 1. Relationship between Processing conditions and stretching stress, bubble stability and thickness uniformity Intern. Polymer Processing XIX (2004) 1 55