BIAXIALLY ORIENTED SHRINKAGE PA6 FILM

Transcription

1 BIAXIALLY ORIENTED SHRINKAGE PA6 FILM M.Takashige (a), *, T.Kanai (b) (a) Idemitsu Unitech Co, Tokyo, Japan (b) Idemitsu Kosan Co, Chiba, Japan ABSTRACT In the market place, a shrinkage film which is adapted to both environmental and barrier problems is desired. Polyvinylidene chloride film is a popular shrinkage film for food packaging, but when this film is incinerated, it generates toxic materials such as chlorine gas and dioxins. In recent times, environmental problems have come into question in the packaging industry. A biaxially oriented polyamide 6 (PA6) film produced by the double bubble tubular film process is suitable for shrinkage film. To produce high shrinkage film, the influence of process conditions and material design on shrinkage property is investigated in this report. The heat setting temperature can easily control the shrinkage, density and other physical properties. The blend of PA6-66 and poly(m-xylylene adipamide) (MXD6) can achieve a barrier as effective as polyvinylidene chloride, and the shrinkage film from this blend has an easy and straight tear, allowing for easy opening of the product. The material and production technology of a PA6 film could solve high shrinkage, environmental and long life packaging problems. 1. INTRODUCTION The biaxially oriented film made of polyvinylidene chloride is widely * Mail address: M. Takashige, Idemitsu Unitech Co.,Ltd. Shinkawa NS bldg, 26-2 Shinkawa 1-chome, Chuo-ku, Tokyo, Japan masao.takashige@iut.idemitsu.co.jp

2 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film used as a shrinkage film for packaging of ham and sausage etc, because of its good shrinkage properties and gas barrier performance /1,2/. In recent years, environmental problems have come into question in the packaging industry. Of these problems, the de-chlorination and the waste reduction in particular have raised concerns in this industry. When this film is incinerated, it generates toxic materials such as chlorine gas and dioxins. The development of oxygen gas barrier film without halogenated compound is desired. The previous report discussed an oxygen gas barrier film which satisfies formability and physical properties by using PA6 and MXD6 blend /3/, PA6/MXD6 multilayer film /4/ and PA6/ethylene-vinylalcohl (EVOH) multilayer film /5/. If there is even a slight clearance between meat and film, meat drippings accumulate which are unsanitary and cause poor appearance. Regarding this point, a shrinkage film which can package foods tightly and is environmental problem free is required. The packaging of sausage and meat uses sausage casing (rocket package) which needs calking with aluminum, but it cannot seal the package perfectly. Furthermore, as aluminum is used, the metal detector cannot be used. There is also a problem of safety and package opening, and a change of the package system is desired. The development of high shrinkage film is desired by using the double bubble tubular film process. There are several reports issued on the double bubble tubular stretching technology [3-25] of formability and the structure analysis for resin such as polyethylene terephthalate (PET), polybuthylene terephthalate (PBT), polyphenylene sulfide (PPS), PA6-12 and PA12 that are reported by White, Kang, Song and Rhee et al /8-17/. However, there are only a few reports on the double bubble tubular technology of the shrinkage film /18-20/. On the other hand, a biaxially oriented film has been widely used as a top sheet of plastic packages. In this field, the biaxially oriented films of PET and PA6 were used, but the top sheet has a rippling problem and also an unsatisfactory appearance after boiling sterilization. In this usage, a controllable weak shrinkage film at low temperature has been required. The biaxially oriented film produced by the tentering process has been developed for this usage /21/, but the container with the top sheet causes distortion /22/ because of its shrinkage anisotropy. So a shrinkage film produced by the double bubble tubular process which has a good shrinkage balance has been desired /23-29/. 2

3 M. Takashige and T. Kanai Journal of Polymer Engineering A film which is adapted to both environmental problems and shrinkage properties was developed in this research. 2.1 Equipment 2. EXPERIMENTAL The apparatus used for the double bubble tubular film process is shown in Fig. 1 /24/. A 40 mmφ extruder (L/D=24) with a circular die 75mm in diameter and a lip clearance of 1mm and with a water-cooling ring 90 mm in diameter was used. While passing through the second blowing process, which is composed of two pairs of pinch rolls and a heating furnace (the far infrared radiation heater is self-contained), this raw film was stretched simultaneously in the machine and transverse directions by using high take up force and the internal bubble air. The stretched film was heat-set using a heat treatment device described in the previous report /25/. 2.2 Material The material was Ube PA 1023FD (PA 6) with a mean molecular weight of 23,000 and relative viscosity of η r =3.5 in 98% sulfuric acid as a solvent. Ube PA 6-66 copolymer 3053 (PA 66 content 15%) with a mean molecular weight of 30,000 and relative viscosity of η r =3.8 in 98% sulfuric acid as a solvent was also used. The material was Mitsubishi Gas Chemical MX PA 6007 (MXD6) with a mean molecular weight of 25,000 and relative viscosity of η r = 2.7 in 96% sulfuric acid as a solvent. 2.3 Experimental Method The melt process conditions of the un-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. The extrusion rate was 17.6 kg/h and the take up velocity was 7.0 m/min. The 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. 3

4 Hopper Infrared Heat Treatment Heater Air Bubble Die Water Cooling Ring Tenter Second Bubble First Bubble Fig.1 Schematic view of double bubble tubular process

5 The standard conditions for the stretching process were set at 310 C for the process temperature (temperature of heating furnace) and MD (Machine Direction)/ (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 heat setting temperature was varied between 150 and 210 C and the relaxation ratio was 10% in. The stretching ratios of both MD and are controlled 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 MD and. The measurement method of stretching stress is described in the previous report /7/. Stretching stress was calculated with the help of the tubular theoretical equation (1) reported by the authors et al. /7/. σ = (ΔP R)/H L (1) where ΔP, R, H L are inside bubble pressure, final bubble radius and final film thickness respectively. Fig.2 shows the measurement method of stretching stress. 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 could be obtained. The heat set temperatures were changed and the shrinkage in hot water as a function of temperature was evaluated. Two conditions, namely the hot steam condition at 120 C (retort treatment)for 30 minutes and the hot water condition at 95 C (boil treatment) for 30 minutes, were tested 2.4 Heating Medium Water and silicone oil were used as a heating medium in order to clarify the influence of water on film shrinkage. The PA6 film is mainly used for food packaging, so sterilization in the boiling treatment at 95 C or retort treatment at 120 C is required to prolong the lifetime of foods. For this reason, shrinkage in hot water is a very important factor. So the film shrinkage was evaluated in water and out of water.

6 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film Fig. 2 Measurement method of stretching stress ΔP: inside bubble pressure, R: final bubble radius, HL: final bubble thickness, T: bubble tension, r: pinch roll radius 2.5 PA6/MXD6 Blend Polymer and PA6-66 Copolymer PA 6 and MXD 6 blend polymers were evaluated to promote barrier property, shrinkage and easy straight cut performance. PA 6-66 copolymer was also evaluated in terms of shrinkage and easy straight cut performance too. PA 6/MXD 6 blend polymers and also PA6-66/MXD 6 ones were prepared by dry blending and mixing in an extruder and then biaxially oriented film was produced by the same process as PA6 film. 2.6 Shrinkage in hot water In the PA 6 film for retort foods, it is soaked in hot water at a high 6

7 M. Takashige and T. Kanai Journal of Polymer Engineering temperature in order to conduct the sterilization and processing. The relationship among heat setting temperature, film shrinkage percentage and film density was examined. The value of the shrinkage percentage pattern in plane was measured by changing the heat setting temperature and cross-direction position. The shrinkage percentages under the pressurized hot water of 120 C/ 30 minutes as the retort treatment and under hot water 95 C / 30 minutes under the boiling treatment are important. At first the gauge length of 100mm in each direction, namely MD,, diagonal 45 and 135, was marked and then the film sample was soaked in pressurized hot water at 120 C for 30 minutes. After conditioning it for 24 hrs under the 23 C 50% relative humidity (RH), the gauge length L S was measured. The shrinkage percentage S was calculated by using equation (2). S = (L 0 L S )/ L (%) (2) The shrinkage data is the average data of MD,, diagonal 45 and Mechanical Properties An evaluation of gas barrier and toughness was also carried out. The oxygen gas permeability was carried out under 23 C, 60%RH condition by using Mocon Oxtran which can measure the gas transmission rate testing of flat films. The toughness was evaluated by using the film impact strength equipment. The tearing resistance was evaluated by the measurement method of ASTM D 1922 using the elemendorf tearing test machine produced by Toyo Seiki. The straight line cut was evaluated by using the stretched film sample. The film was torn at the 200mm length, and the deviation length was measured. 3.1 PA 6 Film 3. RESULTS AND DISCUSSION 1) Heat Setting Temperature and Shrinkage Fig. 3 shows the relationship between heat setting temperature and shrinkage percentage in pressurized hot water at 120 C with 30 minutes as a 7

8 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film retort condition. Shrinkage decreases with increasing heat setting temperature. High heat setting temperature makes high crystallization and low shrinkage. At the same time, film density is proportional to heat setting temperature. Shrinkage is easily controlled by heat setting temperature. At the condition of heat setting temperature 160 C, shrinkages in both MD and at 120 C retort condition were about 30% in hot water ºC, 30min Δ 95ºC, 30min Shrinkage Percentage Heat setting Temperature ( ) Fig.3 Relationship between heat setting temperature and shrinkage percentage of PA6 in hot water 95ºC and 120ºC (PA 6) Fig. 3 shows the relationship between heat setting temperature and shrinkage percentage in hot water at 95 C for 30 minutes as a boiling condition too. At the condition of heat setting temperature 195 C, shrinkages in both MD and at 95 C boiling condition were 5% in hot water. Shrinkage is very much influenced by heat setting temperature. If high shrinkage film is required, low heat setting temperature is preferable. On the contrary, if the low shrinkage film is required, high heat setting temperature is preferred. 8

9 M. Takashige and T. Kanai Journal of Polymer Engineering Table 1 shows the physical properties of biaxially oriented PA 6 for general purpose film, low shrinkage film and high shrinkage film. General purpose film means heat treatment at 210 C. Low shrinkage film means heat treatment at 195 C. High shrinkage film means heat treatment at 160 C. Shrinkage increases with decreasing heat setting temperature (Fig. 4). Low heat setting temperature makes low crystallinity, low modulus and high impact strength. Table 1 Film physical properties of biaxially oriented PA 6 at three different heat setting temperatures Properties Tensile Modulus Tensile Strength Use General Purpose Film Low Shrinkage Film High Shrinkage Film Resin PA6 PA6 PA6 Heat setting 210 C 195 C 160 C Condition ( C) MPa MD 2,400 2,200 2,100 2,100 2,100 2,000 MPa MD Elongation MPa MD Film Impact Strength Shrinkage Percentage Testing Method ASTM- D882 ASTM- D882 ASTM- D882 J/m 93, , ,000 IPC Method % MD MD C 30min 120 C 30min 9

10 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film % 6 MD Heat setting temperature 210ºC General purpose film 135 Heat setting temperature 6 MD ºC Low shrinkage film 20 MD Heat setting temperature 160ºC High shrinkage film Fig.4 Shrinkage percentage patterns in plane under the different heat setting temperature 210ºC, 195 ºC and 160 ºC ( hot water temperature 95 ºC, 30minutes) 10

11 M. Takashige and T. Kanai Journal of Polymer Engineering 1,155 1,150 Film density (kg/m 3 ) 1,145 1,140 1,135 1, Heat setting temperature ( ) Fig. 5 Relationship between heat setting temperature(heater set temperature) and film density for PA6 The relationship between heat setting temperature and film density is plotted in Fig. 5. The film density increases with a rise in heat setting temperature. The crystallinity increased with increasing temperature. It is necessary to reduce the shrinkage percentage of a film to less than 5% for the application of retorting. It is necessary to set the heat setting temperature near 210 C for this reason. (General purpose film) 2) Processability Figure 6 shows the process temperature of stretching furnace influences on stretching stress. The stretching stress increases with decreasing process temperature. 3.2 PA 6-66 Film 1) Processability The relationship between the set temperature of the heating/stretching furnace and stretching stress of PA 6-66 copolymer and PA 6 is shown in Fig.6. PA 6-66 copolymer has a lower stretching stress than PA 6, because 11

12 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film the hydrogen bond and the crystallization of PA 6-66 may be suppressed by copolymerization. Stretching of PA6-66 is possible in the wide temperature range shown in Fig.6. PA 6-66 copolymer has good continuous processability without any breaks Stretching Stress (MPa) PA 6-66 PA Process Temperature ( ) Fig.6 Relationship between process temperature (heater temperature) and stretching stress for PA 6 and PA ) Impact Strength Table 2 shows the physical properties of PA 6 and PA 6-66 film, compared with PA6. PA 6-66 film has a higher impact strength and elongation than PA 6 film. PA 6-66 has lower modulus. 3) Shrinkage Property in hot water Table 2 shows the physical properties of biaxially oriented PA6-66 copolymer. PA6-66 film has lower tensile modulus and higher elongation, impact strength and shrinkage than PA6. 12

13 M. Takashige and T. Kanai Journal of Polymer Engineering Table 2 Film physical properties of biaxially oriented PA6 and PA 6-66 copolymer Properties Use High Shrinkage Film High Shrinkage Film Resin PA6 PA6-66 copolymer Heat setting Condition ( C) 160 C 160 C Testing Method Tensile Modulus Tensile Strength MPa MPa MD MD Elongation MPa MD Film Impact Strength Shrinkage Percentage 2,100 2, ,900 1, ASTM-D882 ASTM-D882 ASTM-D882 J/m 105,000 12,4000 IPC Method % MD MD C 30min in hot water 120 C 30min in hot steam Shrinkage percentage pattern at the retort condition of 120 C for 30 minutes in pressurized hot water is shown in Fig. 7. Shrinkage decreases with increasing heat setting temperature. Fig. 7 shows that shrinkage percentage in MD and is 39% under the condition of heat setting temperature 160 C and the retort condition of 120 C. 13

14 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film Shrinkage percentage (% Heat setting temperature ( ) PA6-66 PA6 Fig. 7 Relationship between heat setting temperature and shrinkage percentage of PA6 and PA6-66 in pressurized hot water 120,30minutes Shrinkage percentage of PA 6-66 and PA 6 as a function of hot water temperature is shown in Fig. 8. Shrinkage linearly increases with increasing hot water temperature between 50 and 120 C. PA 6-66 has 10% higher shrinkage at the retort condition than PA 6 (Fig. 8). This value is almost the same level as polyvinylidene chloride (39%). 14

15 M. Takashige and T. Kanai Journal of Polymer Engineering Shrinkage percenta PA 6 PA Temperature of hot water ( ) Fig. 8 Relationship between temperature and shrinkage percentage of PA 6 and PA 6-66 in hot water 3.3 Shrinkage in hot medium Shrinkage of biaxially oriented film produced at the different heat setting temperatures is measured in both hot water and silicone oil. Figure 9 shows heat setting temperature in a hot medium influences the shrinkage. Shrinkage in water is larger than that in silicone oil. Water makes the hydrogen bonds of PA 6 weaken. From Fig. 9, water existence is equal to 40 C higher shrinkage temperature effect. 15

16 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film Shrinkage Percentage (% High shrinkage film (water) High shrinkage film (silicon oil) General purpose film (water) General purpose film (silicon oil) Heating Medium Temperature ( ) Fig. 9 Relationship between heating medium temperature and shrinkage percentage of high shrinkage film and general purpose film in hot water and silicone oil (Heat setting temperature; high shrinkage film 160 C, general purpose film 210 C) 3.4 PA 6 / MXD 6 Blend Film The blend film of PA 6 and MXD 6 was studied with an expectation of shrinkage and easy and straight cut properties /3/. Figure 10 shows the relationship between heat setting temperature and shrinkage percentage at the retort condition in 120 C pressurized hot water. Shrinkage decreases with increasing heat setting temperature because the heat setting promotes the crystallinty and the thermal dimension is stabilized. Shrinkage percentage in MD and is 28% under the condition of heat setting temperature 160 C and the retort condition of 120 C. As expected, this blend film has an easy and straight cut property the same as we obtained in the previous report /3/. High performance shrinkage film was developed. 16

17 M. Takashige and T. Kanai Journal of Polymer Engineering Shrinkage Percentage (% PA6/ MXD6=70/ 30 PA6-66/ MXD6=70/ Heat setting Temperature ( ) Fig. 10 Relationship between heat setting temperature and shrinkage percentage of blend film in hot water Blend film: PA 6 / MXD 6 = 70 / 30 and PA 6-66 / MXD 6 = 70 / 30 Figure 11 shows the measurement method of the straight line cut in film. Figure 12 shows the relationship between the MXD6 blending ratio and the straight line cut. The deviation length of MD drastically decreased with increasing MXD6 blending ratio. In PA6 film, the deviation length was about 35 mm. But in 30% MXD6 blending film, the deviation length was almost 0 mm in MD. It was found that PA6/MXD6 film blended between 20% and 40% has a straight line cut in MD. The property of the straight line cut disappeared, when MXD6 blend ratio was below 10%. 17

18 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film PLEASE SEND THIS FIGURE AS ONE UNIT Currently it is in many parts and we would have to reconstitute it thereby introducting errors into the final diagram Fig. 11 Straight line cut measurement method (High shrinkage film) 18

19 M. Takashige and T. Kanai Journal of Polymer Engineering MD Straight Line Cut (Deviation) (mm MXD6 Blend Ratio (%) Fig. 12 Relationship between MXD6 blend ratio and straight line cut property (Deviation) ( PA6/MXD6 blend high shrinkage film) 3.5 PA 6-66 / MXD 6 Blend Film As polyvinylidene chloride film has higher shrinkage than PA 6 / MXD 6 blend film, PA 6-66 copolymer / MXD 6 blend film was studied to obtain high shrinkage. Figure 13 shows the relationship between the set temperature of the heating/stretching furnace and stretching stress for PA 6-66/MXD 6 and PA 6/MXD 6. PA 6-66/MXD 6 has low stretching stress and good stretchability without any breaks, compared with PA6/MDX6. The relationship between heat setting temperature and shrinkage percentage in hot water for PA 6-66/MXD 6 blend film is shown in Fig. 10. PA 6-66/MXD 6 has 10% higher shrinkage than PA 6/MXD 6. Shrinkage percentage in MD and is 39% under the condition of heat setting 19

20 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film temperature 160 C and the retort condition of 120 C. PA 6-66/MXD 6 blend film has almost the same shrinkage percentage as polyvinylidene film. 160 Stretching Stress (MP PA6/ MXD6=70/ 30 PA6-66/ MXD6=70/ Process Temperature ( ) Fig. 13 Relationship between process temperature and stretching stress (Blend film PA 6/MXD6=70/30 v.s. PA 6-66/MXD6=70/30) Table 3 shows easy tearing properties and shrinkage of three shrinkage films, namely PA6, PA6/MXD6 and PA6-66/MXD6. It was found that easy and straight cut property of PA6-66/MDX6 blend film is obtained as is the case with PA 6/MXD 6 blend film. 20

21 M. Takashige and T. Kanai Journal of Polymer Engineering Table 3 Easy tearing properties of biaxially oriented PA6, PA6/MXD6 blend and PA6-66/MXD6 blend shrinkage films Properties Straight line cut (Deviation) Tearing resistance Shrinkage Percentage Use High Shrinkage Film High Shrinkage Film Resin PA6 PA6/ MXD6 (70/30) Heat setting temperature C High Shrinkage Film PA6-66/MXD6 (70/30) Testing Method MD(mm) 30 ~40 0 ~ 2 0 ~ 3 IPC method (%) (N/cm) IPC method MD MD C water C steam 4. CONCLUSION The shrinkage films of PA 6 and PA 6-66 copolymer were developed by using the double bubble tubular film process. Shrinkage of PA 6 film was controlled by the heat setting temperature. The shrinkage percentage increases with decreasing heat setting temperature. Shrinkage percentages of PA 6 and PA 6-66 in MD and under the condition of heat setting temperature 160 C and the retort condition of 120 C are 28% and 38% respectively. The blend film of PA 6-66/MXD 6 has a 10% higher shrinkage than that of PA 6/MXD 6. Furthermore the blend film of PA 6-66/MXD 6 can achieve a barrier as effective as polyvinylidene chloride, and it has an easy and straight cut, allowing for easy opening of the product. 21

22 Vol. 27, No. 9, 2007 Biaxially Oriented Shrinkage PA6 Film The material and production technology of a PA 6 film maintain the compatible performance of high strength and high shrinkage and they could solve both environmental and barrier free problems. Furthermore, shrinkage is easily controlled by the heat setting temperature. If the heat setting temperature is 195 C, low shrinkage film is obtained and applied to the top film of the tray which requires both good shrinkage balance and low shrinkage. REFERENCES 1. U.S.Patent 2,409,521 (1946), Wiley,R.M. 2. U.S.Patent 2,452,080 (1948), Stephenson,W.T, 3. Takashige M, Kanai T, Yamada T (2004) :Int Polym Process 19: Takashige M, Kanai T (2005) Int Polym Process 20: Takashige M, Kanai T (2006) Int Polym Process, 21: Kanai T, Takashige M (1985) Seni-gakkaishi 41: Takashige M, Kanai T (1990) Int Polym Process 5: Kang HJ, White JL (1990) Polym Eng Sci 30: Kang HJ, White JL, Cakmak M (1990) Int Polym Process 1: Kang HJ, White JL (1990) Int Polym Process 5: Rhee S, White JL (2001) Int Polym Process 16: Rhee S, White JL (1999) Polym Eng Sci 39: Song K, White JL (2000) Polym Eng Sci 40: Song K, White JL (2000) Int Polym Process 15: Song K, White JL (2000) Polym Eng Sci 40: Rhee S, White JL (2001) SPE ANTEC Tech Papers 59: Rhee S, White JL (2001) SPE ANTEC Tech Papers 59: Uehara H, Sakauchi K, Kanai T, Yamada T (2004) Int Polym Process 19: Uehara H, Sakauchi K, Kanai T, Yamada T (2004) Int Polym Process 19: Uehara H, Sakauchi K, Kanai T, Yamada T (2004) Int Polym Process 19: Japan Patent S Takashige M (1999) Film Processing, Kanai T, Campbell G. (Eds), Progress in Polymer Processing Series Hanser, Munich. 23. Takashige M, Kanai T Yamada T (2003) Int Polym Process 18:

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