Easy Tear Film of Biaxially Oriented PA 6/MXD 6Blend by Double Bubble Tubular Film Process
In the market, a film which adapted to environmental and bar-rier problems is desired. This report discusses the stretchingstability and physical properties for biaxially oriented PA 6/MXD 6 blending film produced by double bubble tubular filmprocess. As MXD 6 blending ratio increased, the stretchingstress decreased. Hydrogen bond is due to be suppressed bythe steric hindrance of the MXD 6 resin. Blending film can bestretched stably and oxygen gas permeability is more excellentthan PA 6. Further the film blended between 20 % and 40 %as MXD 6 blend ratio has the property of easy straight linecut. It is found that the plate-like cylinder structure was formedin the observation of TEM. However, thickness uniformity dete-riorates when MXD 6 blending ratio exceeds 40 %. To improvethe thickness uniformity, the influence of the kneading condi-tions was studied. When the melting point of MXD 6 decreased,the stretching stability deteriorated and mechanical propertiesdrastically lowered. The excess reaction of the blending resinsis not desirable. Stretchability, easy tear property and oxygengas barrier property are maintained by following process con-ditions. In case of dry blending, MXD 6 30 % is proper content.And in case of premixing, MXD 6 melting point keeps over236 °C. It was confirmed that there is the condition satisfyingall properties. The blending film keeps the compatible perfor-mance of strength and easy tear property. Namely, the materialand production technology of a film could solve environmentaland barrier free problems.
1 Introduction
In recent years, environmental problems have come into ques-tion in the packaging industry. These problems, the de-chlori-nation and waste reduction, have brought concerns in this in-dustry, especially waste reduction which has been a seriousproblem. It is the desire that the waste reduction problem issolved. In order to achieve this waste reduction, there is a rapidincreasing shift from a bottle to a standing pouch for repackag-
ing use in an effort to utilize resources effectively. For the pro-vision of this standing repackage pouch, thin and strong biaxi-ally oriented PA6 film is indispensable.
PA 6 film has inferior oxygen gas barrier performance. Kcoated film, which coats the surface of biaxial stretching PA 6film with polyvinylidene chloride in order to raise the oxygengas barrier performance until now, was used. When this filmis incinerated, it generates toxic gas such as chlorine gas anddioxin. The development of oxygen gas barrier film is desired.An oxygen gas barrier film that satisfies formability and physi-cal property using blending technology was developed. As agas barrier resin, poly (m-xylene adipamide) (MXD 6) was se-lected.
The application of biaxial stretching PA 6 film is for in-creased toughness in the environment. This film is affixed withthe LLDPE sealant film, and the package is very strong. How-ever, when the package is opened, it is not easily cut becauseof the large tearing resistance and not cut in straight line.
The bag is simple and safe to open, for children and even forelderly people. In the medical field, there are many of applica-tions, where we cannot use a pair of scissors for safety and hy-giene reasons, so the development of a film with the easy openaccess is desired. The development of the film with compatibleperformance of toughness and opening-ability is the goal. Apolymer blend is a useful method in order to obtain the newmaterial from existing material [1 to 4]. Research on blendingbetween PA 6 and other resins has been made. It has beenfound that the mixed and stretched PA 6 resin and MXD 6 resinfilm at fixed compounding ratio can make a film with easiertear property [5]. It was found that blending film has thestraight line cut as a new property. Blending samples of PA 6and MXD 6 have been studied by changing the melting condi-tion by Takeda [6 to 8]. It is known that this system involvesreactive blending from research in the past. Shibayama hasmade reaction analysis of PA 6 and MXD 6 [9 to 10].
There are several reports issued on double bubble tubularstretching technology [11 to 22] of formability and structureanalysis for resin such as PET, PBT, PPS, PA 6– 12 and PA 12that are reported by White, Kang, Song, Rhee et al. [13 to 22].However, there are no reports on double bubble tubular tech-nology of the blending resin. The analyses of deformation be-havior of PA 6 were reported previously according to stretch-ing stress analyses [23]. This study was carried out to clarify
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FILM
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M. Takashige et al.: Easy Tear Film of Biaxially Oriented PA 6/MXD 6 Blend
the relationship between blending composition and biaxialstretching formability structure. It should aim at the develop-ment of a film with open-easiness and straight line cut keepingthe toughness of biaxial stretching PA 6 film. The developmentof straight line cut film and open-easiness was examined by thecombination of tubular biaxial stretching technology and resinblending technology. Detailed studies under the various pro-cess conditions, governing the film processability in doublebubble tubular PA 6/MXD 6 blending film process are de-scribed below.
2 Experimental
2.1 Equipment
Apparatus used for the double bubble tubular film process isshown in Fig. 1. By using 40 mm 1 extruder (L/D = 24) witha circular die of the diameter of 75 mm and the lip clearanceof 1 mm and with a water-cooling ring having the diameter of90 mm.
While passing through for second blowing, which is com-posed of two pairs of pinch rolls and a heating furnace (a far in-frared radiation heater is self-contained), this raw film wasstretched simultaneously in the machine and transverse direc-tions by using internal bubble air. The stretched film was heat-set using a heat treatment device.
2.2 Material
The material was Ube PA 1023FD (PA 6) with mean molecularweight of 23 000 and the relative viscosity of gr = 3.5 in 98 %sulfuric acid as a solvent. The material was Mitsubishi GasChemical MX PA 6007 with mean molecular weight of 25 000and the relative viscosity of gr = 2.7 in 96 % sulfuric acid as asolvent. The chemical structure of both PA 6 and MXD 6 isshown in the Fig. 2.
2.3 Experimental Method
The melt process conditions of the un-stretched film were270 °C for resin temperature at the die exit, 1.2 for blow up ra-tio, and 6.0 for draw down ratio respectively. The extrusion ratewas 17.6 kg/h and the take up velocity was 7.0 m/min. Filmwas quenched in water at 20 °C to suppress crystallization.The stretching device consists of a heating/stretching furnaceand an air ring. The air ring, which injects air downward at anangle of 45°, was installed at the upper part of the heating fur-nace.
The standard condition for the stretching process was 330 °Cfor process temperature (temperature of heating furnace) andMD (Machine Direction)/TD (Transverse Direction) = 3.0/3.2for stretching ratio respectively. The thickness the un-stretchedfilm was 130 lm, which became 13.5 lm after stretching. The
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Fig. 1. Schematic view of double bubble tubular film processa: extruder, b: die, c: cooling bath, d: take up roll, e: air ring, f: heatingfurnace, g: annealing, h: winding
Fig. 2. Chemical structure of PA 6 and MXD 6
Fig. 3. Measurement method of stretching stress (the internal bubblepressure was measured using the digital manometer)
stretched film was heat treated to prevent shrinkage, using aheat treatment device of the tenter process.
The thickness uniformities of non-stretched film and heat-treated stretched one were measured. The thickness uniformitywas normalized by the mean thickness as:
Fig. 3 shows the measurement method of stretching stress.Stretching stress was calculated with the help of the tubulartheoretical equation reported by the authors et al. [23]. Themaximum stress at the end point of stretching may be obtainedby the following equation.
r ¼ DP � D=2t; ð2Þ
where r: stretching stress in the TD, DP: internal bubble pres-sure, D: bubble diameter at end point of stretching, t: filmthickness at end point of stretching.
Internal bubble pressure during the stretching was measuredby using the digital manometer in the double bubble tubularprocess, and the stretching stress was calculated.
The comparative evaluation of stretching stress and form-ability was done using the resins of different blending ratio.Formability and physical properties were evaluated by thechange of blending ratio of PA 6 and MXD 6. The blending ra-tio was made to change with 0%, 10 %, 20 %, 30 %, 40 % inMXD 6 in dry blending.
An improvement of thickness uniformity by enhancing thekneading level was obtained with a twin screw extruder (Ike-gai; PCM-45) in premixing. A comparative evaluation ofstretching stress and formability was done using the resins pre-pared under various kneading conditions. The blending ratiowas fixed 40 % in MXD 6.
2.4 Observation of TEM
Phosphorus wolframic acid was used to dye the film. A thinsection was cut down using the microtome. The phase separa-tion structure was evaluated by transmission electron micro-scopy (TEM; Nihon Denshi JEM-200CX). The film in whichthe blending ratio of the PA 6 and MXD 6 resin changed wasused. Edge view and end view were selected as a measurementdirection (Fig. 4). Phase separation structure difference be-tween the kneading conditions was observed using the electronmicroscopy too.
2.5 Observation of SALS
The phase separation structure was evaluated by using the lightscattering device (SALS; Opteck GP5DA, He-Ne laser beam).
2.6 DSC
The melting point was evaluated by using DSC (Seiko Instru-ments DSC210). The melting point is influenced by the degreeof co-polymerization of the two kinds of resins.
2.7 Mechanical Properties
An evaluation of gas barrier and toughness was also carriedout. The oxygen gas permeability was carried out under 23 °C,60 % RH condition by using Mocon Oxtran. The toughnesswas evaluated by using the film impact strength equipment.The tearing resistance was evaluated by using the elemendorftearing test machine.
The straight line cut was evaluated by using the stretchedfilm sample. The film was torn 200 mm length, and the devia-tion length was measured.
The laminated film was composed of the next composite(blending film/LLDPE sealant film = 15/50 lm). A straightline cut was evaluated by using the package film sample (lami-nated bag). The film was torn 100 mm length, and the deviationlength was measured.
3 Results and Discussion
3.1 Blend Ratio (Dry Blending)
3.1.1 Stretchability
The samples of different MXD 6 blending ratios were pro-duced, and the stretching stability was evaluated. Fig. 5 showsthe relationship between blending ratio and stretching stress.The stretching stress lowered, as MXD 6 blending ratio in-creased. Fig. 6 shows the relationship between process tem-perature (temperature of heating furnace) and stretching stress.Temperature dependency of the stretching stress for blendingfilm is larger than one of PA 6 film. It was found that the blendfilm had only a narrow stretchability region. This may be be-cause the temperature dependency of MXD 6 resin is large.
PA 6 is also characterized by strong hydrogen bondingowing to the molecular structure having polyamide bondingintrinsic to the resin, and therefore it is better simultaneousbiaxial stretching than sequential stretching. Hydrogen bondis due to be suppressed by the steric hindrance of the MXD 6resin. As a result, stretching stress of blending film decreased.
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M. Takashige et al.: Easy Tear Film of Biaxially Oriented PA 6/MXD 6 Blend
3.1.2 Physical Properties
The physical properties were evaluated by the change of blend-ing ratio of PA 6 and MXD 6. Fig. 7 shows the relationship be-tween blending ratio and oxygen gas permeability. Oxygen gaspermeability of the blending sample of PA 6 and MXD 6 wasmore excellent than the PA 6 film. The oxygen gas permeabil-ity was also improved, as blending ratio of the MXD 6 resin in-creased.
Fig. 8 shows the relationship between blending ratio andfilm impact strength. The film impact strength lowered, as
MXD 6 blending ratio increased. Because hydrogen bondingis due to be suppressed by the steric hindrance of the MXD 6resin, and MXD 6 resin is rigid.
The high gas barrier property of MXD 6 is demonstrated inthe blend film without hurting PA 6 ductility. It is shown thatthe complementarity’s of dynamic property and gas barrierproperty by the blending has skillfully carried out.
Both PA 6 and MXD 6 are crystalline resins. Elastic modu-lus and gas barrier are better in MXD 6 than PA 6. The ductilityof PA 6 is higher than that of MXD 6. It is possible to develop amaterial that compensates for the weak point of the each otherby blending these two components.
3.1.3 Easy Tear Properties
For the easy tear properties, tearing resistance and straight linecut were measured. Fig. 9 shows the relationship betweenMXD 6 blending ratio and tearing resistance. The tearing resis-tance in MD and TD were measured. The tearing resistancelowered was shown, as MXD 6 blending ratio increased. Itshowed the same tendency in MD and TD direction. TheMXD 6 blending film showed the smaller tearing resistancethan PA 6 film.
Fig. 10 shows the measurement method of straight line cutin film. The straight line cut level was expressed using the de-viation length. Fig. 11 shows the relationship between blending
150 Intern. Polymer Processing XIX (2004) 2
Fig. 5. Relationship between MXD 6 blending ratio and stretchingstress
Fig. 6. Relationship between process temperature and stretching stress
Fig. 7. Relationship between MXD 6 blending ratio and oxygen gaspermeability
Fig. 8. Relationship between MXD 6 blending ratio and film impactstrength
Fig. 9. Relationship between MXD 6 blending ratio and tearing resis-tance
ratio and straight line cut. The deviation length of MD drasti-cally decreased with increasing MXD 6 blending ratio. Butthe deviation length of TD was not changed in all MXD 6blending ratio. In PA 6 film, the deviation length was about35 mm. But in 30 % MXD 6 blending film, the deviation lengthwas about 0 mm in MD.
It was found that MXD 6/PA 6 film blended between 20 %and 40 % has the straight line cut in MD. The property of thestraight line cut disappeared, when MXD 6 blend ratio is below15 %.
The laminated film (Blending film/LLDPE sealant film =15/50 lm) was measured the straight line cut too. Fig. 12shows the relationship between blending ratio and straight linecut in package film (laminated bag). The straight line cut levelwas expressed using the deviation length. The deviation lengthof MD drastically decreased with increasing MXD 6 blendingratio. In PA 6 film, the deviation length was about 30 mm. Butin 30 % MXD 6 blending film, the deviation length was about2 mm in MD.
It was verified that MXD 6/PA 6 film blended between 20 %and 40 % has the straight line cut in MD in package film (lami-nated bag), too.
3.1.4 TEM Observation
TEM photographs of biaxially stretched MXD 6 blending filmat the various blending ratio were shown in Fig. 13. The dyeingmechanism by phosphorus wolframic acid is not clear. It wasjudged that the bright section was a region of MXD 6 resin.The edge view and end view were defined in Fig. 4. It wasfound by the electron microscopy that the blending films hada plate-like cylinder structure consisting of MXD 6 domains.The MXD 6 domain is highly oriented. The difference of tear-ing anisotropy is partially due to this domain structure.
MXD 6 forms the disperse phase in the blend film. It wasproven that the slender MXD 6 domain is several hundred nm
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Fig. 10. Straight line cut measurement method (film)
Fig. 11. Relationship between MXD 6 blending ratio and straight linecut
Fig. 12. Straight line cut measurement method (package) Fig. 13. Observation of TEM (biaxially stretched film)
M. Takashige et al.: Easy Tear Film of Biaxially Oriented PA 6/MXD 6 Blend
width and several thousand nm length from the electron micro-scopy. And the domain has lined up in MD.
It was found that it broke rectilinear, when it was torn alongthe MD in the slender MXD 6 domain. It did not break recti-linear, when it was perpendicularly torn with the MD in theslender MXD 6 domain right angles.
In a PA 6 film, the anisotropy of such tearing was not ob-served. It is estimated that the 100 nm order structure causedthe macroscopic dynamics property of the tearing. It was foundthat this cylinder structure has produced the straight line cut.Tearing resistance of MD and TD was also decreased by the cy-linder structure.
As a result of extension observation of the phase separationstructure, the number of the cylinder structure decreased withdecreasing MXD 6 blending ratio, and the longitudinal lengthshortened with decreasing MXD 6 blending ratio. The numberof cylinder structure increased with increasing MXD 6 blend-ing ratio, and the longitudinal length lengthened with increas-ing MXD 6 blending ratio. And the straight line cut becamegood.
TEM photographs of non-stretched blend film at the variouscompounding ratio are shown in Fig. 14. The continuity of theMXD 6 layer in MD was confirmed in being a stage of thenon-stretching. It was considered that the continuity of theMXD 6 layer in the non-stretched film occurs under the extru-sion processing.
3.1.5 SALS Observation
Fig. 15 shows the photograph of light scattering image exam-ple. The light scattering intensity increased with increasingthe MXD 6 blending ratio. It was confirmed that MXD 6 phaseseparation structure had lined up in the MD, because it ap-
peared in the TD resistant. This streak reflects the anisotropicshape, which was extended in MD of the MXD 6 layer. Whileonly an isotropic diffuse scattering was observed in PA 6 film.
3.1.6 Mechanism of Property Appearance
Fig. 16 shows the mechanism for property appearance inblending film. It was found that each phase controlled manyproperties. A plate-like cylinder structure was formed in MD.
MXD 6 phase controls the straight line cut, tearing resis-tance and oxygen gas barrier performance. PA 6 phase is con-tinuous, and PA6 phase controls the strength. A high strength
film was achieved, because it was oriented biaxially using thedouble bubble tubular process. The blending film keeps thecompatible performance of strength and easy tear property.
3.1.7 Thickness Uniformity
Fig. 17 shows the relationship between MXD 6 blending ratioand thickness uniformity. The thickness uniformity deterio-rates with increasing MXD 6 blending ratio. Especially, thethickness uniformity deteriorates when MXD 6 blending ratioexceeds 40 %.
There was a condition for satisfying all properties in near30 % blended MXD 6 in case of dry blending.
3.2 Kneading Condition (Premixing)
3.2.1 Stretchability (Melting Point of MXD 6)
The improvement of the thickness uniformity by the change ofthe kneading level was studied. It was examined by selecting40 % as MXD 6 blending ratio. The sample of changing thekneading condition was prepered. The kneading was carriedout using the twin extruder with co-rotation screws. MXD 6melting point was used as a scale of the kneading level.
Fig. 18 shows the relationship between pre mixing andthickness uniformity. It was confirmed that the thickness uni-formity improved, when MXD 6 melting point went down.
The relationship between melting point of MXD 6 andstretching stress is shown in the Fig. 19. The stretching stressdrastically lowered, as MXD 6 melting point lowered. In thelow melting point of MXD 6, stretching bubble was notstable.
3.2.2 Physical Properties
Table 1 shows the relationship between melting point ofMXD 6 and physical properties. When MXD 6 melting pointis lowered, the stretching formability deteriorated, and thephysical properties also lowered drastically. In the physicalproperties, the film impact strength lowered, the gas perme-ability deteriorated and the straight line cut performance dis-appeared.
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Fig. 17. Relationship between MXD 6 blending ratio and thicknessuniformity
Fig. 18. Relationship between premixing condition and thickness uni-formity
Fig. 19. Relationship between MXD 6 melting point and stretchingstress
M. Takashige et al.: Easy Tear Film of Biaxially Oriented PA 6/MXD 6 Blend
3.2.3 Structure Analyses
The phase separation structure perfectly disappeared over300 °C resin temperature in the kneading process. (Fig. 20). Itwas shown by the evaluation of DSC that the peak of the melt-ing point became one (210 °C). Namely the co-polymerizationoccurred perfectly (Fig. 21).
In addition, the stretching was not stabilized, so the filmsample could not obtained.
It is known that this system is the reactive blending from theresearch in a past. Cutting and recombination of amide group inmolecular chain are generated under the high temperature overmelting point on both components, and the co-polymerizationof the sample advances. The excess reaction of the blending re-sins is not desirable.
In case of pre mixing, MXD 6 melting point keeps over236 °C. It was confirmed that there is the condition satisfyingall properties.
4 Conclusion
The stretching stress lowered, as MXD 6 blend ratio increased.Hydrogen bond is due to be suppressed by the steric hindranceof the MXD 6 resin.
It was confirmed that MXD 6 blending film blended be-tween 20 % and 40 % has the straight line cut in MD. The prop-erty of the straight line cut disappeared, when MXD 6 blendratio is below 15 %. It is found that the plate-like cylinderstructure was formed in the observation of TEM. The cylinderstructure produced the straight line cut. Tearing resistance ofMD and TD also decreased. The cylinder structure was ob-served even in a non-stretched film. The thickness uniformitydeteriorates when MXD 6 blend ratio exceeds 40 %.
The improvement of the thickness uniformity by the changeof the kneading level was studied. In the condition in whichMXD 6 melting point lowered, the stretching formability deter-iorated, and the mechanical properties also drastically lowered.The phase separation structure perfectly disappeared over300 °C as the resin temperature in the kneading process. It wasshown by the evaluation of DSC that the peak of the meltingpoint became one. Namely the co-polymerization occurred per-fectly. The excess reaction of the blending resins is not desirable.
PA 6 phase controls the strength, and the MXD 6 phase con-trols the straight line cut.
A high strength film was achieved, because it was stretchedbiaxially using the double bubble tubular process.
Stretchability, easy tear property and oxygen gas barrierproperty are maintained by next process conditions. In case ofdry blending, MXD 6 30 % is proper content. And in case ofpre mixing, MXD 6 melting point keeps over 236 °C. It is con-firmed that there is the condition for satisfying all properties.
The blending film keeps the compatible performance ofstrength and easy tear property. Namely, the material and pro-duction technology of a film could solve environmental andbarrier free problems.
References
1 Nishi, T., Wang, T. T.: Macromolecules 8, p. 909 (1975)2 Chuang, H. K, Han, C. D.: J. Appl. Polym. Sci. 30, p. 165 (1985)3 Greco, R., Malincoico, M.: Polymer 29, p. 1418 (1988)4 Jo, W. H., Kim, G., Chae, S. H.: Polymer Journal 25, p. 1023 (1993)5 U. S. Patent 5 541 011 (1996), Takashige, M., Hayashi, T.6 Takeda, Y., Paul, D. R.: Polymer 33, p. 899 (1992)7 Takeda, Y., Keskkula, H., Paul, D. R.: Polymer 33, p. 3394 (1992)8 Takeda, Y., Paul, D. R.: Polymer 32, p. 2771 (1991)9 Shibayama, M., Uenoyama, K., Oura, J., Iwamoto, T.: Polymer 36,
p. 4811 (1995)10 Shibayama, M., Oura, J., Iwamoto, T.: Kobunshi Ronbunsyu 53, p. 453 (1996)11 Kanai, T., Takashige, M.: Seni-gakkaishi 41, p. 272 (1985)12 Takashige, M.: Film Processing, in: Kanai, T., Campbell, G. (Eds.), Pro-
gress in Polymer Processing Series. Hanser, Munich (1999)13 Kang, H. J., White, J. L.: Polym. Eng. Sci. 30, p. 1228 (1990)14 Kang, H. J., White, J. L., Cakmak, M.: Int. Polym. Process 1, p. 62 (1990)15 Kang, H. J., White, J. L.: Int. Polym. Process 5, p. 38 (1990)16 Rhee, S., White, J. L.: Int. Polym. Process 16, p. 272 (2001)17 Rhee, S., White, J. L.: Polym. Eng. Sci. 39, p. 1260 (1999)18 Song, K., White, J. L.: Polym. Eng. Sci. 40, p. 902 (2000)19 Song, K., White, J. L.: Int. Polym. Process 15, p. 157 (2000)20 Song, K., White, J. L.: Polym. Eng. Sci. 40, p. 1122 (2000)21 Rhee, S., White, J. L.: SPE Antec Tech. Papers 59, p. 1446 (2001)22 Rhee, S., White, J. L.: SPE Antec Tech. Papers 59, p. 1451 (2001)23 Takashige, M., Kanai, T.: Int. Polym. Process 5, p. 287 (1990)
Date received: October 23, 2003Date accepted: November 1, 2003
154 Intern. Polymer Processing XIX (2004) 2
Premixing(low level)
Premixing(high level)
Resin temperature (°C) 286 300Melting point of MXD 6 (°C) 236.6 232.3
Stretching formability Good BadStretching stress (MPa) 70 36
Film impact strength (J/m) 60 000 43 000Oxygen gas permeability
(cc/m2 · 24 h)15 24
Strait line cut Good BadObservation of TEM Phase
separationstructure
Nothing
Thickness uniformity (%) ± 6.0 (%) ± 4.5 (%)
Table 1. Melting point of MXD 6 vs. physical properties
