Adv Polym Technol. 2018;1–11. wileyonlinelibrary.com/journal/adv | 1© 2018 Wiley Periodicals, Inc.

1 | INTRODUCTION

There are two processes used to produce biaxially oriented films, namely the tentering process and the double bubble tubular film process.[1,2]

The tentering process is divided into two methods of a se-quential stretching method (a step by step stretching method) and a simultaneous stretching one. These two methods are employed independently in accordance with the characteris-tics of resins, but the sequential stretching method is glob-ally the most commonly used because of high productivity.

A sheet produced by a sheeting machine is stretched in the longitudinal direction and then stretched in the transverse direction. Another process is the simultaneous stretching method, where the film is stretched in the longitudinal di-rection and the transverse direction at the same time. This process is mainly used for polyamide 6 (PA6) and ethylene vinyl alcohol copolymer which are difficult to be stretched uniformly by the sequential biaxial stretching because of high crystallization speed and strong hydrogen bond. The tenter-ing process has the advantage of being suitable for mass pro-duction, but the machine costs are high.[1–6]

Received: 14 November 2017 | Accepted: 8 February 2018

DOI: 10.1002/adv.21961

O R I G I N A L A R T I C L E

Dynamics and structure development for biaxial stretching polyamide 6 films

Toshitaka Kanai1 | Yoshimune Okuyama2 | Masao Takashige3

1KT Poymer, Sodegaura, Chiba, Japan2Graduate School of Natural Science & Technology, Kanazawa University, Kanazawa, Japan3Idemitsu Unitech Co., Ltd, Sodegaura, Chiba, Japan

CorrespondenceToshitaka Kanai, KT Poymer, Sodegaura, Chiba, Japan.Email: toshitaka.kanai@ktpolymer.com

AbstractThe biaxial stretching film of polyamide 6 (PA6) is widely used for food and indus-trial products packaging. It is expected to improve stretchability, impact strength, tensile strength, and thermoformability for a larger market. So, in this research, the stretching methods were investigated in terms of dynamics and structure develop-ment for the biaxial stretching films. There are three different stretching methods which can produce PA6 films, namely the simultaneous biaxial stretching, the se-quential biaxial stretching, and the double bubble tubular process. The physical prop-erties of PA6 film are very much influenced by the biaxial stretching process. But there are no reports which described the difference among the three stretching meth-ods and film properties. For this reason, the higher order structure and stretching behaviors of the three stretching methods were studied. As a result, the sequential biaxial stretching showed strong stretched effect and molecular orientation in the transverse direction (TD) and poor film thickness uniformity, because the strong hydrogen bonds were created during the machine direction (MD) stretching and high stretching stress in the TD stretching was required. In the simultaneous biaxial stretching, the molecular orientation was equal both in the MD and in the TD of the stretched plane. The double bubble tubular process produced well- balanced film and equal orientation in any direction in terms of the phase difference in the plane of film. The biaxially oriented PA6 film having good stretchability and high physical proper-ties was also investigated.

K E Y W O R D Sfilms, polyamides, stretchability, structure–property relations

2 | KANAI et Al.

The double bubble tubular film process is composed of the tubular film process, followed by the preheating, and the biax-ial stretching process in the second bubble, which is inflated biaxially with much higher force by both the inside bubble pressure and the take- up force below the melting tempera-ture than the first bubble. This process has the advantage of low- cost machines and various size productions using only one machine can be made. Furthermore, as this process is si-multaneous stretching, it can produce a good balanced film in machine direction (MD) and transverse direction (TD) and a high stretching stress during the process, which is appropriate for shrinkage film and high strength film.[1,2,7–28]

The biaxial stretching PA6 film is widely used for food and industrial products packaging, because it has good phys-ical properties, such as high toughness, high tensile strength, hard wearing properties, and gas barrier properties.[1,2]

For this reason, PA6 has become more important for in-dustrial usage. So there are many papers which have reported on PA6 film.[25–37] Recently, it is expected to improve the stretchability, film physical properties, and formability for a larger market such as food packaging usage of high barrier and high strength retort film, Li- ion battery packaging film, and medical usage of PTP long life packaging. So, in this research, the stretching methods which influenced the film physical properties were investigated in terms of dynamics and structure development for biaxial stretching films.

There are three different stretching methods which can produce PA6 films, namely the simultaneous biaxial stretch-ing,[2] the sequential biaxial stretching,[1,3] and the double bubble tubular process.[1,4–10] There are no reports which described the difference among the three stretching meth-ods and film properties, so it was not clarified why the films have big differences in their properties. For these reasons, this study was to investigate the higher order structure and stretching behaviors of the three stretching methods and to develop the biaxial stretching film of PA6 which has good stretchability and good physical properties.

2 | EXPERIMENT

2.1 | Experimental equipmentThe film samples of PA6 were used to study the biaxial stretcha-bility and the superstructure formation using a biaxial stretcher. The biaxial stretcher SDR- 527K, which was designed by our group and made by Etoh Corporation, was used.[2,38,39]

The schematic view of this equipment is shown in Figure 1. The apparatus was designed and constructed to carry out the simultaneous and the in situ measurement of stress, birefrin-gence, and three- dimensional refractive indexes during the bi-axial stretching process. The test machine was equipped with a stretching unit and XY mapping controlled and driven by a computer, load cells for the measurement of stress of the

biaxial stretching film, and two optical measurements systems. These systems had the vertical and inclined incidence of laser beams using photo- elastic modulators (PEM) called the dou-ble birefringence measurement systems for the measurement retardations and three- dimensional molecular orientations. The light source axis was adjusted to overlap at the center of the biaxial stretching film on the stretching unit. The strain was calculated from the biaxially stretched distance which was stretched by the stretching control unit. All of the simultane-ous measurement data can be obtained in real time during the biaxial stretching process.[39] Various data could be obtained at the same time, such as the stress–strain curve (S–S), refrac-tive indexes in three directions, light scattering data during the biaxial stretching, and retardation distribution of the biaxial stretched film. The stretching conditions are shown in Table 1.

The samples of the double bubble tubular film were also produced. The process conditions are described in detail in 2.3 experimental conditions.

The distribution of the phase contrast at whole rotation angles for the biaxial stretched film was measured by RETS 100 Otsuka Electronics Co., Ltd.

The optical anisotropy of film was observed using po-larizing plates. Films were inserted between two polarizing plates crossed with each other to examine the photographic image by applying the incident light from below.

The biaxial stretching film was dyed with fluorescent dyestuff WhitexRP (Sumitomo Chemical), and the orienta-tion of PA6 amorphous chains in the biaxial stretching film was evaluated by the Fluorescence Spectrophotometer Jasco FOM- 1 produced by Jasco. The excitation light wavelength was 365 nm, and fluorescence wavelength was 420 nm. The polarized fluorescence intensity in- plane (I|| and I⊥) as a func-tion of angle which was measured by rotating a polarizer and an analyzer was obtained. I|| denotes that a polarizer and an analyzer are parallel. I⊥ denotes that a polarizer is perpendic-ular to an analyzer. The excitation light was entered from the backside of the sample.

F I G U R E 1 Schematic diagram of biaxial stretching machine

| 3KANAI et Al.

The film crystalline orientation was evaluated using the wide- angle X- ray diffractometry produced by Rigaku (Rotary Flex RU- 200).

2.2 | MaterialsPolyamide 6 was used for this research. PA6 was Ube Nylon 1023FD with a mean molecular weight of 23,000 and the relative viscosity of ηr = 3.5 in 98% sulfuric acid as a solvent.

2.3 | Experimental conditions

2.3.1 | Simultaneous and sequential biaxial stretched filmsThe 50- μm- thickness PA6 film samples were produced using an extruder with a die extruded at a resin temperature of 265°C and chilled water bath of 18°C to prevent crystalliza-tion. Using a 40φ 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, nonstretched film was produced at a resin temperature of 265°C and a blow- up ratio of 1.2.

The film samples were stretched using the biaxial stretcher under the conditions of preheating time 2 min-utes, standard stretching temperature 110°C, standard stretching speed 10 mm/s, and various stretching ratios. The original size of PA6 specimens was 67 × 67 mm, and the chuck distance for the biaxial stretching was set at 38 mmx38mm. In situ measurement of stress and strain curves and birefringence was obtained. The stretching ratio conditions of the sequential stretching were MD × TD 1.5 × 1, 2 × 1, 2.5 × 1, 3 × 1, 3 × 1.5, 3 × 2, 3 × 2.5, 3 × 3, respectively. The stretching ratio conditions of the simultaneous stretching were MD × TD 2 × 2, 2.5 × 2.5, 3 × 3, respectively.

2.3.2 | Double bubble tubular stretched filmsThe double bubble tubular film was also produced to com-pare with tenter stretching films. Figure 2 is the schematic view of the double bubble tubular film extrusion.

The double bubble tubular film process consists of two stages of bubbles. In the first stage, the bubble is blown from a molten state. In this step, the crystalline polymer is rapidly cooled by temperature- controlled water to reduce the crystal-linity. The first bubble is flattened out by a set of nip rolls and re- inflated into a larger second stage bubble at a higher tem-perature than the softening temperature. The second bubble is stretched biaxially in the infrared heater oven, and after stretch-ing, the bubble is cooled by air. The second bubble is usually stretched biaxially at the same stretching ratio in MD and TD. The second bubble is also flattened out by a set of nip rolls. It is slit along both edges, and each film is wound separately.

Nonstretched films for the tenter biaxial stretched film and double bubble stretched film were produced under the same extrusion conditions. The standard stretching ratio condition of the double bubble tubular stretching using a production line was MD × TD 3 × 3. Each sample was also measured by the polarizing microscope and three- dimensional birefringences.

3 | RESULTS AND DISCUSSION

The photographic images through the polarizing plate for both the simultaneous biaxial stretching film and the se-quential biaxial stretching film are shown in Figure 3. The photographic image taken through the polarizing plate of the simultaneous biaxial stretching film shows uniform bi-axial stretching, but the photographic image taken through the polarizing plate of the sequential biaxial stretching film shows both stretched part and nonstretched part (kink stretching). The results of the simultaneous biaxial stretch-ing film, the sequential biaxial stretching film, and the dou-ble bubble tubular film are described in the next three parts.

3.1 | Simultaneous biaxial stretching filmThe simultaneous biaxial stretching results are shown in Figure 4. It shows the stress and strain curve and retardation as

T A B L E 1 Stretching conditions

Stretching mode Sequential or simultaneous biaxial stretching

Stretching speed 10 mm/s

Stretching ratio MD 3 times × TD 3 times (standard condition)

Stretching temp. 110°C

Preheating time 2 min

F I G U R E 2 Schematic view of double bubble tubular film process

4 | KANAI et Al.

a function of total strain ratio (surface ratio) under the simul-taneous biaxial stretching and the stretching ratios MD × TD 3 × 3. The simultaneous biaxial stretching shows almost the same stress of MD and TD and keeps very low retardation for vertical incidence. The retardation for vertical incidence laser beam R0 is almost 0, and the retardation for 30° inclined inci-dence laser beam Rφ is larger than R0. It is because the refrac-tive indexes of MD (Nx) and TD (Ny) are much larger than the refractive index of the normal direction ND (Nz). These results mean the simultaneous biaxial stretching is equal in biaxial stretching and plane orientation in MD and TD.

Figure 5 shows three- dimensional refractive indexes as a function of total strain ratio under the simultaneous biaxial stretching. The refractive indexes of Nx and Ny increase with increasing total strain ratio. The refractive index of Nz de-creases with increasing total strain ratio. It means that the simultaneous biaxial stretching increases both the MD and TD molecular orientations in the parallel to film plane.

The crystalline orientation of the simultaneous biax-ial stretching film was analyzed by the wide- angle X- ray

diffraction measurement. Figure 6 shows that the X- ray diffraction patterns from the three directions of the through view perpendicular to the film plane, the end view per-pendicular to MD, and the edge view perpendicular to TD measured at each stretching ratio. From these results, the diffraction patterns of the through view are uniform and ring- shaped, but the diffraction intensities from the end view and the edge view show strong diffraction intensities in the equator direction which increase with increasing the stretching ratio. The biaxial stretching film is in planar ori-entation in MD and TD plane.

The stretchability, stretching stress, and orientation were evaluated by changing the stretching temperature. The relationship between stretching stress and stretching ratio at various stretching temperatures is shown in Figure 7. The stretching stress increases with decreasing the stretch-ing temperature. The stress at low stretching temperature shows strain hardening and high stretching stress at the final stretching ratio. The stress–strain curves below and above 100°C shows different behavior, because 100°C is

F I G U R E 3 Observation of the simultaneous biaxial stretching sample (left) and the sequential biaxial stretching sample (right) using polarizing plates

Non stretched Part(Kink Part)

Stretched part

Stretching Ra�o: MDxTD 3x3

Uniform stretching

F I G U R E 4 Stress total stretching ratio curves and retardation total stretching ratio curve of the simultaneous biaxial stretching

F I G U R E 5 Refractive index and total stretching ratio curve of simultaneous biaxial stretching

| 5KANAI et Al.

the boiling temperature of water. PA6 is always influenced by water or humidity. High humidity makes high hydrogen bond and high crystallinity of PA6 and then causes high yield stress.

The plane orientation P was defined by the following equation;

where Nx is the refractive index in MD, Ny is the refractive index in TD, and Nz is the refractive index in the thickness direction ND.

The plane orientation as a function of stretching tempera-ture is shown in Figure 8. The plane orientation increases with decreasing the stretching temperature, because the stretching stress increased with decreasing the stretching temperature and showed the strain hardening at higher stretching ratio and then polymer chains are oriented in the plane of MD and TD. The influence of the stretching temperature on the plane ori-entation was not significant.

The stretchability, stretching stress, and orientation were evaluated by changing the stretching speed. The relation-ship between stretching stress and stretching ratio at differ-ent stretching speeds is shown in Figure 9. The stretching stress decreases with increasing the stretching speed. This means that increasing stretching speed reduces time to orien-tate polymer chains during the stretching process. In case of

PA6, the faster the stretching speed is, the lower the stretch-ing stress is. This result is opposite to conventional results such as PP, PE, and PS, because the stretching stress of PA6 influences both the creation of the hydrogen bond and the increase in the crystallization during the stretching. The slow stretching speed, which takes more time in stretching, creates high crystallinity and strong hydrogen bonds. For this reason,

(1)P= [(Nx+Ny)∕2]−Nz

F I G U R E 6 The result of wide- angle X- ray diffraction patterns of simultaneous biaxial stretching at different stretching ratios

1.5 1.5 2.0 2.0 2.5 2.5 3.0 3.01.0 1.0Through view

Edge view

End view

Y MD

X TD)Z ND)

X-ray

Y TD

Z ND)X-ray

Y TDX MD)

X-ray

F I G U R E 7 Stretching stress surface ratio curves of simultaneous biaxial stretching at different stretching temperature

0

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40

011

Stre

tchi

ng st

ress

[MPa

]

Surface ratio [-]

80℃90℃100℃110℃120℃130℃

6 | KANAI et Al.

the stretching stress of PA6 film decreases with increasing stretching speed.

The plane orientation P as a function of stretching speed is shown in Figure 10. The plane orientation increases with de-creasing the stretching speed, because polymer chains at the slow stretching speed are oriented strongly by high stretching stress.

The stretchability, stretching stress, and orientation were evaluated by changing the stretching ratio. The relationship between stretching stress and stretching ratio at different stretching ratios is shown in Figure 11. The stretching stress increases with increasing the stretching ratio, and the stress at high stretching ratio shows strain hardening. The film breaks sometimes occurred at high stretching ratio 3.5 × 3.5, be-cause of high stretching stress.

The plane orientation P as a function of stretching ratio is shown in Figure 12. The plane orientation increases with

increasing the stretching ratio, because polymer chains at high stretching ratio are oriented strongly by high stretch-ing stress. The stretching ratio strongly influences stretching stress and plane orientation.

Figure 13 shows that the X- ray diffraction patterns for the simultaneous stretching film from the three directions of the through view perpendicular to the film plane, the end view perpendicular to TD, and the edge view perpendicular to MD measured at each stretching ratio. From these results, it was found that the diffraction patterns of the through view are uniform, but the diffraction patterns from the end view and the edge view show strong diffraction intensities in the equa-tor direction which increase with increasing the stretching ratio. This means that the biaxial stretching film is in planar orientation in MD and TD plane and especially the plane ori-entation in the crystalline layer increased with increasing the stretching ratio.

F I G U R E 8 Relationship between plane orientation degree and stretching temperature for simultaneous biaxial stretching

F I G U R E 9 Stretching stress surface ratio curves of simultaneous biaxial stretching at different stretching speed

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011

Stre

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ress

[MPa

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Surface ratio [-]

3mm/s5mm/s10mm/s12mm/s15mm/s

F I G U R E 1 0 Relationship between plane orientation degree and stretching speed for simultaneous biaxial stretching

F I G U R E 1 1 Stretching stress surface area ratio curves of simultaneous biaxial stretching at different stretching ratio

| 7KANAI et Al.

3.2 | Sequential biaxial stretching filmThe sequential biaxial stretching results are shown in Figure 14. It shows the stress–strain curve under the sequen-tial biaxial stretching and the stretching ratios MD × TD

3 × 3. During the MD stretching, MD stress increases rapidly and then increases gradually. After finishing the MD stretch-ing, the TD stretching begins and the TD stress increases rap-idly and then increases gradually. The TD stretching stress finally became larger than MD stretching stress.

Figure 15 shows three- dimensional refractive indexes as a function of the total strain ratio of the sequential biaxial stretching. MD stretching increases only Nx which means the molecular orientation in MD. During the TD stretching, there are two parts existed as shown in Figure 3. In the stretched zone, TD stretching increases Ny which means TD molecular orientation proceeds. In the unstretched zone (kink part), the refractive index during the TD stretching did not change and kept orientation almost constant.

From these results, it was found that the sequential biaxial stretching shortened the distance between the hydrogen bonds and made the hydrogen bonds stronger and then increased TD stretching stress. Owing to this strong TD stretching stress, it is considered that it is difficult to stretch uniformly.

The plane orientation P as a function of stretching ratio is shown in Figure 16. The plane orientation of the stretching part increased with increasing the stretching ratio, but the plane ori-entation of the kink part only increased slightly with increasing

F I G U R E 1 2 Relationship between plane orientation degree and stretching ratio for simultaneous biaxial stretching

F I G U R E 1 3 Results of wide- angle X- ray diffraction patterns of simultaneous biaxial stretching at different stretching ratio

edge viewY(MD)

X(ND)

end viewY(TD)

X(ND)

Through viewY(TD)

X(MD)

3.0 3.0 3.3 3.3 3.5 3.5

8 | KANAI et Al.

the stretching ratio, because polymer chains at the kink part were not oriented strongly by the stretching ratio as shown in Figure 3.

The crystalline orientation of the sequential biaxial stretch-ing film was analyzed by the wide- angle X- ray diffraction measurement. The X- ray diffraction patterns for the stretched part and the kink part from the through view perpendicular to the film plane were measured at various stretching ratios shown in Figure 17. At the first step, the relative intensity (200) plane integrated along the meridian direction increased with increasing MD stretching ratio. It means that MD orien-tation proceeds. At the second step, the relative intensity inte-grated along the meridian direction during the TD stretching and then the relative intensity integrated along the equator direction became stronger after the TD stretching ratio more than 2. The kink part occurred after the TD stretching ratio became more than 2. The crystalline structure of the kink part did not change even with increasing the TD stretching ratio.

As a result, the crystalline orientation in the kink part kept the MD preferred orientation. But the crystalline orientation in the stretched part was the TD preferred orientation after the TD stretching ratio was more than 3. The relative intensity of

the through view was not changed, but the relative intensities of the end view and edge view increased with increasing the stretching ratio. This means that the plane orientation espe-cially in the crystalline layer increased with increasing the stretching ratio.

From the above results, it was found that the sequen-tial stretching of PA6 showed ununiform stretching such as stretched parts and kink parts, because the strong hydrogen bond was created after MD stretching. Even though the same stretching ratio in MD and TD was performed, the stretch-ing parts were stretched well and unstretched part was not stretched well. The sequential stretching produced unbal-anced film and less uniform film.

3.3 | Double bubble tubular stretching filmThe third biaxial stretching method is the double bubble tubular film process. After the machine suddenly stopped, samples were taken out and measured. The stretching defor-mation behaviors were analyzed. Bubble diameter (D) and film thickness (H) shown in Figure 18 were measured, and then, the stretching ratio pattern (MD and TD) and birefrin-gence were evaluated.

Figure 18 shows the bubble of the double bubble tubular extrusion. The stretching stresses in TD and MD were ob-tained by the following Equations (2) and (3)[2,7];

(2)σTD =ΔP ⋅RL∕HL

(3)σMD =FL∕(2πRLHL)

F I G U R E 1 4 Stretching stress surface ratio curves of sequential biaxial stretching

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1 10

Stre

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MD stress

TD stress

F I G U R E 1 5 Refractive index surface ratio of sequential biaxial stretching

F I G U R E 1 6 Plane orientation degree surface ratio of sequential biaxial stretching

| 9KANAI et Al.

where σTD, stretching stress in TD; σMD, stretching stress in MD; ΔP, bubble inside pressure; RL, final bubble diameter; HL, film thickness; and FL, bubble take- up tension.

Figure 19 shows the stretching ratio and stretching stress curves. The stretching ratio was changed under the same MD and TD stretching ratio. MD and TD stresses were almost the same, and film was stretched uniformly in any direction by the inside bubble pressure. This is different from the tenter stretching methods which only stretch MD and TD by chucks. The double bubble stretching method shows higher stretching stresses than the tenter stretching methods.

Figure 20 shows stretching ratio and birefringence pattern as a function of the distance from an air ring. MD stretching

ratio was larger than TD at the initial stage, and then, MD and TD stretching ratios became the same value. Birefringence is large at the initial stage, and then, birefringence reaches al-most 0. This means that MD orientation at the initial stage was dominant and then became balanced between MD and TD.

Figure 21 shows orientation using the fluorescence spec-trophotometer. The polarized fluorescence strength as a func-tion of angle (amorphous orientation) is shown.

The double bubble tubular process shows uniform orienta-tion in all of the directions, but the simultaneous stretching and the sequential stretching show high orientation in MD and TD directions. This is because the film was clipped and stretched in MD and TD during the tenter process and the tubular bubble

F I G U R E 1 7 The result of wide- angle X- ray diffraction patterns of sequential biaxial stretching at different stretching ratio

MD

TD

MD Stretching

F I G U R E 1 8 Evaluation method of double bubble tubular process

D

H

Dis

tanc

e fr

om A

ir rin

g

Evaluations(1) Stretching Ratio Pattern (MD,TD)(2) Stretching Ratio Difference (MD-TD)(3) Birefringence

(Air ring)

After the machine suddenly stopped, samples were taken out and bubble diameter (D) and film thickness (H) were measured.

Schematic view of double bubble tubular process

10 | KANAI et Al.

was stretched by the inside pressure and stretching tension at the same time during the double bubble tubular process. This is a very big difference between the tenter stretching and the double bubble stretching. This difference influences the sec-ondary process, especially thermoformability.

Figure 22 shows the distribution of the phase contrast at the three different stretching methods. This figure shows the distribution of the phase contrast at different stretching meth-ods. The double bubble tubular stretching showed low phase contrast, and the sequential stretching showed high phase

contrast. This means the double bubble tubular stretching can easily produce balanced film in any direction and the sequen-tial stretching produces unbalanced biaxial stretching film in MD, 45° direction, TD, and 135° direction.

Tensile properties such as tensile strength at break, elon-gation at break, are influenced by the biaxial stretching

F I G U R E 1 9 Relationship between stretching ratio and stretching stress for double bubble tubular process

Stretching Ratio (MD=TD)

Stre

tchi

ng st

ress

(MPa

)

F I G U R E 2 0 Stretching ratio and birefringence pattern for double bubble tubular process

F I G U R E 2 1 Orientation using polarized fluorescence method for three different biaxial stretching methods

MD

Simultaneous Stretching

Sequen�alStretching

Double Bubble Tubular Process

MD

Uniform Orienta�on in all of the direc�ons

High Orienta�on in MD and TD Direc�ons

Polarized fluorescence intensity as a func�on of angle (Amorphous Orienta�on)

MD

F I G U R E 2 2 Distribution of phase contrast at different stretching methods

| 11KANAI et Al.

process.[2,7] The double bubble tubular film has a balanced tensile property. By contrast, the tenter stretched film does not show uniform tensile properties in the film plane. The results of the tenter stretched films are owing to the stretching effect which is not stretched in any direction uniformly and in addition the bowing phenomenon.

4 | CONCLUSIONS

The three different stretching methods, which can produce PA6 films, namely the simultaneous biaxial stretching, the sequen-tial biaxial stretching, and the double bubble tubular stretch-ing, were investigated. The following results were obtained.

• Simultaneous biaxial stretching process: The polymer chains were oriented in MD or TD and almost the same orientation in MD and TD, so the orientation bal-ance was good. The stretching ratio influenced the stretch-ing stress and the plane orientation. The stretching stress increased with decreasing the stretching speed.

• Sequential biaxial stretching process: Both the stretched parts and the unstretched parts existed, owing to the hydrogen bond during the TD stretching. The stretched parts show high TD orientation. The unstretched parts show low TD orientation and high MD orientation, be-cause of very low TD stretching effect. The ununiform part causes retardation distribution and the film balance to worsen.

• Double bubble tubular process: Uniform stretching stress and orientation in every direc-tion and good balanced film in every direction were ob-tained. It was found that highly oriented film with high physical properties was obtained under the low stretching temperature, slow stretching speed, and high stretching ratio. Furthermore, the double bubble tubular film process, which can produce good balanced film in every direction, is the most suitable process for requiring high physical properties and the secondary forming process such as bat-tery packaging and PTP medicine packaging.

ORCID

Toshitaka Kanai http://orcid.org/0000-0001-6441-0218

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How to cite this article: Kanai T, Okuyama Y, Takashige M. Dynamics and structure development for biaxial stretching polyamide 6 films. Adv Polym Technol. 2018;00:1–11. https://doi.org/10.1002/adv.21961