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In situ simultaneous measurement of stress, retardation, and three-dimensional refractive indexes during biaxial stretching experiments under various preheating times

  • Kentaro Egoshi EMAIL logo , Toshitaka Kanai and Kazuhiro Tamura
Published/Copyright: February 20, 2018
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Journal of Polymer Engineering
From the journal Journal of Polymer Engineering Volume 38 Issue 7

Abstract

An evaluation method for the stretchability of a biaxially oriented film during the stretching process was recently developed using an in situ measurement test machine. Stress, retardation, three-dimensional refractive indexes, light-scattering image, and birefringence distribution of films could be obtained in a short time. This stretching test machine was applied to examine the film stretchability of both semicrystalline polymers, such as polypropylene, and noncrystalline polymers, such as polystyrene, under various preheating times. From the measurements, the stress of semicrystalline polymers increased with increasing preheating times before stretching the film. However, the stress of noncrystalline polymers did not increase with increasing preheating times. This means that semicrystalline polymer is required to set up an optimum stretching condition of the preheating time for a satisfactory biaxially oriented film. Furthermore, the birefringence distribution and thickness uniformity of the stretched film were measured simultaneously. It was found that the stretchability of polypropylene and polystyrene films could be evaluated with a small piece of the sample using the biaxial stretching test machine.

Keywords: biaxial stretching process; birefringence distribution; preheating time; stretchability; thickness uniformity

1 Introduction

Biaxial stretching films are widely used for both industrial products and for our daily necessities. Recently, films with thin and uniform thicknesses were required for the reduction of raw materials and the production of high-quality stretching films. In the production lines for stretching films, one needs to understand not only the properties of the stretchable resin but also the optimum stretching conditions for the film resin. Furthermore, evaluating the stretchability of the films becomes important in the development of the stretching film. The stress-strain curve, retardation-strain curve, three-dimensional molecular orientations, and crystallization of semicrystalline polymers are key points for the evaluation of stretchability during biaxial stretching [1]. Previous research studied the relationship between the stretching force and stretching ratio for various samples [2], [3], [4], [5], [6], [7], [8].

Moreover, the simultaneous measurements of stresses, birefringence, and three-dimensional molecular orientations are also essential for the evaluation of stretchability. Several retardation measurement methods have been developed [9], [10]. However, the relationships among the stresses, deformation of spherulite, three-dimensional molecular orientations, and retardation behavior as a function of the stretching ratio during the biaxial stretching process have not been reported. The spherulite structure of semicrystalline polymers was measured using light scattering [11], [12], [13]. The growth rate of spherulites from several copolymers after solidification at the same crystallization temperature was examined using a four-leaf clover pattern. Additionally, the relationship between the spherulite diameter and crystallization time was shown but the deformation of spherulite caused by stresses during the stretching process has not yet been studied. The relationship between birefringence and stress with a separate measurement at the static state of stretched films has been reported [14].

On the other hand, the in situ measurement of stress and retardation during uniaxial stretching using photoelastic modulator (PEM) has been previously proposed [15], [16], [17]. This method is superior in terms of high sensitivity and short measurement time. With the advantages of PEM, stress and retardation in simultaneous measurements during the uniaxial stretching process can be measured even if the stretching speed is fast [1], [17].

Above all, there are no reports on the simultaneous measurements of stress, birefringence, three-dimensional refractive indexes, and light-scattering during the biaxial stretching process. Additionally, there are no reports on the stretchability of both semicrystalline polymer and noncrystalline polymer under stretching conditions such as preheating time.

In this article, we describe the simultaneous measurements of stress, retardation, three-dimensional refractive indexes, and light-scattering of semicrystalline polypropylene samples and noncrystalline polystyrene samples under several preheating time conditions during the sequential biaxial stretching process obtained with a newly developed biaxial test machine.

2 System construction

The biaxial stretching test machine developed recently by our group has been described in detail in previous articles [18], [19]. A schematic diagram of this test machine is shown in Figure 1. The apparatus was designed and constructed to carry out the simultaneous and in situ measurement of stress, birefringence, three-dimensional refractive indexes, and light-scattering during the biaxial 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 stresses of the biaxial stretching film, two optical measurement systems (which have vertical and inclined incidences of laser beams using PEMs called the double birefringence measurement systems for the measurement of retardations and three-dimensional molecular orientations), and it also included a light-scattering system for monitoring the spherulite conformation and spherulite size. The light source axis was adjusted to overlap at the center of the biaxial stretching film on the stretching unit. The strain is calculated from the biaxially stretched distance that is stretched by the stretching control unit. All the simultaneous measurement data could be obtained in real-time during the biaxial stretching process.

Figure 1: A diagram for simultaneous measurement of stresses, retardations, three-dimensional refractive indexes, and light-scattering during the biaxial stretching process.
Figure 1:

A diagram for simultaneous measurement of stresses, retardations, three-dimensional refractive indexes, and light-scattering during the biaxial stretching process.

3 Samples and experimental conditions

The sequential stretching method is the most popular in the production of biaxially stretched films [1]. For this reason, the sequential biaxial stretching mode was chosen in this study. As shown in Figure 1, sequential biaxial stretching for a square polymer sample using this new machine was performed in two sequential stretching steps: the first step was to stretch one side of the sample, which is called the machine direction (MD) stretching, and then the second step was to stretch the other side of the sample, which is called the transverse direction (TD) stretching by the stretching unit. Therefore, the stress that is obtained by the load cell in the MD stretching process is called the MD stress and the stress obtained in the TD stretching process is called the TD stress.

3.1 Semicrystalline sample

Polypropylene is one of the semicrystalline polymers that have excellent features such as high formability, heat resistance, high transparency, and cost performance, and thus polypropylene film is widely used for food packaging and industrial films. Polypropylene is also a semicrystalline polymer that has a highly ordered molecular structure.

The resin of isotactic polypropylene (iPP) sample, having melt flow rate (MFR)=3 g/10 min, Mw=3.6×105 g/mol, Mw/Mn=5.0, and Tm=160°C, was produced by Prime Polymer (Japan), and the sheet was produced under a resin temperature of 250°C at the die exit and a chill roll temperature of 30°C. The nonstretching sheet with 500 μm film thickness was cut to size at 85 mm2 of the piece in use. The test sample direction is constantly set at the same film orientation for each experiment. The stretching conditions were fixed with the chamber temperature at 162°C, stretching speed at 25 mm/s, total stretching ratio (multiply MD stretching ratio and TD stretching ratio) at MD5×TD6, and the preheating time at 90, 120, 150, and 180 s before stretching.

3.2 Noncrystalline sample

Polystyrene is a noncrystalline polymer that has excellent features such as high transparency, high rigidity, high formability, and cost performance; therefore, polystyrene is widely used for electronic products, food containers, and display cases. Polystyrene is an amorphous polymer that does not have an ordered molecular structure.

The PS sample has MFR=4 g/10 min. The nonstretching sheet with 1200 μm as a film thickness was cut to a size of 85 mm2. The stretching conditions were fixed with the chamber temperature at 124°C, stretching speed at 25 mm/s, total stretching ratio at MD3.8×TD4.2, and preheating time at 120, 180, and 240 s before stretching.

4 Results and discussion

4.1 Polypropylene samples

4.1.1 Simultaneous measurement of stresses and retardations during the sequential biaxial stretching

The simultaneously measured results of stresses and retardations during the sequential stretching process under several preheating times can be explained by the following: the MD and TD stresses that were measured by the load cells were established in MD and TD stretching axis as shown in Figure 2A and B, respectively. The retardations of both vertical incidence and inclined incidence of laser beams were measured by the double birefringence measurement systems as shown in Figure 2C and D, respectively.

Figure 2: Simultaneous measurement of stresses and retardations of polypropylene film during sequential biaxial stretching process at several preheating time conditions. (A) Relationship between MD stress and total stretching ratio; (B) relationship between TD stress and total stretching ratio; (C) relationship between vertical incidence angle of retardation and total stretching ratio; (D) relationship between inclined incidence angle of retardation and total stretching ratio; (E) variation of vertical incidence angle of retardations of polypropylene film during TD stretching process; (F) variation of inclined incidence angle of retardations of polypropylene film during TD stretching process.
Figure 2:

Simultaneous measurement of stresses and retardations of polypropylene film during sequential biaxial stretching process at several preheating time conditions. (A) Relationship between MD stress and total stretching ratio; (B) relationship between TD stress and total stretching ratio; (C) relationship between vertical incidence angle of retardation and total stretching ratio; (D) relationship between inclined incidence angle of retardation and total stretching ratio; (E) variation of vertical incidence angle of retardations of polypropylene film during TD stretching process; (F) variation of inclined incidence angle of retardations of polypropylene film during TD stretching process.

From the results of the stresses, it was found that the MD and TD stresses differently increased with increasing preheating times. As an example, the MD and TD stresses and the ratio of TD stress to MD stress at the end of the stretching process: MD4.7×TD5.5 are summarized in Table 1. From the results, it was clearly found that the MD and TD stresses increased with increasing preheating times, and the ratio of TD5.5 stress to MD4.7 stress (TD5.5 stress divided by MD4.7 stress) was more than 2.5 times. Additionally, the ratio of TD (4.7×5.5) stress to MD (4.7×5.5) stress was more than 2.4 times. This means that the semicrystalline polypropylene undergoes strain hardening at the last stage of TD stretching, and the stress in TD is greater than in the MD due to the sequential biaxial stretching process.

Table 1:

Stresses at the end of the stretching process and the ratio of TD stress to MD stress of polypropylene.

Preheating time (s)90120150180
MD4.7 stress (MPa)0.790.840.840.99
TD5.5 stress (MPa)2.112.252.192.49
TD5.5/MD4.72.692.682.612.52
TD(4.7×5.5)/MD(4.7×5.5)2.422.492.522.79

Moreover, to investigate the deformation of spherulite in the polypropylene sample during the sequential stretching under a preheating time of 120 s, the spherulite of the semicrystalline polymer was observed as a four-leaf clover on the screen using the light-scattering system. Similar to the results of light-scattering shown in Figure 2A and B, it was found that the polypropylene has spherulite, which was deformed by increasing stresses and stretching ratio, and then broken up at MD5×TD3.

From the results of retardations as shown Figure 2C and D, it was found that both the vertical incidence and the inclined incidence of retardations increased with increasing preheating times similar to the increasing of stresses. For more clarity, the variation of both the vertical and inclined incidences of retardations during the TD stretching process can be replotted in Figure 2E and F, respectively. From the replotted results, it was clearly found that the variation of retardations during TD stretching process increased with increasing preheating times. This means that the molecular orientation of semicrystalline polypropylene is influenced by the increase of preheating time.

4.1.2 Refractive indexes of polypropylene samples during sequential biaxial stretching

The refractive indexes can be immediately calculated after stretching from the results of retardations, which were measured in real-time with the maximum sampling time at 1 ms during the sequential biaxial stretching using the double birefringence measurement systems, as shown in Figure 3A and B. The values of nx and ny are the refractive indexes of MD and TD stretching direction, respectively, and nz is the refractive index of film thickness direction. The relationship between refractive indexes and sequential stretching ratio is also shown: in the beginning of MD stretching, nx is increasing due to increases in the MD stretching stress, and nz is decreasing due to the decreasing molecular orientation in the film thickness direction. Next, in the beginning of TD stretching, ny is increasing due to the increasing of TD stretching stress and nz is still continuously decreasing due to the decreasing of film thickness during the TD stretching process. Additionally, ny become greater than nx at the beginning of TD stretching because of the high molecular orientation of the sequential biaxial stretching.

Figure 3: Refractive indexes of polypropylene film during sequential biaxial stretching process at two preheating time conditions. (A) Preheating time condition at 120 s; (B) preheating time condition at 180 s.
Figure 3:

Refractive indexes of polypropylene film during sequential biaxial stretching process at two preheating time conditions. (A) Preheating time condition at 120 s; (B) preheating time condition at 180 s.

Compared with the results of the refractive indexes behavior under different preheating time conditions, it was found that the degree of plane orientation (ΔP) at 120 s of the preheating time was smaller than at 180 s. This means that the preheating time at 120 s provides lower stretching stresses than at 180 s.

Where the degree of plane orientation (ΔP) is defined as Equation 1:

(1)ΔP=nx+ny2−nz

4.1.3 Retardation distribution of stretched film

The retardation distributions at the center area of the stretched film were easily measured by XY mapping controlled system as shown in Figure 4A, and the standard deviation (SD) of retardation distribution can be calculated from the measured results, as shown in Figure 4C. In the measured results, it was found that the retardation distribution shows the difference in color all over the image. Nonuniform retardation distributions were shown in color all over the image at both short and long preheating times. On the contrary, more uniform distribution was shown at 120 s of the preheating time, which means more uniform retardation distribution. It shows optimum preheating time conditions for better uniformity of the biaxial stretching film.

Figure 4: Results of retardation distribution and film thickness of polypropylene stretched film at several preheating time conditions. (A) Retardation distribution images at the center area of stretched film; (B) film thickness distribution images at the center area of stretched film; (C) correlation of SD of film thickness and thickness uniformity and SD of retardation of polypropylene stretched film at various preheating times.
Figure 4:

Results of retardation distribution and film thickness of polypropylene stretched film at several preheating time conditions. (A) Retardation distribution images at the center area of stretched film; (B) film thickness distribution images at the center area of stretched film; (C) correlation of SD of film thickness and thickness uniformity and SD of retardation of polypropylene stretched film at various preheating times.

Furthermore, the birefringence distribution can be calculated as the measured retardation distribution divided by the thickness of the stretched film. In other words, the birefringence distribution of stretched film can also be used as data for the discussion of stretched film thickness uniformity.

4.1.4 Thickness uniformity of stretched film

The film thickness at the center area of the stretched film was measured using the dial gauge Teclock PG-01J (Mitsutoyo, Kanagawa, Japan) as shown in Figure 4B. The uniformity of thickness was calculated with Equation 2 from the measured results of film thickness as shown in Figure 4C. From the calculated results, it was found that the thickness uniformities showed bad values at the short and long preheating times such as 90 and 180 s, and they showed good values at the middle preheating times such as 120–150 s. This means that the stretching conditions under the optimum preheating time provides better thickness uniformity for the stretched film.

(2)Thickness uniformity [%]= (maximum thickness−minimum thickness)/ (2×average thickness)×100

Moreover, the SD of the measured thickness values at each preheating time was shown in Figure 4C. It was also found that the SD of the measured thickness values showed bad values in the short and long preheating times such as 90 s and 180 s, and they showed good values at the middle preheating times such as 120–150 s, such as the thickness uniformity and SD of retardation distribution shown in the Figure 4C. In other words, the SD of the measured thickness values is also suitable for the discussion of thickness uniformity.

4.1.5 Relationship between SD of retardation distribution and thickness uniformity

From the results shown in Figure 4C, the SD of retardation distribution and the thickness uniformity of the stretched film showed the same behavior at several preheating times. The results showed that the small SD distribution is the optimum thickness uniformity. This means that they are closely related. In other words, the thickness uniformity is easily evaluated by the measurement of retardation distribution of the stretched film.

4.1.6 Differential scanning calorimetry measurement of the nonstretched polypropylene film

The preheated nonstretch sample at each preheating time was heated from –20°C to 220°C at 20°C per minute for differential scanning calorimetry (DSC) measurement, as shown in Figure 5A. According to the results of DSC measurement, it was found that the low melting temperature component decreased and the endothermic peak moved to higher temperatures whereas the latent heat of fusion increased with increasing preheating time. This means that the increasing preheating times made both the crystallinity and the melting temperature become higher. As the DSC measured results, the crystallinity and the melting temperature were increased by the preheating time, which caused the increasing of stretching stresses, retardations, and degree of plane orientation, as shown in Figures 2 and 3, respectively.

Figure 5: Semicrystalline polypropylene sample under increasing preheating time conditions. (A) DSC measurement of nonstretched film of polypropylene; (B) crystalline structure of semicrystalline polymer (PP); (C) surface temperature measurements during preheating time.
Figure 5:

Semicrystalline polypropylene sample under increasing preheating time conditions. (A) DSC measurement of nonstretched film of polypropylene; (B) crystalline structure of semicrystalline polymer (PP); (C) surface temperature measurements during preheating time.

4.1.7 Schematic crystalline structure of polypropylene

Figure 5B explains the changing of polypropylene crystalline lamella under increasing preheating time conditions. As the crystalline lamella became thicker and the amorphous region became thinner due to the increasing preheating time, this caused the endothermic peak to move to higher temperatures, as shown in Figure 5A, and the increasing of stresses and retardation as shown in Figure 2A–F.

4.1.8 Stretchability of polypropylene under preheating time conditions

Standard deviation of retardation distributions and the films’ thicknesses are shown in Figure 4A–C, whereas the crystalline lamella under increasing preheating times and the measured surface temperatures of nonstretched samples during the preheating time are shown in Figure 5A–C. From these data, it was found that both the SD of retardation distribution and the thickness uniformity of the stretched film showed bad values at short preheating times due to the nonuniformity of internal temperature distribution of the sample by the shortage of preheating time, and they also showed bad values at long preheating times because the crystalline structure lamella became thick and the amorphous region became thin, which caused the melting temperature, crystallinity, stresses, and retardations to become higher. On the contrary, both the SD of retardation distribution and thickness uniformity showed good results at intermediate preheating times. This means that an optimum preheating time for polypropylene stretchability exists in the stretched film process.

4.2 Polystyrene sample

4.2.1 Simultaneous measurement of stresses and retardations of polystyrene during sequential biaxial stretching

Figure 6A showed the results of the MD stress, Figure 6B showed the TD stress, Figure 6C showed the vertical incidence of retardation, and Figure 6D showed the inclined incidence (30°) of retardation behavior during the sequential biaxial stretching process under several preheating time conditions. As the results of stresses and retardations, it was found that all of the preheated samples showed the same results of stresses and retardation behaviors at several different preheating time conditions. This means that the stresses and retardations of polystyrene are not greatly influenced by the preheating time conditions during the sequential biaxial stretching process. The variation of retardation in the TD stretching process is almost the same as the variation of retardation in the MD process. This means that the polystyrene sample has the same degree of plane orientation during the sequential biaxial stretching process at several different preheating time conditions.

Figure 6: Simultaneous measurement of stresses and retardations of polystyrene film during sequential biaxial stretching process at several preheating time conditions. (A) Relationship between MD stress and total stretching ratio; (B) relationship between TD stress and total stretching ratio; (C) relationship between vertical incidence angle of retardation and total stretching ratio; (D) relationship between inclined incidence angle of retardation and total stretching ratio.
Figure 6:

Simultaneous measurement of stresses and retardations of polystyrene film during sequential biaxial stretching process at several preheating time conditions. (A) Relationship between MD stress and total stretching ratio; (B) relationship between TD stress and total stretching ratio; (C) relationship between vertical incidence angle of retardation and total stretching ratio; (D) relationship between inclined incidence angle of retardation and total stretching ratio.

For more clarity, the MD stress and TD stress and the ratio of TD stress to MD stress at the end of the stretching process: MD3.5×TD4.2 are summarized as shown in Table 2. As the summarized results show, it was found that the MD and TD stresses do not increase with increasing preheating times, and the ratio of TD4.2 stress to MD3.5 stress (TD4.2 stress divided by MD3.5 stress) is less than 1.0 times. Additionally, the ratio of TD (3.5×4.2) stress to MD (3.5×4.2) stress is more than 1.1 times. This means that in the case of noncrystalline polymers, the stretching stresses, and the degree of plane orientation are not greatly influenced by the preheating time.

Table 2:

Stresses at the end of the stretching process and the ratio of TD stress to MD stress of polystyrene.

Preheating time (s)120180240
MD3.5 stress (MPa)0.840.850.77
TD4.2 stress (MPa)0.790.830.71
TD4.2/MD3.50.930.970.93
TD(3.5×4.2)/MD(3.5×4.2)1.151.221.11

4.2.2 Refractive indexes of polystyrene sample during biaxial stretching process

The refractive indexes can be calculated from the results of retardations at several different preheating time conditions, which were measured by the double birefringence measurement systems, as shown in Figure 7A and B. The relationship between refractive indexes and sequential stretching ratio is shown: in the beginning of MD stretching, nx is decreasing and nz is increasing due to increasing MD stretching stresses. Next, in the beginning of TD stretching, ny is decreasing and nx is increasing due to the increasing TD stretching stress during the TD stretching process. Additionally, ny become smaller than nx in the last of the TD stretching processes (MD3.8×TD4.2) for the molecular orientation of the sequential biaxial stretching. As the results of refractive indexes behavior, it was shown that polystyrene has a negative birefringence that is different from polypropylene, which has a positive birefringence, as shown in Figure 3A and B.

Figure 7: Refractive indexes of polystyrene film during sequential biaxial stretching process at two preheating time conditions. (A) Preheating time condition at 120 s; (B) preheating time condition at 240 s.
Figure 7:

Refractive indexes of polystyrene film during sequential biaxial stretching process at two preheating time conditions. (A) Preheating time condition at 120 s; (B) preheating time condition at 240 s.

Compared with the results of refractive indexes behavior under different preheating time conditions, it was found that all the preheated samples showed the same degree of plane orientation behavior at several preheating times. This means that, in the case of noncrystalline polystyrene, the degree of plane orientation of stretched film was not greatly influenced by the preheating time condition, which is different from the semicrystalline polypropylene.

4.3 Differences of stress and retardation behaviors between polypropylene and polystyrene films

During the biaxial stretching, MD and TD stresses, vertical incidence of retardation (Δnd(0)), inclined incidence of retardation (Δnd(30)), and three-dimensional molecular orientation behavior of semicrystalline polypropylene and amorphous polystyrene are shown in Figure 8A (1–3) and in Figure 8B (1–3), respectively.

Figure 8: Stresses (1) and retardations (2) and three-dimensional molecular orientations (3) behavior during the sequential biaxial stretching process. (A) Semicrystalline polypropylene film; (B) amorphous polystyrene film.
Figure 8:

Stresses (1) and retardations (2) and three-dimensional molecular orientations (3) behavior during the sequential biaxial stretching process. (A) Semicrystalline polypropylene film; (B) amorphous polystyrene film.

As shown in Figure 8A (1) and Table 1, the semicrystalline polypropylene sample demonstrates that the TD stress is much greater than MD stress at the end of the sequential biaxial stretching. It was also found that the semicrystalline sample has the strain hardening at the end of stresses-strain curve and, as shown in Figure 8A (2), the variation of retardations in the TD stretching process is greater than in the MD stretching process at the early TD stretching and, as shown in Figure 8A (3), the semicrystalline polypropylene has a high plane orientation during the sequential biaxial stretching process. This means that nx is less than ny at the beginning of TD stretching due to the semicrystalline polypropylene sample, which has a large difference between the MD and TD stresses and strain hardening by the sequential biaxial stretching process.

On the contrary, as shown in Figure 8B (1) and Table 2, the amorphous polystyrene sample shows that the TD stress is almost the same as the MD stress at the end of sequential biaxial stretching. Additionally, it was also found that the amorphous polystyrene sample does not have strain hardening at the end of stress-strain curve, which is different from the semicrystalline polypropylene. Furthermore, as shown in Figure 8B (2), the variation of retardation in the TD stretching process is greater than the variation of retardation in the MD stretching process at the late TD stretching. As shown in Figure 8B (3), the noncrystalline polystyrene has the low plane orientation during the sequential biaxial stretching process. This means that nx is greater than ny at the last stage of TD stretching due to the amorphous polystyrene, which has a small difference of MD and TD stresses according to the sequential biaxial stretching process.

5 Conclusions

A recently developed test machine for the evaluation of the biaxially oriented films during the stretching process can simultaneously measure the stress, retardation, three-dimensional refractive indexes, light-scattering image, and birefringence distribution of semicrystalline polypropylene and noncrystalline polystyrene films. It was found that the stretchability of polypropylene and polystyrene films could be obtained as follows:

The stretchability of semicrystalline polypropylene sample was influenced by increasing the preheating time. To obtain good stretchability for the semicrystalline film, production under the optimum preheating time condition is very important.

  1. The semicrystalline polypropylene film has the strain hardening of stresses-strain curve, and TD stress is much greater than MD stress in the TD stretching process of the sequential biaxial stretching.

  2. The molecular orientation of the semicrystalline polypropylene film shows an increase in the TD stretching process, and is influenced by the preheating time.

  3. The stretchability of polystyrene is not greatly influenced by increasing the preheating time.

  4. As the thickness uniformity of the biaxially oriented film is closely related to the SD of retardation distribution, the thickness uniformity can be easily evaluated by the measurement of retardation distribution.

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Received: 2017-08-30
Accepted: 2018-01-04
Published Online: 2018-02-20
Published in Print: 2018-08-28

©2018 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/polyeng-2017-0313
Keywords for this article
biaxial stretching process; birefringence distribution; preheating time; stretchability; thickness uniformity

Articles in the same Issue

  1. Frontmatter
  2. Material properties
  3. Effects of nano-silicon dioxide surface modification on the morphology and mechanical properties of ABS/PMMA blends
  4. Efficient enhancement in polyethylene biodegradation as a consequence of oxidative fragmentation promoted by pro-oxidant/pro-degradant metal stearate
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