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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2017.06.012
Open Access
Barrier improvement of the biaxial oriented polypropylene films using passive mechanisms
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Hamid Reza Shayanipour
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shayanipour@gmail.com

Corresponding author.
, Reza Bagheri
Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran
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Tables (6)
Table 1. Characterization of polypropylene, cation exchanged layered clay (organoclay11 and organoclay18), and polypropylene/clay nanocomposites.
Table 2. Oxygen transmition rate of the produced composites.
Table 3. Oxygen transmition rate of the produced film.
Table 4. Oxygen transmition rate of the produced film.
Table 5. Composition and mechanical properties of the BOPP nanocomposites.
Table 6. Composition and transparency properties of the BOPP nanocomposites.
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Abstract

Barrier properties of the biaxial oriented polypropylene (BOPP) films are the most important properties in food packaging industry. In this study different compatibilizers and materials were examined in order to find an appropriate blend composition to minimize the oxygen permeability of the films. It was observed that using organoclay and nanocomposits have positive effect on the oxygen permeability although the kind of compatibilizer is important. Through all evaluated materials the PVA (Poly(vinyl alcohol)) and MA (Maleic Anhydride) were the most successful selection for this goal. The BOPP film blended by PVA achieved the lowest oxygen transfer rate (300 cc 20μm/m2 24h atm) in compared with all other conventional BOPP films. In a novel action, both modified clay and PVA were used to produce a resistant blend. As each component was successfully improved the barrier properties of the BOPP film, it was tried to examine the synergistic effect of using both of them at the same time. The best oxygen permeability (220 cc 20μm/m2 24h atm) was found for the PP/gPP/PVA+Organoclay with the composition of 60/10/20+10 weigh %. X-ray analysis and scanning electron microscope were used to identify the occurred event in the polymers.

Keywords:
BOPP
Blend
Nanocomposite
Oxygen permeability
Full Text
1Introduction

Fillers are normally used to improve particular properties of the polymers and the polymer/nanocomposites [1]. The nanoclays have recently gained attention due to their ability to improve thermal, mechanical, barrier and fire retardant properties [2]. Nanosized fillers have been used in a wide range of applications from providing photo-catalyst activation and conductivity to improving moisture or oxygen barrier properties [3–5].

The special nanoparticles characteristic are due to their size and high relative ratio of surface area to volume [6]. For example, one particular characteristic of the nanoparticles is their optical clarity, which is better than that of its equivalent conventional-size particles, because the diameter is smaller than the wavelengths of the light. Therefore they can be used in application, which the film transparency is very important [7].

One of the greatest challenges in using nanosized fillers in the polymer network are their uniform distribution in the polymer matrix. In fact, it is a challenge to generate a favorable interaction between the polymer and the nanofiller and avoiding phase separation and agglomeration [8–10]. As an example, the natural layered clay delaminates entirely in water or solutions like polyamide but it does not spontaneously disperse in non-polar polyolefin like polypropylene. There are some methodologies for improving compatibility of the components like using suitable compatibilizer [10,11]. Compatibilizers are added especially to polyolefin/nanoclay composites or polymer blends prepared by melt blending. The interfacial adhesion between the compatibilizer and the clay galleries is influenced by the functionality and its concentration, the molecular weight and the molecular weight distribution (MWD) of the compatibilizer as well as the mass ratio of the compatibilizer to the clay [9].

In this study, the oxygen permeability of the BOPP film was reduced using passive mechanisms (nanocomposits and blend). Various nanoclays and blends were evaluated by appropriate compatibilizers in order to obtain acceptable BOPP film with the minimum oxygen permeability. The type of the blend or nanoclay and also their concentration are a big challenge due to the various limitations like mechanical properties of the final film and its transparency [12]. In this study the best oxygen permeability beyond all standards was obtained.

2Experimental

Different melt blending with various compositions were prepared to introduce the PP-based composites. The components were mixed in a container before the melt blending. All composites were prepared with a co-rotating twin-screw midi extruder (DSM) with a capacity of 16cm3, a screw length of 150mm and a screw speed of 60rpm. The temperature of the extrusion and injection molding was 200°C. The melt blending time was 15min.

PVA and 98.5% purity glycerin, used as plasticizer, were mixed in a super mixer at 1000rpm for 3min and then dried in a dehumidifying dryer at 90°C for 3h. A 40% purity Perkadox 14 compatibilizer and 1 phr Maleic Anhydride (MA) were mixed with 100 phr of PP in a super mixer, and then the dried PVA was added at 1000rpm for 3min. The PP/PVA mixture reacted in a twin screw extruder and the PP/PVA master batch (M/B) was prepared from the die-face cutter. At this time, the temperature distributions of a twin screw extruder were 140, 180, 205, 210 and 200°C, and the screw speed was 200rpm. In order to obtain a 3-layer casting sheet, the blends were inputted into the main single extruder, and the pure PP polymer was inputted into two side single extruders, and then extruded from a 3-layer slit die. The temperatures of main extruder were set to 180, 190, 200 and 210°C, and the die was 210°C. The screw speed was kept at 50rpm.

X-ray diffraction patterns were analyzed for the films dried naturally in room temperature by Rigaku X-ray diffractometer. The X-ray source was Ni-filtered Cu Kα radiation (40kV, 30mA). The film samples were scanned from 2̊ to 60̊ degrees of 2θ at a scanning rate of 3̊min−1. Scanning electron microscopy (SEM) was also used to identify the mechanism of barrier improvement resulted by using nanoclay or blends. The morphology of the fractured surfaces of the specimens was investigated with a JEOL JSM-6335F scanning electron microscope. Before fracturing, the samples were cooled in liquid nitrogen. The fractured samples were sputter-coated with chromium under argon. The electron micrographs were recorded with use of an acceleration voltage of 5.0kV.

3Results and discussion

XRD analysis was performed and the layer distances were calculated and reported. The desired fully-exfoliated clay structure was achieved with high concentrations of the compatibilizer and the organoclays (Table 1). The intercalated clay structure was obtained with both compatibilizers with organoclay18 with the low concentrations of compatibilizer and clay. The space between the clay sheets of the organoclay11 was not widened with low concentrations of compatibilizers and clays.

Table 1.

Characterization of polypropylene, cation exchanged layered clay (organoclay11 and organoclay18), and polypropylene/clay nanocomposites.

Materials  Composition  Interlayer distance (A°) 
PP  100   
Natural clay  100  9.7 
Organoclay 11  100  14.0 
Organoclay18  100  18.8 
PP/PP-g-DEM/organoclay11  90/5/5  14.2 
PP/PP-g-DEM/organoclay18  90/5/5  25.7 
PP/PP-g-MA/organoclay11  90/5/5  14.3 
PP/PP-g-MA/organoclay18  90/5/5  33.0 
PP/PP-g-DEM/organoclay11  70/20/10  29.4 
PP/PP-g-DEM/organoclay18  70/20/10  38.2 
PP/PP-g-MA/organoclay11  70/20/10  Exfoliated 
PP/PP-g-MA/organoclay18  70/20/10  Exfoliated 

It seems that a sufficiently high concentration of the compatibilizer relative to the modified clays enables the exfoliation regardless of the space created by the alkyl amine cation [13]. When the lower concentration of the compatibilizer relative to the clays was used, again the spaces between the layers determined the clay structure in the polymer composite. However, the X-ray scattering techniques reveal the characteristics of the organized structure (Figs. 1 and 2). In other words, the intercalated and original structure of the clay can be detected with this technique. The exfoliation may be detected based on the absence of signals for organized structures. Although the elimination of the peaks could be associated with displacement (nanoclay intercalation, likely to higher nanoclay contents) or equipment sensibility, but it can be a sign of exfoliation. To confirm this claim other analysis like morphology considerations must be performed. Moreover, the relative proportion of the different structures of the clay cannot be determined by X-ray measurements, and so the structure of the polymer composites needed to be analyzed by other means as well.

Fig. 1.

WAXS intensities of the neat PP, organoclay11, and PP/organoclay11 composites: (a) reflection 001 from the basal spacing of organoclay11, (b) reflection 300 of the _ form of isotactic PP, (c) reflection 040 of the _ form of isotactic PP, (d) intraplanar reflections 02 and 11 from organoclay11, and (e) reflection 004 from the basal spacing of organoclay11.

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Fig. 2.

WAXS intensities of the neat PP, organoclay18, and PP/organoclay18 composites: (a) reflection 001 from the basal spacing of organoclay18, (b) intraplanar reflections 02 and 11 from organoclay18, and (c) reflection 003 from the basal spacing of organoclay18.

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Fig. 1 (a), (d) and (e) reveal the characteristics of the organized structure of the organoclay 11 and Fig. 1(b) and (c) reveal the characteristics of the organized structure of the neat PP. It is observable that the organoclay signals have disappeared by increasing the concentrations of the compatibilizer and the modified organoclay. This behavior can be related to the well distribution of the organoclay in the PP network. The same observations are seen for the organoclay 18 which is shown in Fig. 2. Again it can be seen that the organoclay signals in XRD spectrum have disappeared when the mixing phenomenon is well occurred.

The clay structures were studied by SEM microscope, Fig. 3. Three types of clay structure, original (Fig. 3(a)), intercalated (Fig. 3(c)), and exfoliated (Fig. 3(d)), were found in the composite samples. As mentioned earlier, the exfoliated structure does not exhibit X-ray deflections. Nanostructure was found in the clays in those samples with the PP-g-DEM compatibilizer on the fracture surface SEM images of the composite. More exfoliated and intercalated clay structure could be detected by observing the fracture surface of the PP-g-MA sample with higher concentration of compatibilizer and organoclay.

Fig. 3.

SEM images of the fracture surfaces of (a) PP, (b) PP/organoclay18 (90/10wt %), (c) PP/PP-g-DEM/organoclay18 (90/5/5wt %), and (d) PP/PP-g-MA/organoclay18 (70/20/10wt %).

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A rough surface at the fracture indirectly indicates a great relative proportion of exfoliated clay. The fracture surface was smoother in the SEM images when the concentration of the PP-g-DEM compatibilizer was equal to the concentration of the modified clay than when it was double of the concentration of the modified clay, which indicates a cohesion fracture. The result was in line with the X-ray analysis. Interpenetration of the PP-g-DEM compatibilizer and the matrix is possible, as seen in the PP/PP-g-DEM/modified clay, and this indicates that the PP-g-DEM favors interactions with the outer surface of the modified clay particles, and only secondarily interacts with the inner surfaces of the clays. This means that it embeds the clay layers first and then penetrates between the layers, and the polypropylene matrix follows this behavior. The PP-g-MA compatibilizer seems to act in an opposite manner. It interacted with the interlayers of the clay more than the PP-g-DEM compatibilizer and resulted in more exfoliated and intercalated structures in all composites in the SEM images than when the PP-g-DEM compatibilizer was used [14–16].

Fig. 4 also illustrates the SEM images taken from the surface of the films. As can be seen the best distribution and compatibility between the clays and the base matrix occurs when the M.A is used as compatibilizer. It can be seen that in order to have a uniform and homogenous film with the best mechanical properties, parameters like the type of compatibilizer and clays have significant importance. Fig. 4(c) and (d) apparently illustrate the difference between the morphology of the PP/PP-g-DEM/organoclay 18 and the PP/PP-g-MA/organoclay18. More miscibility and uniformity in the polymer grafted with MA is observable.

Fig. 4.

SEM images of (a) PP, (b) PP/organoclay18 (90/10wt %), (c) PP/PP-g-DEM/organoclay18 (90/5/5wt %), and (d) PP/PP-g-MA/organoclay18 (70/20/10wt %).

(0.64MB).

The most important feature in food packaging applications is the barrier properties due to the significant role on food healthsome. Table 2 presents the oxygen permeability of the produced composites with various compositions. As it can be seen, permeability studies also indicates that the best result will be obtained when the M.A is selected as compatibilizer and the organoclay 18 is used as the clay for the composite. Morphological and structure studies in previous sections confirm the permeability observations because it was shown that the best miscibility of the clays with the PP matrix is obtained when the M.A compatibilizer and also the modified organoclay 18 were used.

Table 2.

Oxygen transmition rate of the produced composites.

Materials  Composition  OTR (cc 20μm/m2 24h atm) 
BOPP  100  1630 
PP/PP-g-DEM/organoclay11  90/5/5  1460 
PP/PP-g-DEM/organoclay18  90/5/5  1370 
PP/PP-g-MA/organoclay11  90/5/5  1420 
PP/PP-g-MA/organoclay18  90/5/5  1315 
PP/PP-g-DEM/organoclay11  70/20/10  690 
PP/PP-g-DEM/organoclay18  70/20/10  510 
PP/PP-g-MA/organoclay11  70/20/10  612 
PP/PP-g-MA/organoclay18  70/20/10  408 

Besides using different clays, using other types of materials like copolymers may improve the desired properties of the final film [17]. Previously, polyvinyl alcohol (PVA) was considered and discussed to use in BOPP structure in order to improve the barrier properties of the final film [18]. First of all, PVA was individually added to the polymer network and then a combination of PVA and the optimized clay were used in the BOPP composition. The results are presented as following.

X-ray diffraction (XRD) is a proven tool to study crystal lattice arrangements and yields very useful information on degree of sample crystallinity. XRD patterns of PP–PVA blends are shown in Fig. 5. The diffraction peak of this film is at around 20̊ and 40̊ degrees of 2θ [18]. These peaks are belonging to PVA as was previously demonstrated by other researchers [19]. From this diffractogram, it is obvious that the blend film is crystallized and a combination of an amorphous and crystallized structure is formed. It can be seen that increasing the PVA content affects the peak intensity. More sharp peaks are observed in high levels of PVA content.

Fig. 5.

X-ray diffraction patterns of PP-PVA film.

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Fig. 6 illustrates the morphology of biaxially oriented PP/PVA blend film with 25–30μm of thickness. It can be seen that a well-developed laminar morphology, which was similar to multi-layer film or laminated film, is produced [20]. Fig. 7 also indicates the longitudinal cross section of the mentioned film. As can be seen the PVA is appropriately distributed through the middle layer and a very nice multilayer structure has been produced.

Fig. 6.

SEM result of the fracture surfaces of biaxial oriented PP/PVA 30%.

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Fig. 7.

SEM result of the longitudinal section of biaxial oriented PP/PVA.30%.

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In Fig. 8, the oxygen permeability is shown as a function of the amount of PVA. However, above 50wt% PVA, the blend film was not prepared because of the rupture of blend film during two-dimensional stretching. When the amount of PVA increased, the oxygen permeability was reduced. This result indicates that the number of PVA platelet increased with increasing the amount of PVA, and a permeating path of a permeant molecule was elongated [21,22]. However, the barrier property was not sensitively changed at above 25wt% PVA.

Fig. 8.

Oxygen transmission rate (cc 20μm/m2 24h atm) of PP/PVA blend films as a function of the amount of PVA.

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As reported, different compatibilizers and materials were examined in order to find an appropriate blend composition. Through all evaluated materials the PVA and MA were the most successful selection for this goal. Oxygen permeability of the produced film was selectively measured and compared in Table 3. As can be seen the transmition rate of the oxygen molecules is reduced significantly by lamination from 1630 to 910 cc 20μm/m2 24h atm. The metalized film has very low OTR but it is not applicable in all functions. It is observable that using organoclay and nanocomposits also have positive effect on the oxygen permeability although the kind of compatibilizer is important. It is observed that MA is a better compatibilizer and it is more compatible with the BOPP films. The most interesting founding in this study is the PVA effect on the oxygen permeability. The low oxygen transfer through the BOPP/PVA blend is less than all other films except metalized films.

Table 3.

Oxygen transmition rate of the produced film.

Produced film  OTR (cc 20μm/m2 24h atm) 
BOPP  1630 
Laminated BOPP film  910 
Metalized BOPP film  52 
PP/PP-g-DEM/Organoclay 18(70/20/10)  510 
PP/PP-g-MA/Organoclay 18(70/20/10)  408 
BOPP/DEM/Organoclay 11(70/20/10)  690 
BOPP/MA/Organoclay 11(70/20/10)  612 
BOPP/MA/PVA(60/10/30)  300 

In a novel action both modified clay and PVA were used in a single master batch and the result was evaluated by SEM images. As each component have successfully improved, the barrier properties of the BOPP film, it was tried to examine both of them, at the same time in order to discover the synergistic effect of these two methodologies on each other. Sometimes employing two methodologies in a single composite shows noticeably improvement in barrier properties. Therefore both PVA and organoclay additives were added to the polymer network and examined by various analyses. Fig. 9 illustrates the obtained morphologies from this study.

Fig. 9.

SEM images of (a) PP/PP-g-DEM/PVA & Organoclay 18 (80/10/5–5wt %), (b) PP/PP-g-MA/PVA & Organoclay 18 (80/10/5–5wt %), (c) PP/PP-g-MA/PVA & Organoclay 18 (70/20/5–5wt %), and (d) PP/PP-g-DEM/PVA & Organoclay 18 (70/20/5–5wt %), (e) PP/PP-g-MA/PVA & Organoclay 18 (60/10/20–10wt %), (f) PP/PP-g-DEM/PVA & Organoclay 18 (60/10/20–10wt %).

(1.37MB).

In a consideration rout it was tried to examine various amount of PP, grafted PP and the PVA and modified organoclay contents. A close look at the obtained morphologies, confirms that better compatibility and uniformity were obtained when the M.A was used as compatibilizer. In addition it seems that a more homogenous exfoliated structure was obtained when the grafted PP reaches 20 weight %. It can also be seen that the additives amount significantly change the layers morphology. It is obvious that exceeding the additive percentage from the optimum value cause loss in mechanical or transparency properties. Therefore it seems that mechanical and transparency evaluations beside morphological observations are necessary.

Although more exfoliated structure can improve the barrier properties, other characteristics of the film are important and should be considered beside the barrier improvement [7]. The transparency of the final film and also the mechanical properties, especially the film strength, are important features that will following be reported. The barrier properties were also examined by OTR measurement as shown in Table 4.

Table 4.

Oxygen transmition rate of the produced film.

Produced film  OTR (cc 20μm/m2 24h atm) 
BOPP  1630 
PP/PP-g-DEM/PVA+Organoclay 18(80/10/5+5)  710 
PP/PP-g-M.A/PVA+Organoclay 18(80/10/5+5)  680 
PP/PP-g-DEM/PVA+Organoclay 18(70/20/5+5)  475 
PP/PP-g-M.A/PVA+Organoclay 18(70/20/5+5)  355 
PP/PP-g-DEM/PVA+Organoclay 18(60/10/20+10)  280 
PP/PP-g-M.A/PVA+Organoclay 18(60/10/20+10)  220 

As seen, best oxygen permeability belongs to the PP/gPP/PVA+Organoclay 18 with the composition of 60/10/20+10wt %. Previously it was found that adding 30% PVA can reduce the oxygen transmition rate up to 300 cc 20μm/m2 24h atm. Now using the same proportion of additive (30%) the OTR has decreased until 220 cc 20μm/m2 24h atm. This event confirms that the PVA and organoclay have synergistic effect on each other.

As mentioned, considering just one aspect of properties improvement is not logic and there is a need to evaluate other important properties. Table 5 illustrates strength changes of BOPP films due to changes in their compositions. The important point in this consideration is the negative effect of increasing additives into the polymer network on the film strengths. It is noticeable that the film strengths are decreasing by increasing the weight percentage of the PVA and clay components. Although the strength of the best film (from OTR view) is acceptable, but continuing the PVA or nanoclays increment is forbidden due to the negative effect on mechanical properties.

Table 5.

Composition and mechanical properties of the BOPP nanocomposites.

PP (wt.%)  PPg (wt.%)  Type of clay  Tensile strength (MPa) 
100  –  BOPP  34.3±0.9 
80  10  PP/PP-g-DEM/PVA+Organoclay 18(80/10/5+5) weight %  32.4±0.8 
80  10  PP/PP-g-M.A/PVA+Organoclay 18(80/10/5+5) weight %  33.4±0.7 
70  20  PP/PP-g-DEM/PVA+Organoclay 18(70/20/5+5) weight %  31.8±0.6 
70  20  PP/PP-g-M.A/PVA+Organoclay 18(70/20/5+5) weight %  33.8±0.6 
60  10  PP/PP-g-DEM/PVA+Organoclay 18(60/10/20+10) weight %  28.3±0.5 
60  10  PP/PP-g-M.A/PVA+Organoclay 18(60/10/20+10) weight %  30.1±0.3 

Another important characteristic of the film in food packaging industry is the transparency of the final film. The haze measurements were performed on the produced nanocomposites and the results presented in Table 6. Again in this study it was found that there are serious limitations in using additives without controlling their amount. As can be seen the haze of the films are enhanced when the additives amount are increased. Previous morphological studies indicate that how layer structures will be formed and block the oxygen transmition through the film thickness [23]. Apparently, these kinds of structures affect the light transmition and the haze or transparency of the films. Again, the haze of the best film (from OTR view) is acceptable, but continuing the PVA or nanoclays increment is forbidden due to the negative effect on transparency of the film.

Table 6.

Composition and transparency properties of the BOPP nanocomposites.

PP (wt.%)  PPg (wt.%)  Type of clay  Haze 
100  –  BOPP  0.31 
80  10  PP/PP-g-DEM/PVA+Organoclay 18(80/10/5+5) weight %  0.41 
80  10  PP/PP-g-M.A/PVA+Organoclay 18(80/10/5+5) weight %  0.34 
70  20  PP/PP-g-DEM/PVA+Organoclay 18(70/20/5+5) weight %  0.52 
70  20  PP/PP-g-M.A/PVA+Organoclay 18(70/20/5+5) weight %  0.44 
60  10  PP/PP-g-DEM/PVA+Organoclay 18(60/10/20+10) weight %  0.56 
60  10  PP/PP-g-M.A/PVA+Organoclay 18(60/10/20+10) weight %  0.50 
4Conclusion

There are various methodologies for improving barrier properties of polymeric films, including active and passive mechanisms. In passive methodology, nanocomposites and blend compositions are the subject of many researches in recent publications. Using PVA with a suitable M.A compatibilizer, it was established a successful method for improving barrier properties of BOPP films. The results showed that PVA blend with 70% PP/30% PVA composition can reduce the oxygen transition rate up to 300 cc 20μm/m2 24h atm. Using the same proportion of additive (PVA and organoclay, 30%) the OTR has decreased until 220 cc 20μm/m2 24h atm. This event confirms that the PVA and organoclay have synergistic effect on each other and the barrier properties can be significantly improved in comparison with conventional methodologies.

Conflicts of interest

The authors declare no conflicts of interest.

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Y. Lin, A. Hiltner, E. Baer.
A new method for achieving nanoscale reinforcement of biaxially oriented polypropylene film.
Polymer, 51 (2010), pp. 4218-4224
Copyright © 2019. The Authors
Journal of Materials Research and Technology

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