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thermoplastic starch blend films
Accepted Manuscript Title: Effect of Nanoclay on the Properties of Low Density Polyethylene/Linear Low Density Polyethylene/Thermoplastic Starch Blend Films Author: Maryam Sabetzadeh Rouhollah Bagheri Mahmood Masoomi PII: DOI: Reference:
S0144-8617(15)01235-7 http://dx.doi.org/doi:10.1016/j.carbpol.2015.12.057 CARP 10655
To appear in: Received date: Revised date: Accepted date:
12-10-2015 9-12-2015 22-12-2015
Please cite this article as: Sabetzadeh, M., Bagheri, R., and Masoomi, M.,Effect of Nanoclay on the Properties of Low Density Polyethylene/Linear Low Density Polyethylene/Thermoplastic Starch Blend Films, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.12.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights: Higher nanoclay content improves platelet delamination in LDPE/LLDPE/TPS blends. Both nanoclay and PE-g-MA reduce TPS domain size and deform their particles. LDPE/LLDPE/TPS nanocomposites show higher performance than the pure blends.
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LDPE/LLDPE/TPS nanocomposite films are environmentally friendly materials.
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LDPE/LLDPE/TPS blends with 5phr nanoclay satisfy required packaging properties.
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Effect of Nanoclay on the Properties of Low Density Polyethylene/Linear Low Density Polyethylene/Thermoplastic Starch Blend Films
Maryam Sabetzadeh, Rouhollah Bagheri*[email protected], Mahmood Masoomi
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Chemical Engineering Department, Polymer Group, Isfahan University of Technology, Isfahan 84156 83111, Iran
Abstract
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Tel.: 98 31 3391 5613; fax: 98 31 3391 2677.
The aim of this work is to study effect of nanoclay (Cloisite®15A) on morphology and properties of low-density polyethylene/linear low-density polyethylene/thermoplastic starch (LDPE/LLDPE/TPS) blend films. LDPE/LLDPE blend (70/30 wt/wt) containing 15 wt.% TPS in the presence of PE-grafted maleic anhydride (PE-g-MA, 3wt.%) with 1, 3 and 5 phr of nanoclay are compounded in a twin-screw extruder and then film blown using a blowing machine. Nanocomposites with intercalated structures are obtained, based on the X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. However, some exfoliated single platelets in the samples are also observable. Scanning electron microscopic (SEM) images confirm the ability of both exfoliated nanoclay and PE-g-MA to reduce the size of TPS domains and deform their particles within the PE matrices. As the nanoclay content increases from 1 to 5phr, the tensile strength, tear resistance and impact strength of the films increase, whereas a slight decrease in the elongation at break is observed. The film samples with 5phr nanoclay possess the required packaging properties, as specified by ASTM D4635. These films provide desired 1 Page 1 of 19
optical transparency and surface roughness which are more attractive for packaging applications. Keywords
LDPE, LLDPE, Thermoplastic starch, Clay, Nanocomposite, Film.
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1. Introduction
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Low density polyethylene/linear low density polyethylene (LDPE/LLDPE) blend films with a combination of good processability and superior mechanical properties have attracted much attention in packaging applications (Hemati & Garmabi, 2011). However, raising the problem of environmental pollution related to the accumulation of plastic packaging wastes has caused increasing interest in the use of polymers from renewable resources (Liu et al., 2014; Mortazavi, Ghasemi & Oromiehie, 2014). Among them, starch is a natural, renewable and inexpensive polymer which can be added to polyethylene (PE) to produce environmentally friendly materials (Kahar, Ismail & Othman, 2012; Sabetzadeh, Bagheri & Masoomi, 2012). It is known that native starch-PE materials without a plasticizer and a modifying agent show poor processability and inferior mechanical performance, leading to limit their applications (Sharif, Aalaie, Shariatpanahi, Hosseinkhanli & Khoshniyat, 2013; Thipmanee & Sane, 2012). Starch can be modified in the presence of a plasticizer (such as glycerol) at high temperatures (i.e. TPS) to be processed by conventional plastic-forming equipments and also disperse fairly uniform in the polymer matrix, which is critical for mechanical properties. Several studies have been conducted on PE blends obtained with addition of TPS (Guzmán & Murillo, 2015; Mortazavi, Ghasemi & Oromiehie, 2013; Taghizadeh, Sarazin & Favis, 2013). Even so, these materials have a relatively high interfacial tension arises from the incompatibility between the non-polar PE and the high polar TPS, resulting in poor mechanical properties (Alidadi-Shamsabadi, Behzad, Bagheri & NariNasrabadi, 2015; Cerclé, Sarazin & Favis, 2013). In order to overcome the incompatibility issue and as a consequence improvement in mechanical properties, addition of a suitable interfacial modifier containing reactive groups [e.g. polyethylene-grafted maleic anhydride (PE-g-MA)] has been suggested (Prachayawarakorn, Sangnitidej & Boonpasith, 2010; Taguet, Bureau, Huneault & Favis, 2014). Anyway, the problem of poor mechanical properties in the blends could not be properly overcome (Inceoglu & Menceloglu, 2013). It should be underlined that inclusion of a small amount of nanofiller in a polymer blend provides a composite material with improved mechanical properties (Kim & Cha, 2014). Among inorganic compounds, more attention has recently been paid to clay minerals, especially montmorillonite (MMT) in the research area of nanocomposites, attributed to their environmentally friendly nature, small particle size, high surface area and intercalation properties. The surface of the MMT is modified with certain organic modifiers (e.g. alkylammonium cation) to provide it more compatible with the non-polar polymers. It is well established that surface treatment and the blending process can significantly facilitate 2 Page 2 of 19
dispersion of clay nanoparticles (Kampeerapappun, Aht-ong, Pentrakoon & Srikulkit, 2007; Ludueña, Vázquez & Alvarez, 2013; Schlemmer, Angélica & Sales, 2010).
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In the last few years, increased interest has appeared in PE/starch blends containing clay nanoparticles. Metallocene - catalyzed polyethylene (m-PE)/TPS nanocomposites has been prepared with incorporation of clay reinforced nanocompatibilizer (Chiu, Lai & Ti, 2009). Moreover, nanocomposites based on high-density polyethylene (HDPE)/TPS blends have been studied. It has been demonstrated that addition of nanoclay is in favor of developing nanocomposites with better performance than the native blends (Sharif, Aalaie, Shariatpanahi, Hosseinkhanli & Khoshniyat, 2011). Besides above, low-density polyethylene (LDPE)/starch nanocomposites have been investigated. The most considerable improvement in all properties is achieved in the presence of both clay and compatibilizer (Inceoglu et al., 2013; Manjunath & Sailaja, 2014). Our studies also suggest that LDPE/TPS blends containing nanoclay have desired mechanical properties that would be appropriate for packaging applications (Sabetzadeh, Bagheri & Masoomi, 2014). Recently, HDPE/TPS nanocomposite films have been developed using new compatibilizer systems based on the other clay minerals such as sepiolite (Mir, Yasin, Halley, Siddiqi, Ozdemir & Nguyen, 2013; Samper-Madrigal, Fenollar, Dominici, Balart & Kenny, 2015; Yasin, Nisar, Shafiq, Nho & Ahmad, 2013).
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To date, ternary LDPE/LLDPE/starch blends filled with nanoparticles have rarely been reported in the literature. The present work centers around the use of a nanoclay in a compatibilized LDPE/LLDPE blend film containing 15 wt.% TPS which has been demonstrated to be suitable for packaging applications (Sabetzadeh, Bagheri & Masoomi, 2015). The detailed effect of nanoclay addition on the morphology, mechanical properties, film characteristics such as optical transparency and surface roughness as well as water absorption capacity is investigated.
2. Experimental 2.1. Materials
Low density polyethylene (LDPE) with a density of 0.921 g/cm3, melt flow index (MFI) of 0.75 g/10min and linear low density polyethylene (LLDPE) with a density of 0.920 g/cm3, MFI of 0.9 g/10min (at 190oC, 2.16 kg load) were both film grades and purchased from Petrochemical Commercial Company, Iran. The unmodified corn starch (30 wt.% amylose and 70 wt.% amylopectin) was supplied by Glucosan Company, Iran. Glycerol (99.5% purity) was reagent grade from Hansa Group AG, Germany. Polyethylene grafted-maleic anhydride (PE-g-MA, 1% grafted) was obtained from Pluss Polymers Co, Ltd (India) and used as a compatibilizer. The modified montmorillonite (Cloisite®15A) was obtained from Southern Clay Products, Inc. (USA).
2.2. Sample preparation
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All components were dried in a vacuum oven at 80oC for 24h prior to further treatments. Thermoplastic starch (TPS) was prepared by melt mixing the homogenous compound of native starch and 35 wt% glycerol in a Haake internal mixer (with a volumetric chamber capacity of 300 cm3) at 140oC with rotor speed of 60 rpm for 8 min.
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LDPE/LLDPE (70/30 w/w) blend with 15 wt.% prepared TPS in the presence of 3 wt.% PE-g-MA (Sabetzadeh et al., 2015) were melt-mixed with various contents of nanoclay (1, 3 and 5phr) using a twin screw extruder (model ZSK25, Germany). The extruder had screw diameter (d) of 25mm and the length to diameter ratio (L/D) was 40. The temperature profile along the six heating zones of the extruder barrel was 145-180oC (from feed zone to die) and the screw speed was set at 150 rpm. The prepared compounds were emerged in the form of continuous strands through the die. The strands were cooled using water trough and pelletized. The pellets were then blown into 45μm thick films using Dr Collin single screw extruder (model E45M) with L/D ratio of 25, containing nine zones. The barrel temperature range was 155-170oC and the screw speed was set at 70 rpm. The prepared films designated as nbf0, nbf1, nbf3 and nbf5; containing 0, 1, 3 and 5phr nanoclay, respectively.
2.3. Characterization and measurements
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X-ray diffraction of the film samples was conducted using a Brucker D8 Advance X-ray diffractometer (Brucker, Germany) with Cu Kα radiation (λ=0.154 nm) at a voltage of 40 kV and 30 mA. Scattered radiations were detected in the range of diffraction angle 2θ=2–10° with the scan rate of 1°/min at room temperature. The basal spacing of the nanoclay ,d, was calculated using the Bragg equation, d = λ 2 sin (πθ / 180 ) , where θ is the diffraction angle and λ is the wavelength.
A typical transmission electron microscopy (TEM) image was captured using a Phillips CM200 transmission electron microscope operating at an acceleration voltage of 200 kV. Ultrathin cut (80 nm) from the film sample, using a diamond knife under cryogenic conditions at -100oC was performed. Scanning electron microscope (SEM, TESCAN model Vegall), operating at an accelerating voltage of 15kV was used to study phase morphology of the film samples. Before the test, the film samples were cryogenically fractured in the liquid nitrogen and then sputter coated with a thin layer of gold to avoid electrostatic charging and poor resolution during examination. SEM micrographs of the fracture surfaces were taken at the magnification of 10000×. Tensile properties, such as ultimate tensile strength and elongation at break percentage of the film samples were measured using a Universal Testing Machine Zwick/Roell (model Z 2.5/TH1S), according to ASTM D882. Strip form specimens were cut from the films and strained at a crosshead speed of 200 mm/min at room temperature for both machine and transverse directions. The grip separation was set at 50mm. Tear resistance of the films was also measured in both machine and transverse directions according to ASTM D 1922 standard test method, using a 4 Page 4 of 19
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Ceast ED 300 tear tester (Elmendorf type). The average force required to propagate a tear through a specified length of the film with respect to the thickness determines the tear resistance. In addition, a Ceast falling dart impact test equipment (model 9340) was used to measure the impact strength of the strip shaped film samples, according to ASTM D1709 method at room temperature. The energy that causes films to fail under the specified conditions of impact of a free-falling dart determines the impact strength. For all mechanical tests, at least five replicates were examined for each sample and the average values were determined.
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To evaluate some film characteristics, the optical transparency was determined by the light transmission rate through the films at wavelengths from 400 to 800 nm, using a UV-vis spectrophotometer (Jasco, V550 Series, Japan). Moreover, the surface roughness of the films was examined by atomic force microscopy (AFM, Brucker, Germany) at room temperature. Contact mode was used and the scanning range was 42µm × 42µm in at least five areas for each film sample to calculate the average (Ra) and root mean square roughness values (Rq). These are the useful tools to get information about the particle size and distribution of the clay within the blend matrix.
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In order to study water absorption behavior of the film samples, they were immersed in the distilled water, according to the ASTM D570 method at room temperature. The samples were taken out at regular time intervals and wiped to measure the water absorption percent. Before the test, the samples were dried at 80°C in a vacuum oven for 24h. Water absorption percent at any time (wt.%) was calculated according to wt (%) = [(w2 − w1 ) w1 ]×100 , where w1 and w2
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were the weight of dried sample and the sample after immersion at time t, respectively. 3. Results and discussion
3.1. Structure and morphology
X-ray diffraction patterns of the LDPE/LLDPE/TPS film samples (nbf1, nbf3 and nbf5) as well as the nanoclay (as the reference), in the region of 2θ=2-10° are represented in Fig.1. For pristine nanoclay, a characteristic diffraction peak around 2θ=3.3° is appeared, indicating d-spacing of 2.67nm. The diffraction peak of the sample nbf1 shifts slightly to the lower angle (2θ=2.95°, according to d-spacing of 2.99nm). This peak is also located at 2θ = 2.33° and 2θ=2.13°, corresponding to d-spacing of 3.79nm, and 4.14nm for the samples namely nbf3 and nbf5, respectively. Such increase between nanoclay platelets with increasing nanoclay concentration that leads to diminishing of the characteristic peak intensity can be taken as an indication of nanocomposite structures with good intercalation and/or partial exfoliation of nanoclay in the samples (Kampeerapappun et al., 2007). The XRD results suggest that the polymer chains enter into the nanoclay galleries and separate the platelets. By increasing the nanoclay content, the polymer – particle interactions increase and the stiffening effect of nanoclay intensifies which leads to increasing the melt viscosity of PE matrices. Therefore, more shearing force is applied, resulting the easier intercalation of the 5 Page 5 of 19
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polymer chains into the silicate layers and consequently, an increase in the d-spacing. This causes the clay platelets become disordered and the broader peaks appear in the XRD patterns, indicating intercalation with a good dispersion of nanoclay or some exfoliation in the matrices (McGlashan & Halley, 2003; Tang, Alavi & Herald, 2008). This is more evident in the samples nbf3 and nbf5 which have the similar diffraction profile. A typical TEM image of the sample nbf5 is also captured and shown in Fig.2. In this Figure, the dark lines represent nanoclay, the gray base corresponds to the PE matrices and TPS phase is seen brighter. There are some exfoliated clay platelets, visible as the single and disordered platelets in the sample. It seems that the nanoclay particles are preferentially dispersed within the PE matrices or located at the interface between LDPE-TPS and/or LLDPE-TPS. The obtained XRD and TEM results are similar to those reported previously in the case of PE/TPS blend nanocomposites (Sharif et al., 2011; Chiu et al., 2009).
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SEM micrographs of the fractured surfaces of the film samples (nbf1, nbf3 and nbf5) are captured with the magnification of 10000× and depicted in Fig. 3(a-c). Reaction of MA in PE-gMA with the hydroxyl groups of TPS imparts reduction of interfacial tension between PE matrices and TPS phase which causes a comparatively good dispersion of starch particles in PE matrices for all the samples. It should be explained that starch plasticized by glycerol is a partially miscible mixture of glycerol-rich and starch-rich phases. Low molecular weight glycerol could migrate to the interface and forms a glycerol layer between starch and PE phases, resulting decrease in the interfacial tension of the blend. It seems that PE-g-MA copolymer tends to react with the glycerol layer initially in compatibilized PE/TPS blends and is located at the interface between TPS–LDPE and/or TPS–LLDPE which enhances interfacial interactions in the blends (Sabetzadeh et al., 2015). In the present work, the results indicate that the TPS domains are evidently smaller and have the elongated shapes in the sample nbf5 compared to the other samples (Fig 3c). Here, it is reasonable to assume that the combined effect of PE-g-MA and nanoclay with exfoliated platelets leads to reduction of interfacial tension between PE matrices and TPS effectively and therefore, a more decrease in the starch particles size (Sharif et al., 2011). Based on both TEM and SEM images, Figs 2 and 3c, it can be concluded that the exfoliated nanoclay platelets is more desirable to reduce the size of TPS domains and deform their particles within the PE matrices, compared to the intercalated ones. These results are in accordance with those reported previously in the case of HDPE/TPS nanocomposites (Sharif et al., 2011) and poly (butylene adipate-co-terephthalate)/TPS/clay nanomaterials (Mohanty & Nayak, 2009).
3.2. Mechanical properties Tensile properties of the LDPE/LLDPE/TPS nanocomposite films such as ultimate tensile strength and elongation at break versus nanoclay loading for both machine and transverse directions are represented in Figs 4a and 4b, respectively. For all samples, tensile strength in the machine direction (MD) is higher than that in the transverse direction (TD). The elongation at break has different tendency: it is lower in the machine direction than in the transverse direction. This 6 Page 6 of 19
may be due to the higher degree of orientation that polymer chains undergo inside and outside the die in MD direction. However, in both directions with increasing the nanoclay loading the tensile strength increases; whereas the elongation at break decreases slightly. Figure 5 indicates the tear resistance of the prepared films for both MD and TD directions. It is clear that the tear resistance in TD direction exceeds that in the MD direction. In addition, falling dart impact strength of the films is depicted in Fig 6. Based on the Figs 5 and 6, as the nanoclay content increases, both the tear resistance and impact strength increase.
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It is reported that that with addition of LLDPE into LDPE, tensile properties of the resultant blend film increase, because of the higher performance of LLDPE than LDPE (Lu & Sue, 2002). However, the presence of starch in the blend caused level off the tensile properties of the final materials (Sabetzadeh et al., 2015). In this work, the results show that the mechanical properties of the LDPE/LLDPE/TPS nanocomposites are strongly dependent on nanoclay content and dispersion status. In fact, incorporation of nanoclay as a reinforcing agent has a noticeable effect on the LDPE/LLDPE/TPS blend films which is more pronounced in the samples with higher nanoclay loading. In particular, 35% and 45% increase in tensile strength for the sample nbf5 compared to the pure blend film control (i.e. nbf0) are achieved in MD and TD directions, respectively. In addition, for this sample the tear resistance increases by 54 and 39 % in MD and TD directions, respectively and 38% increase in impact strength is obtained in comparison with the control. However, insignificant decrease in the elongation at break is attained, as the nanoclay content increases.
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It can be deduced that degree of nanoclay dispersion in the blend film is a great contributing factor affecting the mechanical properties of the blends (Chiu et al., 2009; Liao & Wu, 2005; Sharif et al., 2011). In view of this, finer nanoclay dispersion and formation of some exfoliated platelets are the main reasons for achieving the highest tensile strength value for the sample nbf5 among the others. This sample would fulfill the required tensile properties for packaging applications, as evidenced by ASTM D4635 (tensile strength = 16.4MPa, elongation at break percent = 227%) (ASTM D 4635). The highest tear resistance and impact strength are also obtained in the case of this sample. It can be explained that with increasing the nanoclay content, the interactions with incorporated polymers enhance and thicker polymer/nanoclay interphase with higher shear strength and better properties creates which increases the fracture toughness and energy absorption capability (Miyagawa & Drzal, 2004). It has been reported that the exfoliated nanoclay particles in the polymer matrix improve fracture toughness of epoxy/clay nanocomposites which is attributed to creation of the fracture surfaces between the exfoliated platelets (Wang, Chen, Wu, Toh, He & Yee, 2005). It also should be mentioned that the smaller regions of TPS within PE matrices with unique elongated morphology for this sample, owing to the coexistence of PE-g-MA and some exfoliated nanoclay platelets which intensifies the compatibility between polymers, are responsible for the obtained results. It has been suggested that in PE/TPS composites filled with nanoclay, the presence of PE-g-MA causes higher tensile properties in comparison to the counterparts without the PE-g-
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MA inclusion, as a result of finer dispersion of nanoclay induced by the compatibilizer (Sharif et al., 2011).
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It is worthy to note that with increasing the nanoclay content, decrease in elongation at break percent of the samples is not remarkable. This is a significant achievement in the present work and could be due to creation of the thicker polymer/nanoclay interphase with better properties in higher nanoclay loadings, which increases the fracture toughness and delay the tension failure of the samples. However, the stiffening effect of nanoparticles in the polymer matrices which causes restricted mobility of the polymer chains explains the decrease in elongation at break percent of the samples (Sabetzadeh et al., 2014). The obtained results of the mechanical properties due to the effect of nanoclay are in agreement with those reported by other workers for PE/starch nanocomposites (Manjunath et al., 2014; Sharif et al., 2011; Yasin et al., 2013).
3.3. Optical transparency and surface roughness
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Table 1 represents the optical transparency of the studied films, which was examined by acquiring the light transmission percent at selected wavelengths from 400 to 800 nm. The light transmission percent of the nanocomposite films are lower than that of pure blend film control (i.e. nbf0). As the nanoclay content increases, the percent of light transmission increases. This may be due to the homogenous dispersion of nanoclay particles inside the matrix and even smaller particle size than the wavelength of visible light (Hyun, Chong, Koo & Chung, 2003; Sánchez-Valdés, Martínez Colunga, López-Quintanilla, Yañez Flores, García-Salazar & González Cantu, 2008). This result confirms that the nanoclay has a good compatibility with the polymer blend matrix preventing the particle aggregation, thus decreases the irregularity and heterogeneity of the surface and the amount of light scattering to provide the transmittance of visible light through the nanocomposite films. Theoretically, it has been reported that the clay platelets with 1nm thickness less than the visible light wavelength do not inhibit light transmittance. Therefore, the transmittance of a nanocomposite film containing such welldistributed clay platelets should not be significantly changed (Liu et al., 2014; Zeng, Yu, Lu & Paul, 2005). In the present work, the results indicate that the film sample nbf5 with welldeveloped clay platelets have the similar light transmission percent to that of pure blend film control (nbf0). The obtained results are in good agreement with the results of optical transparency of the LDPE/starch nanocomposite films reported previously (Inceoglu et al., 2013). The good transparency of the nanocomposite films is an important property required for packaging applications. In addition, surface roughness data for the film samples are summarized in Table 1. The pure blend film control (nbf0) has a quite smooth surface, with an average roughness (Ra) of 16.1nm and root mean square roughness (Rq) of 24.7nm. For the nanocomposite films, with increase in the nanoclay content, the Ra and Rq values decrease continuously. The lowest Ra and Rq obtained for the sample nbf5 containing the highest nanoclay content is the evidence of more
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uniform nanoclay dispersion in this sample. The above results are in good agreement with morphological studies which discussed earlier.
3.4. Water absorption behavior
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Water absorption behavior of the film samples as a function of both the immersion time and nanoclay content are summarized in Table 2. Based on the results, the nanocomposite films show lower water absorption compared to the pure blend film control (nbf0). About 5% reduction in total water absorption is achieved for the sample nbf1, whereas for the samples nbf3 and nbf5 the water absorption reduction is about 9% and 13%, respectively.
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It is believed that the existence of the starch and/or PE-g-MA lead to the water absorption sensitivity in the starch-based materials (Chiu et al., 2009; Sabetzadeh et al., 2014). In this work, because of the constant TPS (15 wt. %) and PE-g-MA (3wt. %) contents in all formulations, the observed differences in water absorption among the samples can be attributed to the presence of nanoclay. It follows that incorporation of nanoclay and increasing its content would improve the water resistance of the prepared films. This result can be associated with the finer dispersion of nanoclay and formation of the partial exfoliated structure within the matrix, as the nanoclay content increases. It also should be noted that even though the probable microvoids between LDPE and LLDPE would enhance water diffusion inside the samples (Sabetzadeh et al., 2015), the tortuous path constructed by the dispersed nanoclay acts as a barrier for diffusion of water molecules (Chiu et al., 2009; Manjunath et al., 2014). In addition, it has been found that conversion of the interior nanoclay surface from hydrophilic to hydrophobic; owing to alkylammonium ion exchange is responsible for decreasing the water absorption of the nanoclay-based composites. Similar results have also been reported by other workers (Cyras, Manfredi, Ton-That & Vázquez, 2008; Pérez, Alvarez, Mondragón & Vázquez, 2008). 4. Conclusions
In this work, a series of nanocomposite films based on the LDPE/LLDPE/TPS blend by varying nanoclay content of 1-5 phr in the presence of PE-g-MA are prepared in a twin screw extruder followed by an extrusion film blowing process. It has been found that delamination of nanooclay platelets is more effective with higher nanoclay content because of the strong interactions between the blend matrix and clay nanoparticles, as verified by XRD and TEM analyses. On the basis of SEM micrographs, the nanoclay with exfoliated platelets along with PE-g-MA compatibilizer has tremendous effect on morphology and TPS domain size within PE matrices. This result reflects the tensile strength, tear resistance as well as the impact strength of the films whereby, the nanocomposite films attain higher performance over the counterpart conventional blend film. The formulations with 5phr nanoclay content possess the highest mechanical properties among the others which satisfy the required packaging properties, as specified in ASTM D4635. The best clay dispersion degree inside the matrix is responsible to achieve the desired optical transparency and lower surface roughness which are attractive for packaging applications. It is believed that the LDPE/LLDPE/TPS nanocomposite films prepared 9 Page 9 of 19
here, are environmentally friendly materials which is believed to be bioassimilated in the environment after usage.
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McGlashan, S. A., & Halley, P. J. (2003). Preparation and characterisation of biodegradable starch-based nanocomposite materials. Polymer International, 52(11), 1767-1773. Mir, S., Yasin, T., Halley, P. J., Siddiqi, H. M., Ozdemir, O., & Nguyen, A. (2013). Thermal and rheological effects of sepiolite in linear low-density polyethylene/starch blend. Journal of Applied Polymer Science, 127(2), 1330-1337. Miyagawa, H., & Drzal, L. T. (2004). The effect of chemical modification on the fracture toughness of montmorillonite clay/epoxy nanocomposites. Journal of Adhesion Science and Technology, 18(13), 1571-1588. Mohanty, S., & Nayak, S. (2009). Starch based biodegradable PBAT nanocomposites: Effect of starch modification on mechanical, thermal, morphological and biodegradability behavior. International Journal of Plastics Technology, 13(2), 163-185. Mook Choi, W., Wan Kim, T., Ok Park, O., Keun Chang, Y., & Woo Lee, J. (2003). Preparation and characterization of poly(hydroxybutyrate-co-hydroxyvalerate)–organoclay nanocomposites. Journal of Applied Polymer Science, 90(2), 525-529.
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Mortazavi, S., Ghasemi, I., & Oromiehie, A. (2013). Effect of phase inversion on the physical and mechanical properties of low density polyethylene/thermoplastic starch. Polymer Testing, 32(3), 482-491. Mortazavi, S., Ghasemi, I., & Oromiehie, A. (2014). Morphological and rheological properties of (low-density polyethylene)/thermoplastic starch blend: investigation of the role of high elastic network. Journal of Vinyl and Additive Technology, 20(4), 250-259.
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Pérez, C. J., Alvarez, V. A., Mondragón, I., & Vázquez, A. (2008). Water uptake behavior of layered silicate/starch–polycaprolactone blend nanocomposites. Polym. Int, 57(2), 247-253.
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Prachayawarakorn, J., Sangnitidej, P., & Boonpasith, P. (2010). Properties of thermoplastic rice starch composites reinforced by cotton fiber or low-density polyethylene. Carbohydrate Polymers, 81(2), 425-433.
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Sabetzadeh, M., Bagheri, R., & Masoomi, M. (2012). Effect of corn starch content in thermoplastic starch/low-density polyethylene blends on their mechanical and flow properties. Journal of Applied Polymer Science, 126(S1), E63-E69.
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Sabetzadeh, M., Bagheri, R., & Masoomi, M. (2014). Effect of organomodified montmorillonite concentration on tensile and flow properties of low-density polyethylene–thermoplastic corn starch blends. Journal of Thermoplastic Composite Materials, 27(8), 1022-1036.
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Sabetzadeh, M., Bagheri, R., & Masoomi, M. (2015). Study on Ternary Low Density Polyethylene/Linear Low Density Polyethylene/Thermoplastic Starch Blend Films. Carbohydrate Polymers, 119(0), 126-133. Samper-Madrigal, M. D., Fenollar, O., Dominici, F., Balart, R., & Kenny, J. M. (2015). The effect of sepiolite on the compatibilization of polyethylene–thermoplastic starch blends for environmentally friendly films. Journal of Materials Science, 50(2), 863-872. Sánchez-Valdés, S., Martínez Colunga, J. G., López-Quintanilla, M. L., Yañez Flores, I., GarcíaSalazar, M. L., & González Cantu, C. (2008). Preparation and UV weathering of polyethylene nanocomposites. Polymer Bulletin, 60(6), 829-836. Schlemmer, D., Angélica, R. S., & Sales, M. J. A. (2010). Morphological and thermomechanical characterization of thermoplastic starch/montmorillonite nanocomposites. Composite Structures, 92(9), 2066-2070. Sharif, A., Aalaie, J., Shariatpanahi, H., Hosseinkhanli, H., & Khoshniyat, A. (2011). Study on the structure and properties of nanocomposites based on high-density polyethylene/starch blends. J. Polym. Res, 18(6), 1955-1969. Sharif, A., Aalaie, J., Shariatpanahi, H., Hosseinkhanli, H., & Khoshniyat, A. (2013). Fabrication of a novel polyethylene/starch blend through mediation of a high-energy ball milling process: Mechanical properties and formation mechanism. Journal of Applied Polymer Science, 128(1), 145-152. 12 Page 12 of 19
Taghizadeh, A., Sarazin, P., & Favis, B. (2013). High molecular weight plasticizers in thermoplastic starch/polyethylene blends. Journal of Materials Science, 48(4), 1799-1811. Taguet, A., Bureau, M. N., Huneault, M. A., & Favis, B. D. (2014). Toughening mechanisms in interfacially modified HDPE/thermoplastic starch blends. Carbohydrate Polymers, 114(0), 222229.
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Tang, X., Alavi, S., & Herald, T. J. (2008). Effects of plasticizers on the structure and properties of starch–clay nanocomposite films. Carbohydrate Polymers, 74(3), 552-558.
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Thipmanee, R., & Sane, A. (2012). Effect of zeolite 5A on compatibility and properties of linear low-density polyethylene/thermoplastic starch blend. Journal of Applied Polymer Science, 126(S1), E252-E259.
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Wang, K., Chen, L., Wu, J., Toh, M. L., He, C., & Yee, A. F. (2005). Epoxy Nanocomposites with Highly Exfoliated Clay: Mechanical Properties and Fracture Mechanisms. Macromolecules, 38(3), 788-800.
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Yasin, T., Nisar, M., Shafiq, M., Nho, Y.-C., & Ahmad, R. (2013). Influence of sepiolite and electron beam irradiation on the structural and physicochemical properties of polyethylene/starch nanocomposites. Polymer Composites, 34(3), 408-416.
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Zeng, Q. H., Yu, A. B., Lu, G. Q., & Paul, D. R. (2005). Clay-Based Polymer Nanocomposites: Research and Commercial Development. Journal of Nanoscience and Nanotechnology, 5(10), 1574-1592.
Fig. 1 XRD patterns of the pristine nanoclay (●), nbf1 (■), nbf3 (♦) and nbf5 (▲). Fig. 2 Typical TEM image of the sample nbf5.
Fig. 3 SEM micrographs of the LDPE/LLDPE/TPS nanocomposite films: (a) nbf1, (b) nbf3 and (c) nbf5; magnification of 10000×. Fig. 4 (a) Ultimate tensile strength and (b) elongation at break percent of the LDPE/LLDPE/TPS films with different nanoclay loading for machine (♦) and transverse (■) directions. Fig. 5 Elmendorf tear resistance of the LDPE/LLDPE/TPS films as a function of nanoclay content: machine direction (♦), transverse direction (■). Fig. 6 Variation in falling dart impact strength of the LDPE/LLDPE/TPS films with increasing the nanoclay content.
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Table 1 Optical transparency and surface roughness of the prepared films. Surface roughness (nm) Ra
Rq
nbf0
68
74
78
16.1
24.7
nbf1
51
55
58
71.8
85.2
nbf3
58
63
68
52.1
nbf5
65
72
76
24.2
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Sample Light transmission percent at different wavelength (nm) code 400 600 800
64.4
Table 2 Water absorption percent of the prepared films. nbf3
0
0.0
0.0
0.0
3
7.0
6.5
6.0
6
9.5
8.3
7.7
9
10.2
9.5
15 18 21 24 27 30 a
0.0 5.4 7.0
9.0
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nbf1
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nbf0
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Time (daysa)
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8.5
10.6
10.0
9.4
9.0
11.0
10.1
10.0
9.5
11.4
10.5
10.2
9.7
11.6
11.0
10.7
10.0
11.9
11.3
10.9
10.4
12.0
11.5
11.0
10.5
12.0
11.5
11.0
10.5
a day=24h
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S0144-8617(15)01235-7 http://dx.doi.org/doi:10.1016/j.carbpol.2015.12.057 CARP 10655
To appear in: Received date: Revised date: Accepted date:
12-10-2015 9-12-2015 22-12-2015
Please cite this article as: Sabetzadeh, M., Bagheri, R., and Masoomi, M.,Effect of Nanoclay on the Properties of Low Density Polyethylene/Linear Low Density Polyethylene/Thermoplastic Starch Blend Films, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.12.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights: Higher nanoclay content improves platelet delamination in LDPE/LLDPE/TPS blends. Both nanoclay and PE-g-MA reduce TPS domain size and deform their particles. LDPE/LLDPE/TPS nanocomposites show higher performance than the pure blends.
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LDPE/LLDPE/TPS nanocomposite films are environmentally friendly materials.
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LDPE/LLDPE/TPS blends with 5phr nanoclay satisfy required packaging properties.
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Effect of Nanoclay on the Properties of Low Density Polyethylene/Linear Low Density Polyethylene/Thermoplastic Starch Blend Films
Maryam Sabetzadeh, Rouhollah Bagheri*[email protected], Mahmood Masoomi
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Chemical Engineering Department, Polymer Group, Isfahan University of Technology, Isfahan 84156 83111, Iran
Abstract
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Tel.: 98 31 3391 5613; fax: 98 31 3391 2677.
The aim of this work is to study effect of nanoclay (Cloisite®15A) on morphology and properties of low-density polyethylene/linear low-density polyethylene/thermoplastic starch (LDPE/LLDPE/TPS) blend films. LDPE/LLDPE blend (70/30 wt/wt) containing 15 wt.% TPS in the presence of PE-grafted maleic anhydride (PE-g-MA, 3wt.%) with 1, 3 and 5 phr of nanoclay are compounded in a twin-screw extruder and then film blown using a blowing machine. Nanocomposites with intercalated structures are obtained, based on the X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. However, some exfoliated single platelets in the samples are also observable. Scanning electron microscopic (SEM) images confirm the ability of both exfoliated nanoclay and PE-g-MA to reduce the size of TPS domains and deform their particles within the PE matrices. As the nanoclay content increases from 1 to 5phr, the tensile strength, tear resistance and impact strength of the films increase, whereas a slight decrease in the elongation at break is observed. The film samples with 5phr nanoclay possess the required packaging properties, as specified by ASTM D4635. These films provide desired 1 Page 1 of 19
optical transparency and surface roughness which are more attractive for packaging applications. Keywords
LDPE, LLDPE, Thermoplastic starch, Clay, Nanocomposite, Film.
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1. Introduction
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Low density polyethylene/linear low density polyethylene (LDPE/LLDPE) blend films with a combination of good processability and superior mechanical properties have attracted much attention in packaging applications (Hemati & Garmabi, 2011). However, raising the problem of environmental pollution related to the accumulation of plastic packaging wastes has caused increasing interest in the use of polymers from renewable resources (Liu et al., 2014; Mortazavi, Ghasemi & Oromiehie, 2014). Among them, starch is a natural, renewable and inexpensive polymer which can be added to polyethylene (PE) to produce environmentally friendly materials (Kahar, Ismail & Othman, 2012; Sabetzadeh, Bagheri & Masoomi, 2012). It is known that native starch-PE materials without a plasticizer and a modifying agent show poor processability and inferior mechanical performance, leading to limit their applications (Sharif, Aalaie, Shariatpanahi, Hosseinkhanli & Khoshniyat, 2013; Thipmanee & Sane, 2012). Starch can be modified in the presence of a plasticizer (such as glycerol) at high temperatures (i.e. TPS) to be processed by conventional plastic-forming equipments and also disperse fairly uniform in the polymer matrix, which is critical for mechanical properties. Several studies have been conducted on PE blends obtained with addition of TPS (Guzmán & Murillo, 2015; Mortazavi, Ghasemi & Oromiehie, 2013; Taghizadeh, Sarazin & Favis, 2013). Even so, these materials have a relatively high interfacial tension arises from the incompatibility between the non-polar PE and the high polar TPS, resulting in poor mechanical properties (Alidadi-Shamsabadi, Behzad, Bagheri & NariNasrabadi, 2015; Cerclé, Sarazin & Favis, 2013). In order to overcome the incompatibility issue and as a consequence improvement in mechanical properties, addition of a suitable interfacial modifier containing reactive groups [e.g. polyethylene-grafted maleic anhydride (PE-g-MA)] has been suggested (Prachayawarakorn, Sangnitidej & Boonpasith, 2010; Taguet, Bureau, Huneault & Favis, 2014). Anyway, the problem of poor mechanical properties in the blends could not be properly overcome (Inceoglu & Menceloglu, 2013). It should be underlined that inclusion of a small amount of nanofiller in a polymer blend provides a composite material with improved mechanical properties (Kim & Cha, 2014). Among inorganic compounds, more attention has recently been paid to clay minerals, especially montmorillonite (MMT) in the research area of nanocomposites, attributed to their environmentally friendly nature, small particle size, high surface area and intercalation properties. The surface of the MMT is modified with certain organic modifiers (e.g. alkylammonium cation) to provide it more compatible with the non-polar polymers. It is well established that surface treatment and the blending process can significantly facilitate 2 Page 2 of 19
dispersion of clay nanoparticles (Kampeerapappun, Aht-ong, Pentrakoon & Srikulkit, 2007; Ludueña, Vázquez & Alvarez, 2013; Schlemmer, Angélica & Sales, 2010).
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In the last few years, increased interest has appeared in PE/starch blends containing clay nanoparticles. Metallocene - catalyzed polyethylene (m-PE)/TPS nanocomposites has been prepared with incorporation of clay reinforced nanocompatibilizer (Chiu, Lai & Ti, 2009). Moreover, nanocomposites based on high-density polyethylene (HDPE)/TPS blends have been studied. It has been demonstrated that addition of nanoclay is in favor of developing nanocomposites with better performance than the native blends (Sharif, Aalaie, Shariatpanahi, Hosseinkhanli & Khoshniyat, 2011). Besides above, low-density polyethylene (LDPE)/starch nanocomposites have been investigated. The most considerable improvement in all properties is achieved in the presence of both clay and compatibilizer (Inceoglu et al., 2013; Manjunath & Sailaja, 2014). Our studies also suggest that LDPE/TPS blends containing nanoclay have desired mechanical properties that would be appropriate for packaging applications (Sabetzadeh, Bagheri & Masoomi, 2014). Recently, HDPE/TPS nanocomposite films have been developed using new compatibilizer systems based on the other clay minerals such as sepiolite (Mir, Yasin, Halley, Siddiqi, Ozdemir & Nguyen, 2013; Samper-Madrigal, Fenollar, Dominici, Balart & Kenny, 2015; Yasin, Nisar, Shafiq, Nho & Ahmad, 2013).
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To date, ternary LDPE/LLDPE/starch blends filled with nanoparticles have rarely been reported in the literature. The present work centers around the use of a nanoclay in a compatibilized LDPE/LLDPE blend film containing 15 wt.% TPS which has been demonstrated to be suitable for packaging applications (Sabetzadeh, Bagheri & Masoomi, 2015). The detailed effect of nanoclay addition on the morphology, mechanical properties, film characteristics such as optical transparency and surface roughness as well as water absorption capacity is investigated.
2. Experimental 2.1. Materials
Low density polyethylene (LDPE) with a density of 0.921 g/cm3, melt flow index (MFI) of 0.75 g/10min and linear low density polyethylene (LLDPE) with a density of 0.920 g/cm3, MFI of 0.9 g/10min (at 190oC, 2.16 kg load) were both film grades and purchased from Petrochemical Commercial Company, Iran. The unmodified corn starch (30 wt.% amylose and 70 wt.% amylopectin) was supplied by Glucosan Company, Iran. Glycerol (99.5% purity) was reagent grade from Hansa Group AG, Germany. Polyethylene grafted-maleic anhydride (PE-g-MA, 1% grafted) was obtained from Pluss Polymers Co, Ltd (India) and used as a compatibilizer. The modified montmorillonite (Cloisite®15A) was obtained from Southern Clay Products, Inc. (USA).
2.2. Sample preparation
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All components were dried in a vacuum oven at 80oC for 24h prior to further treatments. Thermoplastic starch (TPS) was prepared by melt mixing the homogenous compound of native starch and 35 wt% glycerol in a Haake internal mixer (with a volumetric chamber capacity of 300 cm3) at 140oC with rotor speed of 60 rpm for 8 min.
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LDPE/LLDPE (70/30 w/w) blend with 15 wt.% prepared TPS in the presence of 3 wt.% PE-g-MA (Sabetzadeh et al., 2015) were melt-mixed with various contents of nanoclay (1, 3 and 5phr) using a twin screw extruder (model ZSK25, Germany). The extruder had screw diameter (d) of 25mm and the length to diameter ratio (L/D) was 40. The temperature profile along the six heating zones of the extruder barrel was 145-180oC (from feed zone to die) and the screw speed was set at 150 rpm. The prepared compounds were emerged in the form of continuous strands through the die. The strands were cooled using water trough and pelletized. The pellets were then blown into 45μm thick films using Dr Collin single screw extruder (model E45M) with L/D ratio of 25, containing nine zones. The barrel temperature range was 155-170oC and the screw speed was set at 70 rpm. The prepared films designated as nbf0, nbf1, nbf3 and nbf5; containing 0, 1, 3 and 5phr nanoclay, respectively.
2.3. Characterization and measurements
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X-ray diffraction of the film samples was conducted using a Brucker D8 Advance X-ray diffractometer (Brucker, Germany) with Cu Kα radiation (λ=0.154 nm) at a voltage of 40 kV and 30 mA. Scattered radiations were detected in the range of diffraction angle 2θ=2–10° with the scan rate of 1°/min at room temperature. The basal spacing of the nanoclay ,d, was calculated using the Bragg equation, d = λ 2 sin (πθ / 180 ) , where θ is the diffraction angle and λ is the wavelength.
A typical transmission electron microscopy (TEM) image was captured using a Phillips CM200 transmission electron microscope operating at an acceleration voltage of 200 kV. Ultrathin cut (80 nm) from the film sample, using a diamond knife under cryogenic conditions at -100oC was performed. Scanning electron microscope (SEM, TESCAN model Vegall), operating at an accelerating voltage of 15kV was used to study phase morphology of the film samples. Before the test, the film samples were cryogenically fractured in the liquid nitrogen and then sputter coated with a thin layer of gold to avoid electrostatic charging and poor resolution during examination. SEM micrographs of the fracture surfaces were taken at the magnification of 10000×. Tensile properties, such as ultimate tensile strength and elongation at break percentage of the film samples were measured using a Universal Testing Machine Zwick/Roell (model Z 2.5/TH1S), according to ASTM D882. Strip form specimens were cut from the films and strained at a crosshead speed of 200 mm/min at room temperature for both machine and transverse directions. The grip separation was set at 50mm. Tear resistance of the films was also measured in both machine and transverse directions according to ASTM D 1922 standard test method, using a 4 Page 4 of 19
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Ceast ED 300 tear tester (Elmendorf type). The average force required to propagate a tear through a specified length of the film with respect to the thickness determines the tear resistance. In addition, a Ceast falling dart impact test equipment (model 9340) was used to measure the impact strength of the strip shaped film samples, according to ASTM D1709 method at room temperature. The energy that causes films to fail under the specified conditions of impact of a free-falling dart determines the impact strength. For all mechanical tests, at least five replicates were examined for each sample and the average values were determined.
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To evaluate some film characteristics, the optical transparency was determined by the light transmission rate through the films at wavelengths from 400 to 800 nm, using a UV-vis spectrophotometer (Jasco, V550 Series, Japan). Moreover, the surface roughness of the films was examined by atomic force microscopy (AFM, Brucker, Germany) at room temperature. Contact mode was used and the scanning range was 42µm × 42µm in at least five areas for each film sample to calculate the average (Ra) and root mean square roughness values (Rq). These are the useful tools to get information about the particle size and distribution of the clay within the blend matrix.
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In order to study water absorption behavior of the film samples, they were immersed in the distilled water, according to the ASTM D570 method at room temperature. The samples were taken out at regular time intervals and wiped to measure the water absorption percent. Before the test, the samples were dried at 80°C in a vacuum oven for 24h. Water absorption percent at any time (wt.%) was calculated according to wt (%) = [(w2 − w1 ) w1 ]×100 , where w1 and w2
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were the weight of dried sample and the sample after immersion at time t, respectively. 3. Results and discussion
3.1. Structure and morphology
X-ray diffraction patterns of the LDPE/LLDPE/TPS film samples (nbf1, nbf3 and nbf5) as well as the nanoclay (as the reference), in the region of 2θ=2-10° are represented in Fig.1. For pristine nanoclay, a characteristic diffraction peak around 2θ=3.3° is appeared, indicating d-spacing of 2.67nm. The diffraction peak of the sample nbf1 shifts slightly to the lower angle (2θ=2.95°, according to d-spacing of 2.99nm). This peak is also located at 2θ = 2.33° and 2θ=2.13°, corresponding to d-spacing of 3.79nm, and 4.14nm for the samples namely nbf3 and nbf5, respectively. Such increase between nanoclay platelets with increasing nanoclay concentration that leads to diminishing of the characteristic peak intensity can be taken as an indication of nanocomposite structures with good intercalation and/or partial exfoliation of nanoclay in the samples (Kampeerapappun et al., 2007). The XRD results suggest that the polymer chains enter into the nanoclay galleries and separate the platelets. By increasing the nanoclay content, the polymer – particle interactions increase and the stiffening effect of nanoclay intensifies which leads to increasing the melt viscosity of PE matrices. Therefore, more shearing force is applied, resulting the easier intercalation of the 5 Page 5 of 19
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polymer chains into the silicate layers and consequently, an increase in the d-spacing. This causes the clay platelets become disordered and the broader peaks appear in the XRD patterns, indicating intercalation with a good dispersion of nanoclay or some exfoliation in the matrices (McGlashan & Halley, 2003; Tang, Alavi & Herald, 2008). This is more evident in the samples nbf3 and nbf5 which have the similar diffraction profile. A typical TEM image of the sample nbf5 is also captured and shown in Fig.2. In this Figure, the dark lines represent nanoclay, the gray base corresponds to the PE matrices and TPS phase is seen brighter. There are some exfoliated clay platelets, visible as the single and disordered platelets in the sample. It seems that the nanoclay particles are preferentially dispersed within the PE matrices or located at the interface between LDPE-TPS and/or LLDPE-TPS. The obtained XRD and TEM results are similar to those reported previously in the case of PE/TPS blend nanocomposites (Sharif et al., 2011; Chiu et al., 2009).
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SEM micrographs of the fractured surfaces of the film samples (nbf1, nbf3 and nbf5) are captured with the magnification of 10000× and depicted in Fig. 3(a-c). Reaction of MA in PE-gMA with the hydroxyl groups of TPS imparts reduction of interfacial tension between PE matrices and TPS phase which causes a comparatively good dispersion of starch particles in PE matrices for all the samples. It should be explained that starch plasticized by glycerol is a partially miscible mixture of glycerol-rich and starch-rich phases. Low molecular weight glycerol could migrate to the interface and forms a glycerol layer between starch and PE phases, resulting decrease in the interfacial tension of the blend. It seems that PE-g-MA copolymer tends to react with the glycerol layer initially in compatibilized PE/TPS blends and is located at the interface between TPS–LDPE and/or TPS–LLDPE which enhances interfacial interactions in the blends (Sabetzadeh et al., 2015). In the present work, the results indicate that the TPS domains are evidently smaller and have the elongated shapes in the sample nbf5 compared to the other samples (Fig 3c). Here, it is reasonable to assume that the combined effect of PE-g-MA and nanoclay with exfoliated platelets leads to reduction of interfacial tension between PE matrices and TPS effectively and therefore, a more decrease in the starch particles size (Sharif et al., 2011). Based on both TEM and SEM images, Figs 2 and 3c, it can be concluded that the exfoliated nanoclay platelets is more desirable to reduce the size of TPS domains and deform their particles within the PE matrices, compared to the intercalated ones. These results are in accordance with those reported previously in the case of HDPE/TPS nanocomposites (Sharif et al., 2011) and poly (butylene adipate-co-terephthalate)/TPS/clay nanomaterials (Mohanty & Nayak, 2009).
3.2. Mechanical properties Tensile properties of the LDPE/LLDPE/TPS nanocomposite films such as ultimate tensile strength and elongation at break versus nanoclay loading for both machine and transverse directions are represented in Figs 4a and 4b, respectively. For all samples, tensile strength in the machine direction (MD) is higher than that in the transverse direction (TD). The elongation at break has different tendency: it is lower in the machine direction than in the transverse direction. This 6 Page 6 of 19
may be due to the higher degree of orientation that polymer chains undergo inside and outside the die in MD direction. However, in both directions with increasing the nanoclay loading the tensile strength increases; whereas the elongation at break decreases slightly. Figure 5 indicates the tear resistance of the prepared films for both MD and TD directions. It is clear that the tear resistance in TD direction exceeds that in the MD direction. In addition, falling dart impact strength of the films is depicted in Fig 6. Based on the Figs 5 and 6, as the nanoclay content increases, both the tear resistance and impact strength increase.
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It is reported that that with addition of LLDPE into LDPE, tensile properties of the resultant blend film increase, because of the higher performance of LLDPE than LDPE (Lu & Sue, 2002). However, the presence of starch in the blend caused level off the tensile properties of the final materials (Sabetzadeh et al., 2015). In this work, the results show that the mechanical properties of the LDPE/LLDPE/TPS nanocomposites are strongly dependent on nanoclay content and dispersion status. In fact, incorporation of nanoclay as a reinforcing agent has a noticeable effect on the LDPE/LLDPE/TPS blend films which is more pronounced in the samples with higher nanoclay loading. In particular, 35% and 45% increase in tensile strength for the sample nbf5 compared to the pure blend film control (i.e. nbf0) are achieved in MD and TD directions, respectively. In addition, for this sample the tear resistance increases by 54 and 39 % in MD and TD directions, respectively and 38% increase in impact strength is obtained in comparison with the control. However, insignificant decrease in the elongation at break is attained, as the nanoclay content increases.
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MA inclusion, as a result of finer dispersion of nanoclay induced by the compatibilizer (Sharif et al., 2011).
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It is worthy to note that with increasing the nanoclay content, decrease in elongation at break percent of the samples is not remarkable. This is a significant achievement in the present work and could be due to creation of the thicker polymer/nanoclay interphase with better properties in higher nanoclay loadings, which increases the fracture toughness and delay the tension failure of the samples. However, the stiffening effect of nanoparticles in the polymer matrices which causes restricted mobility of the polymer chains explains the decrease in elongation at break percent of the samples (Sabetzadeh et al., 2014). The obtained results of the mechanical properties due to the effect of nanoclay are in agreement with those reported by other workers for PE/starch nanocomposites (Manjunath et al., 2014; Sharif et al., 2011; Yasin et al., 2013).
3.3. Optical transparency and surface roughness
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Table 1 represents the optical transparency of the studied films, which was examined by acquiring the light transmission percent at selected wavelengths from 400 to 800 nm. The light transmission percent of the nanocomposite films are lower than that of pure blend film control (i.e. nbf0). As the nanoclay content increases, the percent of light transmission increases. This may be due to the homogenous dispersion of nanoclay particles inside the matrix and even smaller particle size than the wavelength of visible light (Hyun, Chong, Koo & Chung, 2003; Sánchez-Valdés, Martínez Colunga, López-Quintanilla, Yañez Flores, García-Salazar & González Cantu, 2008). This result confirms that the nanoclay has a good compatibility with the polymer blend matrix preventing the particle aggregation, thus decreases the irregularity and heterogeneity of the surface and the amount of light scattering to provide the transmittance of visible light through the nanocomposite films. Theoretically, it has been reported that the clay platelets with 1nm thickness less than the visible light wavelength do not inhibit light transmittance. Therefore, the transmittance of a nanocomposite film containing such welldistributed clay platelets should not be significantly changed (Liu et al., 2014; Zeng, Yu, Lu & Paul, 2005). In the present work, the results indicate that the film sample nbf5 with welldeveloped clay platelets have the similar light transmission percent to that of pure blend film control (nbf0). The obtained results are in good agreement with the results of optical transparency of the LDPE/starch nanocomposite films reported previously (Inceoglu et al., 2013). The good transparency of the nanocomposite films is an important property required for packaging applications. In addition, surface roughness data for the film samples are summarized in Table 1. The pure blend film control (nbf0) has a quite smooth surface, with an average roughness (Ra) of 16.1nm and root mean square roughness (Rq) of 24.7nm. For the nanocomposite films, with increase in the nanoclay content, the Ra and Rq values decrease continuously. The lowest Ra and Rq obtained for the sample nbf5 containing the highest nanoclay content is the evidence of more
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uniform nanoclay dispersion in this sample. The above results are in good agreement with morphological studies which discussed earlier.
3.4. Water absorption behavior
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Water absorption behavior of the film samples as a function of both the immersion time and nanoclay content are summarized in Table 2. Based on the results, the nanocomposite films show lower water absorption compared to the pure blend film control (nbf0). About 5% reduction in total water absorption is achieved for the sample nbf1, whereas for the samples nbf3 and nbf5 the water absorption reduction is about 9% and 13%, respectively.
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It is believed that the existence of the starch and/or PE-g-MA lead to the water absorption sensitivity in the starch-based materials (Chiu et al., 2009; Sabetzadeh et al., 2014). In this work, because of the constant TPS (15 wt. %) and PE-g-MA (3wt. %) contents in all formulations, the observed differences in water absorption among the samples can be attributed to the presence of nanoclay. It follows that incorporation of nanoclay and increasing its content would improve the water resistance of the prepared films. This result can be associated with the finer dispersion of nanoclay and formation of the partial exfoliated structure within the matrix, as the nanoclay content increases. It also should be noted that even though the probable microvoids between LDPE and LLDPE would enhance water diffusion inside the samples (Sabetzadeh et al., 2015), the tortuous path constructed by the dispersed nanoclay acts as a barrier for diffusion of water molecules (Chiu et al., 2009; Manjunath et al., 2014). In addition, it has been found that conversion of the interior nanoclay surface from hydrophilic to hydrophobic; owing to alkylammonium ion exchange is responsible for decreasing the water absorption of the nanoclay-based composites. Similar results have also been reported by other workers (Cyras, Manfredi, Ton-That & Vázquez, 2008; Pérez, Alvarez, Mondragón & Vázquez, 2008). 4. Conclusions
In this work, a series of nanocomposite films based on the LDPE/LLDPE/TPS blend by varying nanoclay content of 1-5 phr in the presence of PE-g-MA are prepared in a twin screw extruder followed by an extrusion film blowing process. It has been found that delamination of nanooclay platelets is more effective with higher nanoclay content because of the strong interactions between the blend matrix and clay nanoparticles, as verified by XRD and TEM analyses. On the basis of SEM micrographs, the nanoclay with exfoliated platelets along with PE-g-MA compatibilizer has tremendous effect on morphology and TPS domain size within PE matrices. This result reflects the tensile strength, tear resistance as well as the impact strength of the films whereby, the nanocomposite films attain higher performance over the counterpart conventional blend film. The formulations with 5phr nanoclay content possess the highest mechanical properties among the others which satisfy the required packaging properties, as specified in ASTM D4635. The best clay dispersion degree inside the matrix is responsible to achieve the desired optical transparency and lower surface roughness which are attractive for packaging applications. It is believed that the LDPE/LLDPE/TPS nanocomposite films prepared 9 Page 9 of 19
here, are environmentally friendly materials which is believed to be bioassimilated in the environment after usage.
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References:
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ASTM D 4635. standard specification for polyethylene films made from low-density polyethylene for general use and packaging applications.
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Cerclé, C., Sarazin, P., & Favis, B. D. (2013). High performance polyethylene/thermoplastic starch blends through controlled emulsification phenomena. Carbohydrate Polymers, 92(1), 138-148.
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Chiu, F.-C., Lai, S.-M., & Ti, K.-T. (2009). Characterization and comparison of metallocenecatalyzed polyethylene/thermoplastic starch blends and nanocomposites. Polym. Test, 28(3), 243-250.
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Cyras, V. P., Manfredi, L. B., Ton-That, M.-T., & Vázquez, A. (2008). Physical and mechanical properties of thermoplastic starch/montmorillonite nanocomposite films. Carbohydrate Polymers, 73(1), 55-63. Guzmán, M., & Murillo, E. A. (2015). The properties of blends of maleic-anhydride-grafted polyethylene and thermoplastic starch using hyperbranched polyester polyol as a plasticizer. Polymer Engineering & Science, 55(11), 2526-2533. Hemati, F., & Garmabi, H. (2011). Compatibilised LDPE/LLDPE/nanoclay nanocomposites: I. Structural, mechanical, and thermal properties. The Canadian Journal of Chemical Engineering, 89(1), 187-196. Hyun, K., Chong, W., Koo, M., & Chung, I. J. (2003). Physical properties of polyethylene/silicate nanocomposite blown films. Journal of Applied Polymer Science, 89(8), 2131-2136. Inceoglu, F., & Menceloglu, Y. Z. (2013). Transparent low-density polyethylene/starch nanocomposite films. Journal of Applied Polymer Science, 129(4), 1907-1914. Kahar, A. W. M., Ismail, H., & Othman, N. (2012). Effects of polyethylene-grafted maleic anhydride as a compatibilizer on the morphology and tensile properties of (thermoplastic tapioca starch)/ (high-density polyethylene)/(natural rubber) blends. Journal of Vinyl and Additive Technology, 18(1), 65-70. 10 Page 10 of 19
Kampeerapappun, P., Aht-ong, D., Pentrakoon, D., & Srikulkit, K. (2007). Preparation of cassava starch/montmorillonite composite film. Carbohydrate Polymers, 67(2), 155-163. Kim, S. W., & Cha, S.-H. (2014). Thermal, mechanical, and gas barrier properties of ethylene– vinyl alcohol copolymer-based nanocomposites for food packaging films: Effects of nanoclay loading. Journal of Applied Polymer Science, 131(11), DOI: 10.1002/APP.40289.
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McGlashan, S. A., & Halley, P. J. (2003). Preparation and characterisation of biodegradable starch-based nanocomposite materials. Polymer International, 52(11), 1767-1773. Mir, S., Yasin, T., Halley, P. J., Siddiqi, H. M., Ozdemir, O., & Nguyen, A. (2013). Thermal and rheological effects of sepiolite in linear low-density polyethylene/starch blend. Journal of Applied Polymer Science, 127(2), 1330-1337. Miyagawa, H., & Drzal, L. T. (2004). The effect of chemical modification on the fracture toughness of montmorillonite clay/epoxy nanocomposites. Journal of Adhesion Science and Technology, 18(13), 1571-1588. Mohanty, S., & Nayak, S. (2009). Starch based biodegradable PBAT nanocomposites: Effect of starch modification on mechanical, thermal, morphological and biodegradability behavior. International Journal of Plastics Technology, 13(2), 163-185. Mook Choi, W., Wan Kim, T., Ok Park, O., Keun Chang, Y., & Woo Lee, J. (2003). Preparation and characterization of poly(hydroxybutyrate-co-hydroxyvalerate)–organoclay nanocomposites. Journal of Applied Polymer Science, 90(2), 525-529.
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Fig. 1 XRD patterns of the pristine nanoclay (●), nbf1 (■), nbf3 (♦) and nbf5 (▲). Fig. 2 Typical TEM image of the sample nbf5.
Fig. 3 SEM micrographs of the LDPE/LLDPE/TPS nanocomposite films: (a) nbf1, (b) nbf3 and (c) nbf5; magnification of 10000×. Fig. 4 (a) Ultimate tensile strength and (b) elongation at break percent of the LDPE/LLDPE/TPS films with different nanoclay loading for machine (♦) and transverse (■) directions. Fig. 5 Elmendorf tear resistance of the LDPE/LLDPE/TPS films as a function of nanoclay content: machine direction (♦), transverse direction (■). Fig. 6 Variation in falling dart impact strength of the LDPE/LLDPE/TPS films with increasing the nanoclay content.
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Table 1 Optical transparency and surface roughness of the prepared films. Surface roughness (nm) Ra
Rq
nbf0
68
74
78
16.1
24.7
nbf1
51
55
58
71.8
85.2
nbf3
58
63
68
52.1
nbf5
65
72
76
24.2
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Sample Light transmission percent at different wavelength (nm) code 400 600 800
64.4
Table 2 Water absorption percent of the prepared films. nbf3
0
0.0
0.0
0.0
3
7.0
6.5
6.0
6
9.5
8.3
7.7
9
10.2
9.5
15 18 21 24 27 30 a
0.0 5.4 7.0
9.0
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Time (daysa)
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8.5
10.6
10.0
9.4
9.0
11.0
10.1
10.0
9.5
11.4
10.5
10.2
9.7
11.6
11.0
10.7
10.0
11.9
11.3
10.9
10.4
12.0
11.5
11.0
10.5
12.0
11.5
11.0
10.5
a day=24h
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