Influence of cellulose nanocrystal on strength and properties of low density polyethylene and thermoplastic starch composites

Influence of cellulose nanocrystal on strength and properties of low density polyethylene and thermoplastic starch composites

Industrial Crops & Products 115 (2018) 298–305 Contents lists available at ScienceDirect Industrial Crops & Products journal homepage: www.elsevier...

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Industrial Crops & Products 115 (2018) 298–305

Contents lists available at ScienceDirect

Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop

Influence of cellulose nanocrystal on strength and properties of low density polyethylene and thermoplastic starch composites

T



Narges Graya, Yahya Hamzeha, , Alireza Kabooranib, Ali Abdulkhania a

Department of Wood and Paper Sciences and Technology, Faculty of Natural Resources, University of Tehran, Shahid Chamran Blvd., 31585-4314, Karaj, Iran Département des Sciences du Bois et de la Forêt, Faculté de Foresterie, de Géographie et de Géomatique, Université Laval, 2425, Rue de la Terrasse, Québec, QC, G1V 0A6, Canada b

A R T I C L E I N F O

A B S T R A C T

Keywords: Thermoplastic starch (TPS) Low density polyethylene (LDPE) Cellulose nanocrystals (CNC) Sustainable Composite

Starch is a renewable and readily available material. Adding starch to low density polyethylene (LDPE) is a sustainable way to reduce the dependency on petroleum based polymers. In this study we investigated the strength and barrier performance of low density polyethylene (LDPE)/thermoplastic starch (TPS) nanocomposites reinforced with cellulose nanocrystals (CNC). In order to assure well dispersion of CNC in nanocomposites, initially CNC was added to TPS and then reinforced TPS were blended in extruder with LDPE at various loading levels. Mechanical properties, glass transition temperature (Tg) and melting point (Tm), moisture absorption and barrier properties of the nanocomposites were studied. All mechanical properties, including tensile strength, modulus of elasticity (MOE) and hardness were considerably improved by CNC. Tg and Tm of the nanocomposites were higher in comparison to CNC free nanocomposites. Water absorption showed a significant decrease as a result of addition of CNC to LDPE/TPS blends. The values of water vapor permeability coefficient (WVP) and water vapor transmission rate (WVTR) were reduced by adding CNC, meaning that CNC considerably improved barrier properties of LDPE/TPS composites. Adding 1% CNC to LDPE/TPS blends was the optimal level of CNC loading leading to the highest improvement in the strength and barrier performance of LDPE/TPS blends and satisfied very well required standard tensile strength for extruded and molded LDPE.

1. Introduction In order to reduce worldwide environmental pollution caused by non-biodegradable synthetic polymers, development of ecologically safe polymeric materials continues to receive great attention (Shen et al., 2010; Rhim et al., 2013; Heredia-Guerrero et al., 2017). Low density polyethylene (LDPE) as a low-cost polymer with special properties such as good process-ability and high strength, is used extensively in numerous applications (Sabetzadeh et al., 2015). However, LDPE is a non-degradable polymer and increases accumulation of plastic wastes and environmental pollution (Liu et al., 2013; Mortazavi et al., 2014). Blending of conventional synthetic polymers with biodegradable natural polymers is an economic and versatile approach to enhance the biodegradability of petroleum-based polymers, providing partially biodegradable and sustainable plastics with a wide range of desirable properties. Among the biodegradable polymers, starch is a natural, renewable, low-cost and abundant biopolymer that is applied more and more for the preparation of biodegradable blends and composites (Belhassen et al., 2014; Kaushik and Kaur, 2016). In the case of LDPE, the melt blending of LDPE with a biopolymer such as thermoplastic ⁎

Corresponding author. E-mail address: [email protected] (Y. Hamzeh).

https://doi.org/10.1016/j.indcrop.2018.02.017 Received 6 November 2017; Accepted 6 February 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.

starch (TPS) could make it partially biodegradable (Psomiadou et al., 1997). It is believed that biodegradation of TPS in the mixture with LDPE creates suitable conditions for the LDPE chains attack by microorganisms (Dave et al., 1997; Nguyen et al., 2016). Despite the interesting and promising potential uses of starch to replace conventional polymers, its application faces various drawbacks. The native starch itself cannot be processed with traditional thermoplastic processing technologies because starch melting temperature (Tm) is generally higher than its decomposition temperature (Liu et al., 2008; Mohammadi Nafchi et al., 2013). This problem can be overcome by addition suitable plasticizers to the native starch (García et al., 2011; Wang et al., 2014; Ivanič et al., 2017). So-called thermostatic starch (TPS) is produced by mixing native starch with plasticizer at a temperature above the gelatinization temperature of starch, typically in the 65–90 °C. The plasticizers reduce the intermolecular forces by increasing the chain mobility and improving the flexibility and extensibility of the biopolymer (Parra et al., 2004; Prachayawarakorn et al., 2010; Rico et al., 2016). Challenges of TPS based bio-plastics are related to their water sensitivity and poor mechanical properties, which limit their uses in many

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2.2. Thermoplastic starch (TPS) preparation

applications (Liu et al., 2009; Akrami et al., 2016). TPS is very hygroscopic and its mechanical properties and dimensional stability are strongly affected by moisture since water is a plasticizer for TPS. In addition, in the presence of moisture, the amorphous TPS tends to reform its hydrogen bonds leading to recrystallization and in turn to material embrittlement (García et al., 2011). Many attempts have used starch to produce semi-biodegradable materials with superior mechanical properties, mainly by blending with other compounds such as synthetic polymers (Oromiehie et al., 2013; Peres et al., 2016), reinforcing with natural fibers (Ma et al., 2005), cellulose nanofibers (Jonoobi et al., 2010; Babaee et al., 2015; Karimi et al., 2016), nanowhiskers of cellulose (Angles and Dufresne, 2001) and microcrystalline cellulose (Rico et al., 2016). Although, addition of petroleum based polymers to TPS reduces the water sensitivity of TPS and improves mechanical properties of TPS, performance of LDPE polymer and TPS composites is not satisfying. Particularly at high loading of TPS, a drastic reduction in the mechanical properties has been reported, mainly due to poor phase compatibility and adhesion between the hydrophilic starch and the hydrophobic synthetic polymer (St. Pierre et al., 1997; Sabetzadeh et al., 2012). In this regards, addition of different compatibilizers, i.e., maleic anhydridegrafted-polyethylene (MAPE) and vinyltrimethoxy silane (VTMS) could improve interfacial adhesion and consequently the final product properties (Matzinos et al., 2001; Prachayawarakorn and Pomdage, 2014). However, added compatilizers like VTMS are toxic, and mechanical properties remained still low (Pushpadass et al., 2010). Reinforcing TPS with nano biomaterials is promising alternative to increase mechanical performance, while preserving the sustainable character of final composites (Arun et al., 2012; Alidadi-Shamsabadi et al., 2015). Cellulose nanocrystal (CNC) which is produced by acidic hydrolysis of amorphous parts of cellulose shows an excellent reinforcing potential for wide range of polymers (Kallel et al., 2016; Zhuo et al., 2017). The presence of the well-dispersed cellulose crystallites in various polymer matrices even at low concentrations enhances mechanical strength performance of final product, especially stiffness (Jonoobi et al., 2010; Kaboorani et al., 2016; Kaboorani et al., 2017). Today, cellulose nanocrystal (CNC) as sustainable, odorless, white, fine, and crystalline dry powder that is widely available in quantities suitable for laboratory research and pilot-scale development (Zhang et al., 2013; Li et al., 2013). In this study we examined the preparation and characterization of LDPE/TPS based nanocomposites with various TPS content from 30 to 50% (wt.%) and its effects on the nanocomposites properties. In order to overcome the weakness of LDPE/TPS composites, addition of CNC in 1% and 2% to the composites was studied and microstructural, mechanical, barrier properties, moisture absorption and thermal characterizations of LDPE/TPS composites were also investigated.

In the first phase and to provide the fine dispersion of CNC in starch matrix, a homogenous suspension of CNC (2% wt.) was prepared by mixing CNC (5.5% suspension) with distilled water and the CNC was dispersed for 20 min at ambient temperature using sonication (SONICA 2200EP S3, Italy). Then, in order to transfer the granular starch into TPS containing CNC, for each formulation a given amount of native corn starch and glycerol (33% wt. of starch) were added to given amount of prepared suspension of CNC. The blends were heated and kept at 80 °C for 30 min while stirring (450 RPM), until starch plasticization occurred. The prepared TPS were transformed to LDPE bags and stored overnight. Then, TPS were poured into non-stick plates and dried in an oven under air circulation at 40 °C for 24 h. The resulted TPS containing CNC was palletized to obtain 40–60 mesh pellets. In the second phase of sample preparation, the palletized TPS containing various amounts of CNC (from 0 to 2 wt.% of final composites) was mixed with a LDPE in a granulated form and melt blended using a twin screw extruder (model: TSE 20; Brabender, Germany). The temperature profile along the six heating zones of the extruder from feeder to die was set at 130–140–145–150–155–160 °C at screw speed of 60 RPM. The extruded materials were emerged in the form of continuous strands through the die. The strands air-cooled and then palletized by a pelletizer. Finally, the pellets were dried at 40 °C for 24 h in an oven to remove the moisture and then cooled to room temperature and transferred to LDPE bags and sealed to avoid moisture adsorption before composites fabrication. TPS and CNC contents in the composites varied from 30 to 50 wt.%, and 0–2 wt.%, respectively (Table 1). Combination of hand layup and compression molding technique was used for fabrication of composites with thickness of 2 mm from different formulations (Table 1). The pellets were carefully distributed in all locations of the mold, and allowed to pre-melt at 140 °C in a flat mold for 10 min without pressing. This was followed by hot pressing at 140 °C under pressure of 5 MPa for 10 min. Finally, the samples were cooled to room temperature in a cold press under pressure of 2 MPa.

2.3. Characterization 2.3.1. Tensile strength Specimens of 80 mm × 8 mm × 2 mm dimensions were cut from the compression molded composites for each formulation. At least 10 specimens from each formulation have been made. Measurements of tensile strength were performed according to ASTM 638-03 test method. The tests were performed using an Instron 4486 testing machine equipped with a computerized data acquisition system, in standard conditions of 23 ± 2 °C and 50 ± 5% relative humidity. Measurements were done at 20 mm/min crosshead speed.

2. Experimental Table 1 Sample codes of the LDPE/TPS and the LDPE/TPS/CNC composites.

2.1. Materials Glycerol as a plasticizer was purchased from Hansa Company (Germany). Low-density polyethylene (LDPE) with MFI of 4.7 g/10 min (tested at 21.6 N and 190 °C) was obtained from Karangin Co. Ltd. (Karaj, Iran). Cellulose nanocrystal (CNC), kindly provided by Forest Products Laboratory in Madison, WI., USA and was used as the reinforcing material, (in 5.5% suspension). The properties of used CNC has been reported previously by Kaboorani and Riedl (2015). Native corn starch containing 12% moisture, trace amount of native lipid, 0.7% protein, 0.3% ash, and 26 ± 1% amylose was kindly supplied by Glucosan Co. Karaj, Iran.

299

Treatment code

LDPE (wt.%)

TPS (wt.%)

CNC (wt.%)

a1-TPS30CNC0 a2-TPS29CNC1 a3-TPS28CNC2 b1-TPS40CNC0 b2-TPS39CNC1 b3-TPS39CNC2 c1-TPS50CNC0 c2-TPS49CNC1 c3-TPS48CNC2

70 70 70 60 60 60 50 50 50

30 29 28 40 39 38 50 49 48

0 1 2 0 1 2 0 1 2

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hours until the sample reached a constant weight. In order to calculate WVTR, the curve of weight gain as a function of time was plotted and slope line of the curve was determined. By dividing the slope of each line by surface of sample exposed to water vapor, WVTR was calculated according to Eq. (2). Finally, by using Eq. (3), water vapor permeability coefficient (WVP) was obtained (Babaee et al., 2015).

2.3.2. Hardness The hardness of composites was measured according to ASTM D758 test method by using a Shore D hardness tester. The measurements were made in seven places for each sample. 2.3.3. Surface morphology The surface morphology of obtained TPS, obtained composites, and tensile fractured of selected samples were observed using FE-SEM microscope (model HITACHI S-4160) operated at 20 kV. Prior to characterization, the samples were coated with a gold thin film to avoid electrical charging during the observation.

WVTR =

WVP =

slope (G/t) = A test area

WVTR ×X P(R1 − R2)

(2)

(3)

2.3.4. Moisture absorption Moisture absorption was determined according to ASTM E104 test method. Briefly, the samples were cut to dimensions of 20 mm × 20 mm. Later, samples were dried in an oven at temperature of 55 °C for 24 h. Subsequently, dried samples were kept in a desiccator containing calcium sulfate (0% RH) for 3 days and then the samples were transferred to a desiccator containing potassium sulfate (98% RH). The samples were kept in the desiccator until they reached constant weight. Moisture contents of samples were calculated by using Eq. (1).

In the above equations, WVTR is the water vapor diffusion rate, G is weight change (g), t is time (h), A is area exposed to moisture transfer (m2), G/t = the slope obtained from a chart of weight gain vs. time, WVP is water vapor permeability, X is the thickness of composites samples (2 mm), P is the vapor pressure of water at 25 °C, R1 and R2 respectively are the relative humidity (RH) of desiccator containing calcium sulfate and the relative humidity of desiccator containing saturated solution of potassium sulfate. Linear regression of the data gave a correlation coefficient > 95%.

M1 = ((Wt-W0)/W0) × 100

2.3.6. Glass transition temperature (Tg) and melting point (Tm) Differential scanning calorimetry (DSC) was used to determine Tg and Tm of the composites. The measurement was carried out by using NETZSCH (200F3 Maia, Germany) on a range of temperature of 30–250 °C with a heating rate of 10 °C/min.

(1)

1

t

2.4. Statistical analysis 0 The collected data have been statistically analyzed in a completely randomized design. Duncan’s multiple rang test was used for grouping the means. All comparisons have been made at 95% confidence. 2.3.5. Water vapor transmission rate (WVTR) Water vapor transmission rate (WVTR) was measured according to ASTM E96-95 test method. For the measurement, three grams of calcium sulfate was placed in a container. The sample of composites were fixed on top of the container by using paraffin and the container contents were weighed and were put in a desiccator containing saturated solution of potassium sulfate (RH 98%). Then, the desiccator was placed in a conditioning room and the sample was weighed every few

3. Results and discussion 3.1. Mechanical properties Data on effect of TPS and CNC amounts on MOE of composites are presented in Fig. 1. In general, the effect of TPS on MOE of LDPE was not very pronounced. In contrast, the addition of CNC had a distinctive

Fig. 1. Values of modulus of elasticity (MOE) for LDPE/TPS nanocomposites.

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Fig. 2. Tensile strength of LDPE/TPS nanocomposites.

tensile strength, the optimum CNC and TPS content were found 1% and 30%, respectively. Interestingly, according to ASTM D4976-04 (2018), the LDPE/TPS/CNC composites containing 1% CNC and 29% TPS with tensile strength of 8.6 MPa completely satisfied the required standard tensile strength (i.e., 8.5 MPa) for polyethylene packaging materials. The reinforcing effect of CNC on LDPE/TPS could be attributed firstly to strong compatibility of TPS and CNC due to the present of hydroxyl groups, forming hydrogen bonds and reinforcing the TPS and final LDPE/TPS/CNC composites. Secondly, CNC possibly acts as a binder, enhancing TPS-TPS stress transfer (Meesorn et al., 2017). Thirdly, uniform dispersion of CNC in the TPS also accounts for better reinforcement of final composites, through hydrogen bonding between CNC and TPS. Recent studies showed that CNC with a large aspect ratio, high tensile strength of 7.5–7.7 GPa and MOE from 110 to 150 GPa (Domingues et al., 2014) could improve significantly strengths and thermal properties of polymers and composites (Bras et al., 2011; Oliveira de Castro et al., 2015; Qian Sh Zhang et al., 2018). For instance, Kaboorani et al. (2017) reported that CNC improved MOE and tensile strength of UV-cured acrylate coating. Likewise, Cao et al.

effect on MOE of composites. For instance, at 1% loading of CNC, the increase in MOE was about 40% compared to the control sample. The positive impact of CNC on MOE was more important at lower content of TPS (30%). However, higher loading of CNC (i.e., 2%) gave only about 10% increase in MOE comparing to the control sample. Regarding MOE, the optimum CNC and TPS content was found 1% and 30%, respectively. This indicated that CNC should be used based on end-product quality requirements and in a cost effectiveness manner. These improved properties could be due to reinforcing effects of CNC on TPS (Kaushik and Kaur, 2016). The values of tensile strength for LDPE/TPS nanocomposites are given in Fig. 2. For CNC free composites, increasing TPS content significantly reduced the tensile strength. Indeed, significant increase in tensile strength was observed for reinforced composites even at low CNC content. Similar to MOE, the effect of lower loading of CNC (1%) on tensile strength was better than higher loading one (2%). Adding 1% CNC to LDPE/TPS composites increased tensile strength of the blends as high as two times. At 2% loading of CNC, the reinforcing effect of CNC was lower, especially when TPS content was higher (50%). Regarding

Fig. 3. Hardness of LDPE/TPS nanocomposites.

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Fig. 4. Moisture absorption values of LDPE/TPS nanocomposites.

addition of CNC, and the difference between the moisture absorption values for the composites containing 30%, 40% and 50% TPS significantly decreased. These results are in good agreement with previous studies who reported improved water adsorption for TPS upon addition of CNC (Cao et al., 2008; Slavutsky and Bertuzzi, 2014). This phenomenon could be attributed to the lower mobility of starch chains due to higher crystallinity of CNC (Dufresne and Castano, 2017).

(2007) showed that the Young's modulus and the tensile strength increased with the CNC loading in waterborne polyurethane (WPU-CNC) nanocomposites. Fig. 3 represents the values of hardness for LDPE/TPS blends having, 0, 1 and 2% CNC. Again for CNC free composites, the mean value of hardness decreased by increasing TPS content of blends. On the other hand, CNC increased hardness values of LDPE/TPS blends, irrespective of TPS content in the blends. In contrast to the MOE and tensile strength, increasing CNC loading from 1% to 2% improved hardness of the composites. The enhanced hardness of LDPE/TPS blends as a result of CNC addition could be due to the stiffening effect of the CNC particles (Kaboorani et al., 2012).

3.3. Water vapor transmission rate (WVTR) Water vapor transmission rate (WVTR) is an important property for packaging materials because it is essential in determining the shelf life of products in the package and low WVTR of packaging material allows longer storage time. The results of WVTR and water vapor permeability (WVP) measurement for tested samples are given in Table 2. The values of WVTR and WVP increased by TPS content. In contrast, WVP and WVTR decreased upon addition of CNC and reduction rate depended on the loading of CNC. Although WVTR and WVP were lower at 2% CNC, 1% loading of CNC was more effective for lowering the WVTR and WVP. A good interfacial adhesion between CNC and starch could probably restrict the swelling and moisture diffusion in the composites. Moreover, CNC loading at appreciate quantity creates dense and rigid hydrogen bonded network that acts as a physical barrier to the transport the diffusing molecules. Similar behavior reported for the water vapor permeability (WVP) in agar bio-nanocomposite films upon addition of CNC (Reddy and Rhim, 2014).

3.2. Moisture absorption The values of moisture absorption for the tested samples are showed in Fig. 4. In general, for LDPE/TPS blends without CNC, the values of moisture absorption were proportional to TPS content in the blends. Such result was expected because TPS is hydrophilic and absorbs much more moisture than LDPE which is a hydrophobic polymer (Oromiehie et al., 2013). LDPE/TPS composites containing CNC exhibited much less moisture absorption than LDPE/TPS blends and no significant difference was found between moisture absorption values of the nanocomposites containing 1 and 2% CNC. Interestingly, the adverse effects of TPS content on moisture absorption significantly reduced upon Table 2 The values of water vapor permeability coefficient (WVP), water vapor transmission rate (WVTR), Tg and Tm in LDPE/TPS/CNC nanocomposites. Treatment code

WVTR × 10−7 (g/ m2.day)

WVP × 10−7 (g/ m h Pa)

Tg (°C)

Tm (°C)

a1-TPS30CNC0 a2-TPS29CNC1 a3-TPS28CNC2 b1-TPS40CNC0 b2-TPS39CNC1 b3-TPS39CNC2 c1-TPS50CNC0 c2-TPS49CNC1 c3-TPS48CNC2

1.3 1.12 1.05 1.44 1.17 1.12 1.71 1.39 1.24

22.2 17.5 16.4 22.4 18.3 17.5 26.7 21.6 19.3

60 61.3 63.2 56 58.1 59 53 55.4 56.7

118.8 118.1 119.6 115 116.3 118.5 114 115.1 116.3

3.4. Glass transition temperature (Tg) and melting point (Tm) The DSC thermograms obtained for the nanocomposites are presented in Fig. 5 and Tg and Tm values of them are presented in Table 2. DSC thermograms indicated that increasing the TPS content adversely affected thermal properties of composites. However, CNC increased Tm and Tg values of LDPE/TPS blends, especially at lower content of TPS. This could be attributed to the strong interactions between hydroxyl groups of CNC with TPS, which can reduce the flexibility of polymeric chains in the composite (Dufresne and Castano, 2017). Moreover, due to nucleating effect of the CNC towards the starchy matrix, CNC increases crystallinity of TPS shifting the melting point towards higher temperatures (Mathew and Dufresne, 2002; Kvien et al., 2007).

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suggesting a good and strong adhesion between the TPS and CNC, and also good dispersion of CNC in composites. This could be attributed to the suitable experimental method, e.g., initial mixing of CNC and TPS, and pre-melting of compounds which led to homogenous dispersion of CNC in TPS and in the final composites (Karimi et al., 2014). LDPE/TPS/CNC composites containing 1% CNC exhibited more uniform and continuous surface, without apparent voids or discontinuities in the composites. The morphology of the fractured surface of composite samples (a1, a2 and a3) is presented in Fig. 7. According to SEM micrographs, corn starch completely disrupted and no starch granules could be found in the fractured surface of the samples. In CNC free sample, the two phases were not easily distinguishable; however it was evident that TPS was not uniformly dispersed in LDPE matrix. This could be concluded from the smoother fractured surface of CNC free sample (a1; LDPE/TPS, 70/30) compared with the CNC containing composites, indicating a weak interfacial adhesion between TPS and LDPE. On the contrary, the addition of CNC into the composites resulted in formation of a three-dimensional continuous network that caused rougher fracture surface. The higher loading of CNC resulted in a much rougher surface and some aggregates. Such observations supported all improvements of mechanical, barrier and thermal properties related to 1% CNC incorporation in LDPE/TPS composites. In addition, in all SEMs it was difficult to observe the individual CNC, probably due to their low content and well embedding of CNC in TPS phase.

4. Conclusions In this study, the effect of CNC on performance of LDPE/TPS composites was investigated. CNC enhanced all measured mechanical properties of LDPE/TPS blends including MOE, tensile strength and hardness. Although CNC in all loading levels (1 and 2%) improved the mechanical properties of LDPE/TPS blends, adding 1% CNC was more effective in improving the mechanical properties of nanocomposites. The effect of CNC addition on mechanical properties of nanocomposites was more pronounced in tensile strength where adding 1% CNC increased the tensile strength by as high as two times and the composites containing 1% CNC and 30% TPS fulfilled required tensile strength, according to ASTM D4976-04 (2018). With an exception of hardness, nanocomposites having 1% CNC had better mechanical properties than nanocomposites containing 2% CNC. This is presumably due to the problems associated with poor dispersion of CNC in the matrix (TPS) at 2% CNC loading level. Moisture absorption reduced by introducing CNC to the LDPE/ TPS blends. There was not a significant difference in moisture absorption values of nanocomposites having 1% and 2% CNC. Water vapor permeability coefficient (WVP) and water vapor transmission rate (WVTR) of LDPE/TPS blends were lowered considerably by CNC, leading to improved barrier properties towards water vapor. A small increase in glass transition temperature (Tg) and melting point (Tm) was detected as CNC added to LDPE/TPS blends. The overall results of this study showed that CNC can compensate weakness of LDPE/TPS blends that are caused by adding TPS to LDPE. In summary, the obtained results showed that due to good mechanical properties, low WVRT and WVP, acceptable Tm and Tg, values the LDPE/TPS/CNC nanocomposites could be considered as a sustainable alternative for replacing LDPE in certain packaging materials.

Fig. 5. The DSC thermograms of LDPE/TPS/CNFs nanocomposites.

3.5. SEM analysis The plasticization of starch and dispersion of TPS in LDPE matrix was examined by SEM analysis. As seen in Fig. 6a, the prepared TPS was clear and transparent with a homogenous and smooth surface, indicating good plasticization of starch with the applied technique. This homogenous dispersion and strong adhesion could be due to the good compatibility and similarity in the chemical structure of CNC and starch, as well as hydrogen bonding between the two components (Rico et al., 2016). The morphologies and smoothness of surface of obtained composites depended on the TPS and CNC contents. Increasing TPS from 30% to 50% led to rougher surface, deep cracks and partial phase separation in the surface of composites (Fig. 6 from a1 to c1). CNC was embedded very well in the matrix and appeared as spots in the SEM and no large CNC agglomeration was observed in the surface of composites (Fig. 6a2 and a3). At a constant TPS content, addition of CNC improved surface smoothness and uniformity of composites

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Fig. 6. SEM micrographs of surfaces of neat TPS (a), and LDPE/TPS/CNFs composites.

Fig. 7. SEM micrographs of fractured surface of LDPE/TPS 70/30 composites (a, b and c contain 0, 1, and 2% CNC, respectively).

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Conflict of interest

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