Abstract
Mechanical, thermal, and water barrier properties of poly(lactic acid) (PLA) film reinforced with cellulose nanofibrils (CNF) and bacterial nanocellulose (BNC) were studied and compared. The in-situ formation of sodium carbonate (Na2CO3) on the BNC surface may aid in reducing the interchain hydrogen bonding and agglomeration of BNC fibers. At optimum loading, both CNF/PLA and BNC/PLA nanocomposite films exhibited higher tensile strength and Young’s modulus than the neat PLA without sacrificing its toughness. The BNC/PLA nanocomposite films displayed lower water vapor transmission rate (WVTR) as compared to neat PLA and CNF/PLA films at 0.5 and 1.0 wt%. BNC was found to induce imperfect crystal structures and exhibited higher overall crystallinity than neat PLA and CNF/PLA composites at 0.5 wt%. The BNC/PLA showed higher mechanical properties than CNF/PLA nanocomposites. Nanocellulose derived from plants and bacteria could provide promising solutions to develop high performance biobased-nanocomposites film for packaging application.
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A Comparative Study on the Mechanical, Thermal, and Water Barrier Properties of PLA Nanocomposite Films Prepared with Bacterial Nanocellulose and Cellulose Nanofibrils
Baraka Gitari,a Boon Peng Chang,a Manjusri Misra,a,b,* Alireza Navabi,c and Amar K. Mohanty a,b,*
Mechanical, thermal, and water barrier properties of poly(lactic acid) (PLA) film reinforced with cellulose nanofibrils (CNF) and bacterial nanocellulose (BNC) were studied and compared. The in-situ formation of sodium carbonate (Na2CO3) on the BNC surface may aid in reducing the interchain hydrogen bonding and agglomeration of BNC fibers. At optimum loading, both CNF/PLA and BNC/PLA nanocomposite films exhibited higher tensile strength and Young’s modulus than the neat PLA without sacrificing its toughness. The BNC/PLA nanocomposite films displayed lower water vapor transmission rate (WVTR) as compared to neat PLA and CNF/PLA films at 0.5 and 1.0 wt%. BNC was found to induce imperfect crystal structures and exhibited higher overall crystallinity than neat PLA and CNF/PLA composites at 0.5 wt%. The BNC/PLA showed higher mechanical properties than CNF/PLA nanocomposites. Nanocellulose derived from plants and bacteria could provide promising solutions to develop high performance biobased-nanocomposites film for packaging application.
Keywords: Poly(lactic acid); Nanocomposite films; Bacterial nanocellulose; Mechanical properties
Contact information: a: Bioproducts Discovery and Development Centre (BDDC), Department of Plant Agriculture, Crop Science Building, University of Guelph, 50 Stone Road East, Guelph, N1G 2W1, Ontario, Canada; b: School of Engineering, Thornbrough Building, University of Guelph, 50 Stone Road East, Guelph, N1G 2W1, Ontario, Canada; c: Department of Plant Agriculture, Crop Science Building, University of Guelph, 50 Stone Road East, Guelph, N1G 2W1, Ontario, Canada;
*Corresponding authors: M. Misra, mmisra@uoguelph.ca; A. K. Mohanty, mohanty@uoguelph.ca
INTRODUCTION
The plastic packaging industry consumes the largest market sector of plastic resins, and their products are widely used in food and beverage, pharmaceutical, and other consumer goods (Plastics Europe 2013; Dahlbo et al. 2018). Most of the plastic packaging materials in the form of bottles, film, sheets, etc. are used one time and disposed. The increasing use of these pre- and post-consumer plastic packaging wastes every year contributes to environmental pollution, which affects the ocean and wildlife ecosystems. This phenomenon has reached global concern, and developed countries including Canada, United State, Japan, Germany, France, and China have taken a leading role by implementing new rules and regulations to reduce the plastic bags consumption and waste. Biobased and biodegradable plastic (bioplastic) packaging materials have emerged as an important field of study and development that has the potential to replace non-degradable petroleum-based plastic for packaging applications. In recent years, sustainable bioplastics, e.g. poly(lactic acid) (PLA) synthesis from renewable biomass feedstocks, have been studied as a potential alternative to petroleum-based plastics (Auras et al. 2003; Datta and Henry 2006). Although bioplastic packaging materials from starch-based materials and PLA are already commercially available in the market, the brittleness, poor barrier properties, low impact strength, lack of thermal stability, and high cost of PLA as compared to certain conventional plastics such as polyethylene (PE) and polystyrene (PS) has placed limits on its applications. These drawbacks can be addressed through proper modification of PLA with appropriate additives and processing technique. Auras et al (2003) reported that the mechanical and barrier properties of the two biaxially orientated PLA films are comparable to synthetic plastic films, PS, and polyethylene terephthalate (PET).
Blending of PLA with other thermally stable polymers, filler and fiber has been shown to enhance the properties of PLA (Wang et al. 1998; Kasuga et al. 2003; Huda et al. 2007; Lee et al. 2009a; Quero et al. 2009; Li et al. 2010; Tomé et al. 2011; Lee et al. 2012; Quero et al. 2012; Ma et al. 2013; Fortunati et al. 2015; Abdulkhani et al. 2014; Haafiz et al. 2013; Hubbe et al. 2017). Wang et al. (1998) studied the performance of PLA/poly(ε-caprolactone) (PCL) blends with triphenyl phosphite (TPP) as a coupling catalyst. They found that the elongation at break of the blends was increased significantly (approximately 560%) as compared to neat PLA. However, the tensile strength and elastic modulus decrease dramatically compared with neat PLA. Biodegradable blends from PLA and poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV) also showed a significant increase in elongation at break (from 4% to 220%) as compared with neat PLA (Ma et al. 2013). Similarly, the tensile strength and flexural modulus were also found to be lower than neat PLA after blending with PHBV. Kasuga et al. (2003) used calcium carbonate (vaterite) as a reinforcement in PLA to develop biomaterials for synthetic bone plates. Significant improvements in modulus elasticity and bending strength were observed. However, these improvements come with an increase in porosity and a decrease in toughness. In summary, modification of PLA through co-polymer blending and fillers reinforcement usually increases some properties with some tradeoff in other properties.
Cellulose is one of the fastest growing emerging biomaterials. It is composed of D-glucose linked by β-1,4-glycosidic bonds. It is chemically versatile, biocompatible, hydrophilic, transparent, insoluble in water, and compostable (Auras et al. 2003; Gallegos et al. 2016). The incorporation of small amount of nano-size cellulose (nanocellulose) in polymer can boost the performance of the nanocomposites significantly. Nanocellulose can be extracted from plant and bacteria. A number of works have been conducted by several authors on the reinforcing effect of nanocellulose in polymer biocomposites (Lee et al. 2009a; Quero et al. 2009; Li et al. 2010; Tomé et al. 2011; Lee et al. 2012; Quero et al. 2012; Panaitescu et al.2017), and a summary and thorough review of the use of these nanofibers in polymer have been discussed and reported by Lee et al. (2014).
Cellulose nanofiber (CNF)-reinforced PLA has been shown to have better mechanical properties over the neat PLA (Haafiz et al. 2013; Abdulkhani et al. 2014; Fortunati et al. 2015; Hubbe et al. 2017). However, unmodified nanocellulose is usually not well dispersed in hydrophobic polymers due to incompatibility with the polymer phase resulting in poor mechanical performance (McLean 1982; Poyraz 2018). The dispersion and interaction of the hydrophilic BNC and hydrophobic PLA can be achieved through surface modification and compatibilization (Lee et al. 2009a; Li et al. 2010). Li et al. 2010 developed PLA nanocomposites with grafted maleic anhydride (MA) bacteria nanocellulose (BNC) fiber. The grafted nanocomposites with MA and benzoyl peroxide showed better properties than the unmodified BNC/PLA nanocomposites due to the improved compatibility between the BNC fibers and PLA. Abdulkhani et al. (2014) prepared CNF/PLA cast films and they found that the tensile modulus, tensile strength and elongation at break increased significantly with appropriate CNF loading (Abdulkhani et al. 2014). The water vapor permeability of the nanocomposites films was also comparable to the neat PLA film. A summary of some studies related to nanocellulose-reinforced PLA is presented in Table 1.
Plant-based cellulose has variable properties depending on its growth conditions, plant species, and location in the plant. Additionally, it requires extensive purification steps known as the kraft process to separate and purify cellulose fibers from lignin, hemicellulose, and other biopolymers (Wang et al. 1998; Tomé et al. 2011). BNC is a renewable extracellular nanocellulose produced by acetic acid bacteria in sugar-rich growth mediums (Jozala et al. 2016). It is a viable alternative to plant-based cellulose due to the fewer purification steps required and higher crystallinity (Segal et al. 1959; Chen et al. 2011). In addition, BNC outperforms current in use cellulose in the food industry and technology as effective hydrocolloid additive (da Gama and Dourado 2018).
Table 1. Some Mechanical Properties of Nanocellulose Fiber Reinforced PLA Nanocomposites and their Optimum Fiber Loading
This work aims to study and compare the mechanical, thermal, and water barrier performance of two different feedstock of nanocellulose, i.e. CNF and BNC reinforced PLA films. Different cellulose contents (0.5, 1.0, and 2.0 wt%) were incorporated into PLA through solution casting followed by compression molding. The synthesized BNC was characterized with X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM).
EXPERIMENTAL
Materials and Methods
Bacterial broth preparation
A standard Hestrin Schramm (HS) growth medium was prepared (Hestrin et al. 1947) in 24 Erlenmeyer flasks. 100 mL of the HS medium was added to in each flask. All flasks were capped with aluminum foil and sterilized in an autoclave. The aluminum caps remained on the Erlenmeyer flasks throughout the seeding and growing process to limit oxygen concentrations available to Komagateibacter xylinus and to minimize the risk of contamination.
Growing process
The contents of the Komagateibacter xylinus ATCC 53524 glass vile (American Type Culture Collection, Manassas, VA, USA) were transferred to a 5 mL solution of the growth medium. This inoculation was incubated at room temperature for a week. Next, 0.1 mL of this solution was transferred to each of the Erlenmeyer flasks. The flasks were incubated at 30 C for 14 days. At the end of the growth cycle, the pellicles were collected, and the growth medium was discarded.
Cellulose processing and purification
A standard kitchen blender was used to blend the pellicles in 1 L of deionized (DI) water three times for 20 second intervals. Once blended, ~4 g NaOH (0.1 mol/L) was added into the mixtures. This mixture was then heated and stirred for 10 min on a hot plate at 150 C. The mixture (25 mL) was poured into 50 mL plastic falcon tubes and centrifuged at 8000 rpm for 10 min with a Thermo Scientific Sorvall ST-8 Centrifuge (Thermo Scientific, Mississauga, Canada). The supernatant was decanted, and the precipitate (nanocellulose) was kept. The hydrolysis and centrifuging process were repeated three times. Finally, the precipitate was stored in a refrigerator at 4 °C. The detail purification process of the bacteria nanocellulose (BNC) fiber are presented in Fig. 1. Approximately 1 mL of acetone was added to each tube to prevent the growth of fungi during long term storage.
Hydrated softwood purified cellulose nanofibrils (CNF) were supplied by the University of Maine Nanocellulose Facility (Orono, ME, USA). All nanocellulose samples were dried for 3 days on a Petri dish in the fume hood before cryogrinding. In order to directly compare the properties of CNF/PLA and BNC/PLA nanocomposite films, all fibers used in this study were not surface treated.
PLA nanocomposite films preparation
PLA films fabricated by compression moulding after solution casting have been shown to have higher mechanical properties (Rhim et al. 2006). The neat PLA and its nanocomposite films (0.5, 1.0, and 2.0 wt%) were prepared with solution casting followed by compression molding. The nanocellulose was cryoground to minimize broad size distribution and to obtain a finer and uniform size of BNC. Then, the cellulose was dispersed overnight in 250 mL of acetone. Approximately 20 g of film grade PLA 4043D pellets (NatureWorks, Minnetonka, MN, USA) were dissolved in 400 mL of chloroform (Sigma Aldrich, Oakville, Canada) and stirred overnight. After each solution was mixed overnight, the nanocellulose was added to the chloroform mixture for another 4 h. The obtained final solution was then poured into a standard anti-stick baking pan with dimensions of 450 (l) 275 (w) 30 (t) mm. The solvents were evaporated for 24 h in a fume hood.
The prepared films were placed between metal plates covered with a Teflon film to prevent sticking. All films were degassed by applying and releasing pressure three times to ensure the films were free from air bubbles. The films were compressed using a Carver hydraulic press (Carver, Inc., Wabash, IN, USA) at 180 °C and ~40 MPa for 2 min. The samples were then cooled down while still under pressure for 6 min.
Fig. 1. Synthesis and preparation process of bacteria nanocellulose (BNC)
The films were removed from the mold and cut into dimensions of 150 (l) 10 (w) 0.1 (t) mm and 50 (l) 50 (w) 0.1 (t) mm for tensile testing and water barrier testing, respectively. All films were stored at 25 °C at 50% RH before testing.
Characterization
X-ray diffraction
The cryoground nanocellulose samples were imaged with a Bruker XRD D8 Discover with Davinci design diffractometer (Billerica, MA, USA). A cobalt source was used (λavg = 1.79026 Å) to image the sample between 8° and 45°. Two measurements were taken from a CNF, unwashed BNC, and BNC washed with 1 L DI over a Hirsh Funnel. The amorphous peak was set to 24.1°, and crystal information file (CIF) files were used to define the crystalline peaks. The CIF was based on the March Dollase preferred orientation of the Miller index 100 and 110 directions (Dollase 1986).
Fourier transform infrared (FT-IR) spectroscopy analysis
The functional groups and bonding present on the BNC surfaces with and without sodium carbonate were carried out using Thermo Scientific, Nicolet 6700 Fourier transform infrared spectroscopy (FTIR) from Thermo Fisher Scientific, Waltham, Massachusetts, America. All the samples were scan in the range of 4000 to 600 cm-1 wave number with a 4 cm-1 resolution and 64 scans.
Mechanical testing
The tensile test of the PLA nanocomposite films was performed based on ASTM D882-18 using an Instron Universal Testing Machine (UTM) model 3382 (Norwood, MA) with video extensometer. Each film was clamped into the tensile grips with a 100 mm gauge length, and a 50 mm/min crosshead speed was used. Six films for each sample were tested and the means and standard deviation were reported.
Differential scanning calorimetry (DSC)
The thermal analysis of the PLA films was conducted in a DSC Q200 (TA Instruments, Grimsby, Ontario, Canada). The procedure was as follows: set to equilibrate at 30 °C then ramp 10 °C/min to 200 °C where it was isothermal for approximately 2 min. It was then cooled from 200 °C to 30 °C at 10 °C/min and was isothermal for 2 min. The third cycle was ramped at 10 °C/min to 200 °C. The first heating cycle of the DSC thermograms was used to determine melting temperature (Tm) and crystallization temperature (Tc) as to not erase thermal history when calculating crystallinity. The DSC thermograms of the second heating cycle (third cycle) were used to determine glass transition temperature (Tg).
The degree of crystallinity (Xc) values of all samples were determined from the first heating of the DSC thermograms. The relationship between the area under the curve of the first melting peak of the nanocellulose/PLA composites and 100% crystalline PLA was used to calculate the crystallinity of each sample. The Xc calculation is shown in Eq. 1 below,
(1)
where Hm is the area under the curve (enthalpy) of the melting peak, Hcc is the area under the curve of the cold crystallization peak, H100 is the melt enthalpy of 100% crystalline PLA, which was found to be 93.0 J/g (Nam et al. 2003), and wf is the weight fraction of the nanocellulose loadings.
Water barrier testing
Gravimetric testing was used to determine the barrier properties of each biocomposite film. An Endurance C340 environmental chamber (Associated Environmental Systems, Acton, MA, USA) was set to 37.8 °C and a 90% RH. Approximately 20 to 30 dried silica beads were placed into 3 light metal cups with a 40 mm diameter at the rim and a depth of 50 mm. Each cup was sealed with the prepared film pieces (50 50 0.1 mm) using epoxy glue. The initial weight of each metal cup with dried silica and glued film was measured as an initial reading. Then the cups were placed into the environmental chamber. The weight of the cups was measured every 20 min for the first 2 h and every hour for the subsequent 4 h. Finally, each cup was measured at 24 and 48 h after the first measurement to ensure that the weight of the cups remained consistent. Consistent weight over time indicates that the silica beads and film were saturated with DI water and that the experiment was complete. The slope of time (min) vs. weight (g) for the first 6 points was the water vapor permeability value after conversion to g·mL/m2 ·day.
Thermogravimetric Analysis (TGA)
A TGA Q500 was used (TA Instruments, New Castle, Delaware, America). Approximately 5 mg of dry film sample were placed onto the platinum pan and heated at a rate of 10 °C /min from room temperature to 700 °C.
Statistical analysis
The statistical analysis of the performance of the PLA films was carried out using SAS University Edition version 9.4 (Cary, NC, USA). The experimental design was completely randomized. Each experimental unit was a piece of tensile film. There were 5 experimental units for tensile test (Young’s modulus, tensile strength, and elongation at break), 3 replicates for water vapor transmission rate (WVTR), and 2 for differential scanning calorimetry (DSC). All type I error rates were set at (P = 0.05) unless otherwise stated. Nanocellulose sources, BNC and softwood CNF, and loading (0.5, 1.0, and 2.0 wt%) were independent variables, while Young’s modulus, tensile strength, elongation at break, WVTR, and crystallization percent calculated with DSC thermograms were considered dependent variables. Comparative tests were conducted between CNF/PLA and BNC/PLA composites for tensile strength. Furthermore, contrasting tests were also run between 0.5 and 1 wt% CNF/PLA and BNC/PLA nanocomposites film for WVTR and modulus.
RESULTS AND DISCUSSION
X-Ray Diffraction (XRD)
The X-ray diffraction spectra of CNF and BNC are presented in Fig. 2 (a). The CNF showed customary features distinct to crystalline cellulose (I1 -1 0), (I1 1 0), (IAM), (I002), and (I400), which indicated that the sample was crystalline cellulose (Park et al. 2010; Luo and Wang 2017). Using the March Dollase method (Dollase 1986), the crystallinity of CNF was calculated to be 78% and 80% for the first and second measurements, respectively. These values are similar to those reported by Chen et al. (2011), who employed the Segal method (Segal et al.1959) and found the crystallinity of softwood CNF to be 80.2% ± 4.2%. The crystallinity of the washed BNC sample using the same method was calculated to be 79% and 90% for the first and second measurements, respectively. Bacterial nanocellulose was reported to have higher crystallinity than CNF (Poyraz 2018). However, the unwashed BNC appears to be in an amorphous phase as shown in Fig. 2 (b). This may be due to the presence of other functional groups on the BNC surfaces and hence very little crystalline peaks were observed in the BNC characteristic peaks of (I1 -1 0), (I1 1 0), (I002), and (I400). Furthermore, the peak pattern is consistent with sodium carbonate (Na2CO3), suggesting that there were substantial amounts of Na2CO3 on the sample. To purify the BNC, 0.1 N NaOH was used (Eq. 2). In addition, when blending and mixing the pellicle which has trapped CO2 from cellular respiration of the bacteria (Jozala et al. 2016) and in the presence of air containing, carbon dioxide (CO2) could mix into the solution (Eq. 3). The product of the previous two reactions likely formed Na2CO3 (Eq. 4). However, the amount of Na2CO3 produced is difficult to control as it is formed in-situ during fiber processing and from respiration of the bacteria. The CO2 has been shown to dissolve in water from air and form carbonic acid in soil and natural waters, lowering the pH of the respective medium (McLean 1982).
H2O + NaOH(aq) 
Na+ + OH– + H2O (2)
CO2 + H2O H2CO3 ⇌ 2H+ + CO3– (3)
2Na+ + CO3– Na2CO3 (4)
The formation of carbonate (CO3) as a result of NaOH treatment decreases hydrogen bonding between fibers of nanocellulose, which could favorably affect the dispersion of nanocellulose in its respective matrix (Zhang et al. 2015; He et al. 2016). A proposed schematic drawing of BNC with and without the presence of CO3 is presented in Fig. 3 parts (a) and (b), respectively. CO3 could inhibit the inter-chains hydrogen bonding between BNC. He et al. (2016) synthesized precipitate calcium carbonate hybrid CNF and used to fabricate paper nanocomposites. The produced CNF/paper had a better dispersion and tensile strength due to the presence of calcium carbonate as compared to unmodified CNF/paper composites and conventional paper. This was attributed to the deposition of carbonate on the CNF, which decreases hydrogen bonding between fibers and results in a more dispersed network of CNF in the paper (He et al. 2016). Zhang et al. (2015) conducted a study to determine the effects that milling time, ball mill size, and alkaline pre-treatment with Na2CO3 has on the morphology of nanocellulose fibers. The Na2CO3 in alkaline conditions in conjunction with ball-milling procedure weakens the hydrogen bonds in between cellulose nanofibrils (Zhang et al. 2015).
Fourier Transform Infrared (FT-IR) Characterization
The FTIR spectra of the BNC with and without Na2CO3 are shown in Fig. 4. The BNC with Na2CO3 was prepared according to the cellulose processing method discussed in the methodology section above. For the BNC without Na2CO3 sample, the BNC was filtered and washed several times with distilled water to remove the Na2CO3 from the surface.
Fig. 2. (a) XRD curves of purified BNC and softwood CNF and (b) XRD curves of streamlined process BNC and sodium carbonate
Fig. 3. Proposed interaction of CO32- inhibiting interchain hydrogen bonding left (a) without CO32 and right (b) with CO32
Fig. 4. FTIR analysis of the bacterial nanocellulose with and without the presence of sodium carbonate
From the FTIR spectra, the band at 3344 cm-1 corresponds to the intermolecular hydrogen bonded O-H stretching of cellulose I (Moharram and Mahmoud 2008). The band around 2891 cm-1 is due to the CH2 stretching, while the broad band at 1640 to 1650 cm-1 can be attributed to the H-O-H bending vibration of absorbed water molecules (Mohammadkazemi 2017). The peaks in the range of 1050 to 1059 cm-1 correspond to the ether C-O-C and C-OH stretching vibrations (Gea 2010).
It was found that there is no obvious shifting of the peak and formation of new peak between the BNC with and without Na2CO3. However, the intensity of the hydroxyl band was broadened in the spectrum of BNC with Na2CO3. This may be due to the presence of intermolecular hydrogen bonding between hydroxyl groups of BNCs and CO32- ions as discussed earlier (Fig. 3).
Mechanical Properties
The effects of CNF and BNC in PLA films on the tensile properties are shown in Fig. 5 (a) and (b). Both nanocomposite films showed higher Young’s modulus and tensile strength than the neat PLA. Fibers and fillers usually decrease chain mobility in their respective matrix. This leads to an increase in Young’s modulus and tensile strength and a decrease in elongation (Mathew et al. 2006; Jonoobi et al. 2010). For both CNF/PLA and BNC/PLA nanocomposites, the Young’s modulus increased with increased fiber loadings. The modulus of 0.5, 1.0, and 2.0 wt% CNF/PLA nanocomposites increased from 2539 MPa, 3075 MPa, and 3189 MPa, respectively. On the other hand, the modulus of 0.5, 1.0, and 2.0 wt% of BNC/PLA nanocomposites were 3400 MPa, 3750 MPa, and 3672 MPa, respectively. The hydrogen bonds between PLA and the natural nanocellulose fibers determine its mechanical strength (Nakagaito et al. 2009; Tee et al. 2013). Auad et al. (2008) reinforced polyurethanes (PU) with 1.0 wt% of nanocellulose and found that PU with nanocellulose increases tensile strength and Young’s modulus over neat PU. They attributed the enhancement to the presence of stiff randomly orientated fibers, which additionally decrease the chain mobility of the PU. Similar mechanisms of action have been found in a number of nanocellulose/PLA nanocomposites as well (Lee et al. 2009a; Lee et al. 2009b; Quero et al. 2009; Li et al. 2010; Tomé et al. 2011; Lee et al. 2012; Quero et al. 2012).
The tensile strength of the CNF/PLA nanocomposites increased from 36.7 MPa (0.5 wt%) to 44.2 MPa (1.0 wt%) then decreased to 37.2 MPa for the 2.0 wt% fiber loading. For the BNC/PLA, the tensile strength of the 0.5 and 1.0 wt% were 49.9 MPa and 48.7 MPa, respectively. The tensile strength of the BNC/PLA was decreased at 2.0 wt% loading of BNC. Increasing nanocellulose loadings increases the chance that there are fibers throughout the PLA matrix, which could increase the strength; however, increasing loadings also increases agglomerations (Yeh et al. 2010; Shih et al. 2011). Qu et al. (2010) observed a decrease in tensile strength of BNC/PLA films composites where the fiber loading was greater than 3 wt%.
Neat PLA had an elongation at break at 1.9%. For 0.5 wt% of both CNF and BNC loading in PLA, the films showed similar elongation at break to neat PLA. At 1.0 and 2.0 wt%, the elongation at break of both nanocomposites films showed decreasing trend in elongation. Nanocomposites with 2.0 wt% had lower elongation % at break than the neat PLA. Similarly, the tensile toughness of BNC/PLA decreased with the increasing content of BNC (Fig. 5b). In comparison, the 2.0 wt% CNF/PLA decreased in elongation but to a lesser extent.