PBAT Bionanocomposites with Antimicrobial Natural Rosin for

Article

PLA/PBAT Bionanocomposites with Antimicrobial Natural Rosin for Green Packaging Hesham Moustafa, Nadia El Kissi, Ahmed I. Abou-Kandil, Mohamed S. Abdel-Aziz, and Alain Dufresne ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 29, 2017

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ACS Applied Materials & Interfaces

PLA/PBAT Bionanocomposites with Antimicrobial Natural Rosin for Green Packaging Hesham Moustafa,†,‡ Nadia El Kissi,§ Ahmed I. Abou-Kandil, † Mohamed S. Abdel-Aziz,∥ Alain Dufresne‡* †

Polymer Metrology & Technology Department, National Institute of Standards (NIS),

Tersa Street, El Haram, El-Giza, P.O Box 136, Giza 12211, Egypt. ‡

Univ. Grenoble Alpes, CNRS, Grenoble INP, LGP2, F-38000 Grenoble, France

§

Univ. Grenoble Alpes, CNRS, Grenoble INP, LRP, UJF, F-38000 Grenoble, France

∥ Microbial

Chemistry Department, National Research Centre El-Behoos St.33, Dokki-Giza

12622, Egypt.

KEYWORDS. PLA/PBAT; natural rosin; expanded organoclay; mechanical properties; antimicrobial activity ABSTRACT The use of biodegradable polymers is of great importance nowadays in many applications. Some of the most commonly used biopolymers are polylactic acid (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) due to their superior properties and availability. In this manuscript, we use a facile and green modification method of organoclay (OC) by antimicrobial natural rosin which is considered as a toxicity-free reinforcing material, thus keeping the green character of the material. It increases the interlayer spacing between the clay platelets. This was proven by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) and found to impart antimicrobial properties to PLA/PBAT blends.

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The morphology of the resulting blends was conducted using scanning and transmission electron microscopies (SEM and TEM) and evidence of exfoliation and intercalation was observed. The thermal properties of the blends were studied using differential scanning calorimetry (DSC) and a detailed study of the crystallization of both PLA and PBAT was reported showing cold crystallization behavior of PLA. The final effect on mechanical and antimicrobial properties was also investigated. The obtained results reveal excellent possibility of using expanded OC modified PLA/PBAT polymer blends by adding a green material, antimicrobial natural rosin, for food packaging and bio-membranes applications. INTRODUCTION The minimization of environmental risk and sustainability issues resulting from domestic and industrial wastes obtained from plastics have become a major current concern. Consequently, polymers such as polylactic acid (PLA) have attracted a great interest in a variety of industrial sectors due to their unique properties comparable with classical petroleum-based plastics, in addition to their 100% renewability and biodegradability. Several novel series of biodegradable bioplastics that can be ultimately decayed to carbon dioxide, water and humus have been developed. These advantages open opportunities for their use in a wide spectrum of applications such as food packaging,1,2 membranes for water treatments, drug delivery3-5 and industrial composting.6,7 Nevertheless, the brittleness, low impact resistance and heat distortion temperature of PLA, as well as its high price restrict its applications. To overcome these drawbacks, PLA can be blended with other biopolymers to offer a convenient option for improving these properties or to create new materials with high performance for end use products. Among all biopolymers, highly ductile poly(butylene adipate-co-terephthalate) (PBAT) which is made up from aliphatic-aromatic biopolyester is considered as completely biodegradable with the aid of microorganisms.8 Many studies 9-12 have been conducted on

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blending PLA with PBAT to enhance the processability and the mechanical properties of PLA. Mohanty et al13 have reported that the addition of 25 wt% of PBAT to PLA decreased its brittleness which was reflected through the increase of impact strength from 21.1 J m-1 for PLA to about 50 J m-1 for PLA/PBAT (75/25) blend, whereas the tensile strength decreased. On the other hand, their incompatibility and immiscibility have been the major limitation which causes deterioration in tensile properties, leading to a negative effect for the use of these blends in purposes that require extensive mechanical stress. Siemann and coworkers14 reported that the incompatibility of PLA with PBAT is due to the large difference in the solubility parameters between PLA (10.1 (cal.cm-3)1/2) and PBAT (22.95 (cal.cm-3)1/2), evidencing weak interactions between the constituent biopolymers in the blend. Yeh et al15 have reported that the flexibility and the longer repeating units of PBAT resin when compared to PLA caused a phase separation in PLA matrix which was confirmed by SEM observations. In order to enhance the interfacial adhesion (i.e. improve the compatibility) between both biopolymers, certain plasticizers can be used. Nevertheless, these plasticizers have a detrimental effect on the glass transition temperature (Tg) and tensile properties of the polymer blend, especially at high plasticizer content.16 Organoclay (OC) is often used as reinforcing material in polymer matrices to enhance the mechanical, barrier and flammability properties,17-19 but recently it was also used as compatibilizing agent in polymer blends.20 To fulfill this requirement, the interlayer spacing of clay layers (d-spacing) shall be 3 nm or more depending on the organomodifier (surfactant) content between clay galleries, leading to highly intercalated and/or exfoliated clay platelets in the polymer matrix with expected excellent properties.21,22 As we mentioned in our previous research,20 when the d-spacing distance is equal or higher than 3 nm, part of the organomodifier chemically linked with negative clay layers, while the other part was physically adsorbed into clay layers and counterbalanced with Cl- or Br- anions. However,



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the presence of these kinds of anions in clay galleries can limit its use in a variety of applications because of the toxicity hazard associated to Cl- and Br-, particularly when it is used in food packaging or as membranes for water treatment. Thus, the modification of clay by these types of surfactants contributed to the rebirth of green modification alternatives for clay materials. Indeed, keeping the green aspect of these materials is a key issue. For this reason, our research is focused on an easy and green modification way of organoclay with d-spacing 1.8 nm (i.e. only chemically linked with negative clay layers) to create a highly intercalated and/or exfoliated morphology of clay nanoparticles based on antimicrobial phenolic rosin and long organic fatty acid chains for comparison in biopolymer blends. Gum rosin is a natural material derived from pine trees and it is composed of a mixture of phenolic acids such as abietic and pimaric acids, that have allowed to use rosin and its derivatives in numerous applications such as paper-sizing agent, tackifier in adhesives, emulsifier, and additive agent for printing inks.23,24 The current decade has witnessed quick development of the use of gum rosin as an antimicrobial natural material that can achieve the extending of the shelf life for packaged products as long as possible.19,25,26 In some cases, it is used as a grafting agent for lignin through the esterification reaction to enhance the compatibility between lignin and biopolymers.27 In the present study, a facile green modification of organoclay by antimicrobial rosin to obtain toxicity-free expanded rosin organoclay (ROC) was performed by melt compounding for enhancing the biocompatibility of PLA/PBAT blends at various ratios. The morphology of these bionanocomposites was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermal, viscoelastic and mechanical properties were characterized, as well as the antimicrobial activity. The expanded OC by stearic acid (SA) was also used for comparison (SOC). The proposed novel modification approach of organoclay by antimicrobial natural rosin and compatibilization of


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bionanocomposites processed by melt compounding comply with sustainable development principles.

MATERIALS AND METHODS Materials. Polylactic acid (PLA) with a density of 1.24 g.cm-3 was offered in pellets form from NaturePlast, France, under the commercial name PLE 003 resin. PBAT (PBE 006 resin) with melt Flow Index (190°C; 2.16 kg) of 4-6 g.10 min-1 and density of 1.26 g.cm-3 was offered from NaturePlast, France, and these data were provided by the manufacturer. Nanoclay (OC), surface modified by methyl dihydroxyethyl hydrogenated tallow ammonium, was purchased from Sigma-Aldrich, France. Gum rosin (colophony) is a natural resin product provided from Aldrich Chemicals (Saint Quentin Fallavier, France). It has a molecular weight of 302 g.mol-1 and it is composed of a mixture of aromatic acids such as abietic, levopimaric, dehydroabietic, pimatic, and isopimatic acids. Stearic acid (C18H35O2) with purity 95% was purchased from Sigma-Aldrich, France, and used as an expanding agent between clay galleries. The rosin and stearic acid were used as received without further purification. Modification of organoclay (OC) by gum rosin and stearic acid. In order to increase the interlayer spacing distance (d-spacing) of organoclay (OC), the gum rosin (R) was used as a green modifier material by using melt mixing method instead of in situ or other dissolution methods that use carcinogenic chloroform solvent. Briefly, modified clay and gum rosin were first blended together by using ceramic mortar with weight ratio (1:1). The blend was introduced in an oven at 120°C for 1h but the blend was mixed each 20 min and this step was repeated three times to ensure that the melted rosin reacted well with OC and intercalated into the clay galleries. Afterward, it was cooled at room temperature and grinded



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into a powder form. The grafting of organoclay was confirmed by XRD and FT-IR analysis. The modification of OC by SA was done as described in our previous article.20 Processing of PLA/PBAT/expanded OC bionanocomposites. Before mixing, both biopolyesters (PLA and PBAT) were dried in a laboratory oven at 60°C for 24 h. Also, isothermal TGA experiments for ROC and SOC were conducted at 170°C (processing temperature) in order to monitor any thermal decomposition during the processing of expanded organoclay with the polymers, as shown in Figure 1. No strong degradation was observed within 30 min (1800 s) and the weight loss was lower than 3%. The blending strategy for the preparation of bionanocomposites consisted in first mixing the grafted or expanded organoclay and PLA followed by mixing with PBAT. PLA was premixed at 170°C for 3 min in a closed Brabender Mixer, Model: FD0234H, Germany, before adding the expanded organoclay (ROC or SOC) and mixing for 5 min. Then PBAT was added and mixed for additional 5 min for a total compounding time of 10 min at 80 rpm. The obtained samples were molded by placing them in a molding set consisting of a 100 x 100 x 1 mm3 thick steel spacer placed between two Teflon pressing papers and two steel sheets. The pressing protocol consisted of 5 min pre-heating of the sample in a hydraulic press (Mécanique Outillage, Model Saint-Eloi, France) at 170°C, followed by compression under a pressure of 50 bars for 2 min, and air cooling under a certain load until the mold reached around 60-70°C. The investigated samples with ROC and SOC were labeled according to the ratio of PLA and PBAT in the blend, as reported in Table 1.


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Figure 1. Isothermal TGA for ROC and SOC at 170ºC. Table 1. Composition, mixing protocol and codification of the prepared formulations for virgin PLA, and OC filled bionanocomposites with different PBAT contents and 2.5 wt% of ROC or SOC. Constituents (wt%) Mixing conditions

Specimen code Matrix

Nanofiller

PLA

PBAT

SOC*

ROC*

Temperature [°C]

Screw speed

Mixing time

[rpm]

[min]**

Pristine PLA

100

-

-

-

170

80

10

7525 ROC

75

25

-

2.5

170

80

5+5

5050 ROC

50

50

-

2.5

170

80

5+5

2575 ROC

25

75

-

2.5

170

80

5+5

7525 SOC

75

25

2.5

-

170

80

5+5

5050 SOC

50

50

2.5

-

170

80

5+5

2575 SOC

25

75

2.5

-

170

80

5+5

* The ratio of OC to Rosin or St. acid was 1:1 and 2.5 wt% refers to the OC % in expanded organoclay (EOC) ** 5 min mixing for PLA/EOC and 5 min mixing of PLA/EOC/BPAT in this order

X-ray diffraction (XRD). The d-spacing distance and 2Ɵ position of OC and its modification by rosin or SA were performed using a PANalytical X-ray diffractometer



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(X’Pert Pro MPD), Netherlands, with CuKα radiation (40 kV, 40 mA) by using 2Ɵ range from 1.5° to 15° at a scan speed rate of 2°.min-1, and at a sampling width of 0.02°. Fourier transform infrared spectroscopy (FTIR). FTIR spectra were implemented to detect the chemical grafting between OC and both expanding agents, rosin and SA, using a FTIR Perkin-Elmer 1720X spectrometer, Netherlands, in the spectral range between 4000 cm-1 and 600 cm-1 with a resolution of 4 cm-1 and 32 scans. The specimens were prepared by pressing 100 mg of KBr and 2 mg of powder to give a thin disc. A reference spectrum of pure KBr disc was used. Attenuated total reflectance infrared (ATR-FTIR) spectra were analyzed for the bionanocomposites with a thickness of 150 µm to 200 µm and under the same conditions as mentioned above. Scanning electron microscopy (SEM). The compatibility and dispersion of EOC particles in the PLA/PBAT matrix were investigated using scanning electron microscopy (SEM, High Resolution Quanta FEI 200, Czech Republic). The fractured cross-section of the samples was gold coated prior to the observation to avoid electrostatic charging during observation. SEM images were obtained under high vacuum for both secondary electrons (topography contrast) and backscattered electrons (chemical contrast). The acceleration voltage (5-10 kV) and the electron beam spot size (3.0-3.5) were carefully chosen in order to optimize the quality of the images. Transmission electron microscopy (TEM). The dispersion state of modified nanoclay particles within the PLA/PBAT blends was investigated using transmission electron microscopy (TEM-Philips CM 200) with an accelerating voltage of 200 kV. The samples were prepared by Ultramicrotomy at room temperature. The thickness of ultra-thin sections of the investigated samples was about 80 nm. The sections were deposited on the surface of copper observation grids coated with an amorphous carbon film.


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Differential scanning calorimetry (DSC). The thermal analysis of pristine PLA and its blends with various contents of PBAT in the presence of 2.5 wt% expanded organoclay (ROC or SOC) as a fixed amount was accomplished using differential scanning calorimetry (DSC Q-100, TA Instruments apparatus), equipped with a liquid nitrogen cooling system (LNCS) unit. Each sample (5-10 mg) was investigated using heating and cooling ramps from -50ºC to 200ºC with a scanning rate of 10ºC min-1 and nitrogen flow rate of 50 mL.min-1. During the heating ramp, the glass transition temperature (Tg), crystallization and melting temperatures (Tc and Tm for PLA) and associated crystallization and melting enthalpies (∆Hm and ∆Hc) were determined, while during the cooling ramp, the crystallization temperature and enthalpy (Tc and ∆Hc for PBAT) were determined. The crystallization or melting enthalpies were used to calculate the degree of crystallinity (Xc) of PLA or PBAT in PLA/PBAT bionanocomposites from the following equation:15

% =

∆ , × 100 1 ∅ ∆ ,

Where ∆Hc,m is the crystallization/melting enthalpy of the polymer in the blend and ∆ ,

is the crystallization/melting enthalpy for a 100% crystalline polymer, that was considered to be -/+93/93 J.g-1 for PLA,28 and -/+114/114 J.g-1 for PBAT,29 and ϕ is the weight fraction of the polymer in PLA/PBAT nanocomposites. The measured degrees of crystallinity were therefore normalized to the actual fraction of the polymer in the blend. At least duplicates were recorded for each blend. Viscoelastic characterization. Viscoelastic tests were carried out using a rotational rheometer (Model ARES-G2 mode), equipped with parallel plates with rough surfaces. The geometry was 25 mm in diameter and the gap was fixed to 2 mm. All experiments were performed at 170°C. The frequency sweeps were accomplished within the linear regime that is to say at a deformation of 5% for the pristine polymer and 0.1% for the blends.



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Mechanical properties. The tensile properties of the specimens were conducted using an Instron 5965 Universal Testing Machine, UK, with a load cell of 5 kN and a crosshead speed of 10 mm.min-1, according to ASTM D 638. Type 2 dumbbell test samples were diecut from the molded sheets. The rigidity of the samples was calculated by multiplying the modulus by the thickness. The standard deviation was calculated from five parallel measurements for each sample. All specimens were conditioned at 23±2°C and 50± 5% relative humidity for at least 20 h prior to the test. Antimicrobial activity test. The antimicrobial activities against Pseudononas aeruginosa (Gram-negative), Staphylococcus aureus (Gram-positive) and Candida albicans (fungi) for all investigated samples were conducted by using disc agar plate technique.30 The test procedure was described in our previous literature.31 Both bacterial and fungal test microbes were cultivated on nutrient agar (DSNZ 1) medium (g.L-1): beef extract (3), peptone (10), and agar (20), whereas the fungal test microbe was grown on Szapek-Dox (DSMZ130) medium (g.L-1): sucrose (30), NaNO3 (3), MgSO4.7H2O (0.5), KCl (0.5), FeSO4.7H2O (0.001), K2HPO4 (1) and agar (20). The culture of each microorganism was diluted by sterile distilled water from 104 to 108 CFU.mL-1 to be used as an inoculum. 0.1 mL of each inoculum was used to inoculate 1 L of agar medium (just before solidification) then poured into Petri dishes (10 cm in diameter containing 25 mL). Discs (5 mm in diameter) were located on the surface of the agar plates previously inoculated with the test microbe and incubated for 24 h for bacteria and fungi at 37°C. After incubation period, the results and the diameter of growth inhibition zones were measured and the average values were recorded. RESULTS AND DISCUSSION Nanoclay was modified by natural rosin and SA in order to increase the interlayer distance (d-spacing) between clay galleries. This was evaluated by XRD. The 2Ɵ position and the values of the d-spacing distance corresponding to the basal spacing (d001) as obtained by 10 Environment ACS Paragon Plus

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XRD measurements are shown in Figure 2 and Table 2. It can be seen that the diffraction peak for unmodified organoclay appears at 2Ɵ = 4.9° (d001 = 1.8 nm). On the other hand, the diffraction peak for the rosin modified organoclay (ROC) appears at 2Ɵ = 2.4° (d001= 3.9 nm) and that for SA modified organoclay appears at 2Ɵ = 2.2° (d001 = 4.0 nm). This shift indicates that both the rosin or SA molecules can penetrate clay galleries and increase interplatelet spacing (d-spacing), thanks to the proposed green facile modification. It can be also observed that the diffraction pattern for ROC is mostly amorphous, whereas the other one for SOC is more crystalline.

Figure 2. XRD patterns for organoclay (OC) and expanded organoclay by rosin (ROC) and stearic acid (SOC). Table 2. 2Ɵ position and calculated d-spacing for OC and its expanded organoclay (ROC and SOC) determined by XRD. Specimen

2Ɵ [º]

d-spacing [nm]

OC

4.9

1.8

ROC

2.4

3.9

SOC

2.2

4.0



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The functional properties of pristine OC and OC modified by rosin (ROC) or SA (SOC) were supported by FTIR spectra in transmission mode within the range from 4000 cm-1 to 600 cm-1, as shown in Figure 3. In Figure 3a, the spectrum corresponding to neat rosin is also provided for comparison with ROC. Both spectra display characteristic bands at 2938 cm-1 and 2860 cm-1 (corresponding to asymmetric C-H stretching vibration of CH3 and symmetric C-H stretching vibration of CH2, respectively) and 1700 cm-1 (carbonyl group).32 A broad band observed at about 3440 cm-1 for rosin can be assigned to free O-H groups from acids and humidity. When gum rosin was melted with OC at 120°C for 1h, the intensity of the band at 1700 cm-1 (i.e. C=O) decreased, indicating that the melt intercalation of rosin between the clay galleries was achieved. It can also be noted that the characteristic bands at 3630 cm-1 and 1050 cm-1 for ROC have been assigned to (O-H) group in the crystal structure of organoclay33 and Si-O-Si group, respectively. Similarly, as shown in Figure 3b, after the grafting of OC with SA, new bands were observed at about 1715 cm-1, 1300 cm-1 and 720 cm-1 which can be ascribed to SA that formed after the chemical interaction of SA and OC in the layers. Based on the chemical grafting of OC, rosin and SA should contribute to the compatibilization and dispersion of OC nanofiller within the PLA/PBAT blend to obtain intercalated/exfoliated bionanocomposites.

Figure 3. FTIR spectra for (a) gum rosin and ROC, (b) OC and SOC.


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SEM images exhibit the effect of the treatment of OC with gum rosin or stearic acid on the degree of compatibility between PLA and PBAT in the blends at various contents (i.e. 75/25, 50/50, 25/75), as shown in Figure 4. The incompatibility between PLA and PBAT is well-known and has been broadly reported.9,15 The morphology of the fractured surface of pristine PLA is smooth and uniform. When adding 25 wt% of PBAT to the PLA matrix in the presence of ROC, a rough surface is observed for 7525 ROC composite but PBAT seems to be miscible and more compatible with the PLA matrix when compared to 7525 SOC composite, for which voids and empty spaces can be observed throughout the blend. When the PBAT content is equal to 50 wt%, poor compatibility with phase-separated morphology is observed for both 5050 ROC and 5050 SOC samples, as displayed in Figure 4. The reason is apparently because the repeating units of PBAT resin are longer and highly flexible compared to PLA units, which can induce an immiscible phase with the PLA molecules at this ratio.15 It might also be due to the large difference in the solubility parameters between parent polymers, PLA and PBAT.14 Moreover, the ROC or SOC content is not sufficient to achieve the interfacial adhesion between the two biopolyesters. Nevertheless, for PBAT content higher than 50 wt%, the polymers appear to be compatible without any obvious phase-separation or droplet coalescence in the case of 2575 ROC sample. In contrast, a significant incompatibility between PLA and PBAT is noticed for 2575 SOC, evidencing that the modification of OC by gum rosin is more efficient in its compatibilization role between the two biopolyesters in contrast to the composite containing SOC. Additional information about the nanodispersion state of expanded ROC and SOC within the PLA/PBAT blends was provided by TEM observations as shown in Figure 5. It was found that only intercalated structures are observed in the case of 2575 ROC and 2575 SOC nanocomposites. However, both intercalated and exfoliated structures existed in the polymer matrix when the PLA ratio was 75 wt% (for 7525 ROC and 7525 SOC samples), indicating



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the penetration ability of PLA short chains between clay layers leading to clay exfoliation and to the formation of nanocomposites. It obviously evidences that the dispersion of expanded OC is achieved when PLA content is increased in the blend, which should result in enhanced barrier properties. Otherwise, high PBAT content in the blend can drive to form the clay intercalation.

Figure 4. SEM images of the fracture surface of pristine PLA and its bionanocomposites with different PBAT contents and 2.5 wt% of ROC or SOC.


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Figure 5. TEM images for 7525 ROC, 2575 ROC, 7525 SOC, and 2575 SOC composites. Differential scanning calorimetry (DSC) was used to investigate the thermal characteristics of PLA/PBAT blends. The DSC traces are depicted in Figure 6. The thermal data extracted from these thermograms are presented in Table 3. The DSC thermogram corresponding to pure PBAT was reported in our previous study.2 From the thermograms recorded during the heating ramp a thermal transition in the form of a specific heat increment centered around 61°C can be observed for PLA and its bionanocomposites. It is attributed to the glass transition temperature (Tg) of PLA. The peak that is superimposed to the specific heat increment corresponds to the well-known physical aging or enthalpic relaxation of the polymer. No significant shift of Tg is observed when adding PBAT and clay nanofiller (Table 3). Upon heating, the crystallization of PLA is also observed around 100°C. This exothermic peak is classically observed for PLA and attributed to the so-called cold



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crystallization during heating process. The crystallization temperature (Tc) first increased for 7525 ROC and then decreased for higher PBAT content (5050 ROC) and finally disappeared for 2575 ROC. For bionanocomposites reinforced with SOC a gradual and continuous increase of Tc is reported when increasing the PBAT content. The amount of crystalline structures formed during this cold crystallization process was quantified through the degree of crystallinity determined from the crystallization enthalpy according to Eq. (1) and values (Xcc) are reported in Table 3. It was assumed that this crystalline phase was attributed only to PLA even if contribution from the PBAT phase cannot be excluded. Moreover, normalization was performed to account for the actual PLA content in the blend, but the EOC content was assumed to be negligible. A continuous decrease of Xcc was observed when increasing the PBAT content. This behavior might be due to the reduction of polymer chains mobility because of the longer repeating units of PBAT, in addition to the presence of the reinforcing expanded OC.34,35 At higher temperatures, the crystals formed during this cold crystallization process as well as crystals initially present in the blends melt around 158°C. The temperature associated to this thermal event (Tm) is not impacted by the composition of the blend (Table 3). The degree of crystallinity was determined from the melting enthalpy according to Eq. (1) and values (Xcm) are reported in Table 3. It significantly decreases when increasing the PBAT content in the blend. This result is consistent with that reported elsewhere for PLA/PBAT blends in the presence of phthalic anhydride as compatibilizer.34 The difference between this degree of crystallinity and the one observed for the cold crystallization process normalized by the degree of crystallinity reported during melting (Xcm – Xcc)/ Xcm is also reported in Table 3. It corresponds to the proportion of PLA crystals formed during the cooling step, just after melt processing. Interestingly, it is found to significantly increase when increasing the PBAT content, which


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can explain, at least partially, the decrease of cold crystallization. This observation is probably related to the increased flexibility of PLA chains in the presence of PBAT. Following this heating ramp, cooling of pristine PLA didn’t induce any crystallization as shown in Figure 6. However, a crystallization process in the range 75-80°C occurs when PBAT was mixed with PLA. For comparison, pure PBAT was tested in similar conditions and it exhibited a crystallization peak characterized by a temperature of 72.3°C and a degree of crystallinity of 17.8%. This crystallization process can be therefore attributed to the PBAT phase in the blends and should probably reject PLA chains in the amorphous region.

Figure 6. DSC thermograms for pristine PLA and its blends with PBAT in the presence of 2.5 wt% of expanded OC (ROC and SOC) during heating and cooling ramps (Exo up). Table 3. Thermal properties for pristine PLA and its bionanocomposites obtained from DSC experiments. Heating ramp: glass transition (Tg), crystallization (Tc), and melting (Tm) temperatures of PLA and associated crystallization (∆Hc, Xcc) and melting (∆Hm, Xcm) enthalpies and degrees of crystallinity. (100 × (Xcm - Xcc)/Xcm) corresponds to the proportion of PLA crystals formed during the cooling step, just after melt processing. Cooling ramp: crystallization temperature (Tc) of PBAT and associated crystallization enthalpy (∆Hc) and degree of crystallinity (Xc).

Heating ramp Sample PLA

Tg (°C)

Tc (°C)

∆Hc (J.g-1)

Xcc (%)*

Tm (°C)

∆Hm (J.g-1)

Xcm (%)*

100 × (Xcm - Xcc)/Xcm (%)

60.4

99.6

26.1±0.2

28.1±0.3

158.5

26.1±0.1

28.1±0.2

0



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7525 ROC

60.8

105.9

16.4±0.5

23.5±0.6

158.8

19.6±0.3

28.1±0.4

16±0.3

5050 ROC

60.3

102.8

4.0±0.4

8.6±0.3

158.2

13.0±0.2

28.0±0.1

69±0.2

2575 ROC

60.4

–

–

–

158.6

1.7±0.4

7.3±0.5

100±0.4

7525 SOC

60.3

99.1

16.7±0.3

23.9±0.5

158.5

19.5±0.1

28.0±0.3

15±0.3

5050 SOC

60.4

99.8

5.8±0.4

12.5±0.2

158.7

11.3±0.4

24.3±0.2

42±0.18

2575 SOC

60.1

103.5

0.75±0.2

3.2±0.3

158.0

3.9±0.2

16.8±0.3

81±0.2

Cooling ramp Sample

Tc (°C)

∆Hc (J.g-1)

Xc (%)**

PBAT

72.3

20.3±0.1

17.8±0.1

7525 ROC

76.6

6.0±0.4

20.9±0.3

5050 ROC

78.0

6.1±0.15

10.7±0.2

2575 ROC

81.0

10.5±0.2

12.3±0.1

7525 SOC

73.6

5.9±0.33

20.7±0.3

5050 SOC

78.0

6.2±0.5

10.9±0.4

2575 SOC

80.5

10.8±0.1

12.6±0.2

* Determined using Eq. (1) with ∆ = 93 J.g-1. = 114 J.g-1. ** Determined using Eq. (1) with ∆

The viscoelastic behavior of PLA/PBAT nanocomposites was evaluated by measuring the evolution of the viscoelastic components, i.e. elastic component or storage modulus G', and viscous component or loss modulus G", as a function of frequency. Results are reported in Figure 7. Pristine PLA exhibits a typical viscoelastic liquid-like behavior, with a loss modulus higher than the storage modulus (G" > G') mainly at low frequencies. This behavior is associated with a complex viscosity characterized by a Newtonian plateau at low frequencies, followed by a significant shear thinning behavior with increasing shear rate indicating that the viscosity is a function of the shear rate, as shown in Figure 8. Considering now the nanocomposite blends, the loss and storage moduli are compared to those of pristine PLA in Figures 7a and 7b for ROC and SOC, respectively. It is observed that both G’ and G” are increased in comparison with the values obtained for the pristine


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polymer. It is worth noting that for nanocomposite blends, the loss and storage modulus values are almost matching, G’ being even higher than G” for 5050 SOC. This behavior indicates a solid-like behavior, thus indicating the possible development of a network that can be associated to the existence of a yield stress.

Figure 7. Storage modulus (G', filled symbols) and loss modulus (G", open symbols) for pristine PLA and PLA/PBAT nanocomposites reinforced with ROC (a) and SOC (b) as a function of frequency at 170°C: Pristine PLA (, ), 7525 ROC/SOC (, ), 5050 ROC/SOC (, ) and 2575 ROC/SOC (, ). These results are confirmed when considering the evolution of the complex viscosity as a function of shear rate represented in Figure 8. The complex viscosity is higher for nanocomposite blends. It is almost not affected by the concentration for ROC, whereas differences are more accentuated, mainly at lower frequencies for SOC. The differences in viscosity for SOC loaded blends vs. quasi-non-variation for ROC are most probably related 19 Environment ACS Paragon Plus


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to the intercalation/exfoliation level of OC and compatibility level between both biopolymers. It can be seen from TEM images (Fig. 5) that intercalated domains for 2575 SOC are bigger than for 2575 ROC. Then the morphology of 7525 ROC is more similar to that of 2575 ROC, while the difference in morphology is more accentuated between 7525 SOC and 2575 SOC, which could explain the difference in viscosity for SOC loaded blends. We can also observe that in both cases the 50/50 blend leads to the more prominent increase in viscosity, when compared with pristine PLA.

Figure 8. Complex viscosity as a function of frequency at 170°C for pristine PLA and associated ROC nanocomposite blends (a) and SOC nanocomposite blends (b). The non-linear mechanical properties have been investigated to point out the influence of different ratios of PBAT on the properties of PLA in the presence of 2.5 wt% of ROC or SOC as a fixed content. Typical stress-strain curves for bionanocomposites are presented in Figure 9a. From these curves, it appears that pristine PLA is a fairly rigid and brittle polymer with a tensile strength of 116.3 MPa and a strain at break of 15.5%. When adding 25 wt% of ductile PBAT and 2.5 wt% of expanded OC, the tensile strength obviously reduced to ~ 66.3 MPa for 7525 ROC and ~ 56.9 MPa for 7525 SOC bionanocomposites, but the strain at break increased to 90.3 % and 85.7%, respectively, as shown in Figure 9a. Similar results have been reported elsewhere34,35 when adding 20 wt% of PBAT to PLA matrix. When the ratio of PBAT in the blend reached 50 wt%, both the strength and strain at


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break values decreased. This decrease may be due to the poor compatibility between PLA and PBAT and to the fact that the expanded organoclay (EOC) amount is not sufficient to fulfill the miscibility at 50/50 ratio, thereby leading phase separation that can cause premature break of the blend, therefore decreasing the stress.34 This result is in agreement with SEM observations. However, an increase in the strain at break is noticed up to 1096 % and 1177 % for 2575 ROC and 2575 SOC samples, respectively, in comparison with other samples. Moreover, the tensile strength is clearly reduced to 19.2 MPa and 18.6 MPa, respectively, but it is still higher than for pristine PBAT (i.e. 14.3 MPa).2 This improvement is probably due to the presence of expanded OC which acts as a reinforcing phase as well as a compatibilizing agent. The excess rosin or SA in the clay galleries can possibly react with the hydroxyl terminal groups of polyesters, forming PLA-PBAT mixed chains during the melt-blending process. On the other hand, the rigidity of bionanocomposites significantly decreased as expected when increasing the PBAT content in the blend, as shown in Figure 9b, even if the initial crystallinity was found to increase for these samples. However, it is worth noting that tensile tests were performed at room temperature, i.e. below the Tg of PLA, thus limiting the impact of its crystallinity.



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Figure 9. (a) Typical tensile stress-strain curves and (b) rigidity for pristine PLA and PLA/PBAT nanocomposites reinforced with 2.5 wt% of EOC. The antimicrobial activity for pristine PLA and its bionanocomposites with variable ratios of PBAT containing the same amount 2.5 wt% of ROC and SOC was performed to evaluate these materials for use in green packaging applications. The test was conducted against three different types of microbes: Pseudomonas aeruginosa (Gr-ve bacteria), Staphylococcus aureus (G+ve bacteria) and Candidia albicans (fungi). The obtained results reported in Figure 10 reveal that pristine PLA has no inhibition effect against all the microbes used in the experiment. Moreover, PLA/PBAT nanocomposites reinforced with SOC did not also seem


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to have any effect on the growth of bacteria or fungi. Their inhibition zones in the petri dish are almost zero for all selected microbes. These results obviously prove that the biological activity of these composites is not affected by the addition of OC treated with stearic acid. The same results have been reported by Moustafa et al31 when organoclay was used as nanofiller for ethylene-vinyl acetate (EVA) and ethylene propylene diene monomer (EPDM) blend. Correspondingly, halos of inhibition are observed for PLA/PBAT nanocomposites reinforced by ROC with different diameters ranging from 10 to 22 mm depending on the microbial and composite type as displayed in Figure 10 and Table 4. This improvement is attributed to the presence of the phenolic structures of rosin between OC layers, which are efficiently active in inhibiting the growth of a wide range of Gram-positive and Gramnegative bacteria, as well as fungi through interrupting microbial developments.26 From the figure, it can be also noticed that 5050 ROC composite has a large inhibitory zone as compared to 7525 ROC and 2575 ROC composites. The reason may be due to the lower compatibility and miscibility of PLA and PBAT in this blend (i.e. 50/50 wt%) leading to ROC nanoparticles migrating outside the polymer matrix and killing the microbes, causing a large inhibition zone. This result was confirmed by SEM analysis. The obtained results can open a route to produce new interesting antimicrobial composites for different purposes such as green packaging or bio-membranes for water treatments. Table 4. Antimicrobial activity for pristine PLA and its blends with different ratios of PBAT containing 2.5 w% of ROC or St. Acid against the three microbes. Diameter of inhibitory zone (mm) Sample

Pseudomonas Staphylococcus Candida aeruginosa

Pristine PLA 7525 ROC

aureus

albicans

0

0

0

10±0.3

12±0.1

13±0.4



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5050 ROC

15±0.6

18±0.4

22±0.2

2575 ROC

14±0.2

17±0.2

10±0.3

7525 SOC

0

0

0

5050 SOC

0

0

0

2575 SOC

0

0

0

Figure 10. Antimicrobial activity for pristine PLA and its blends with different ratios of PBAT containing 2.5 w% of ROC or St. Acid against Pseudomonas aeruginosa, Staphylococcus aureus and Candidia albicans. 24 Environment ACS Paragon Plus

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CONCLUSIONS In this work, toxicity-free nanocomposites were prepared from biodegradable PLA/PBAT blends and expanded organoclay (ROC or SOC) by melt blending. The structure of organoclay modified with the proposed novel facile green method was proven by XRD and FTIR, where the results exhibited that the interlayer spacing between the clay layers increased from 1.8 nm to approximately 3.9 and 4 nm for ROC or SOC, respectively. SEM images showed that a better compatibility and nanodispersion between PLA and PBAT were observed when 2.5 wt% of antimicrobial rosin OC was added, especially in the case of 7525 ROC or 2575 ROC samples. Also, TEM visualization proved the existence of intercalated structures in the case of 2575 ROC and 2575 SOC samples, whereas both intercalated and exfoliated structures existed in the polymer matrix when the PLA ratio was 75 wt% (for 7525 ROC and 7525 SOC samples), indicating the penetration ability of PLA short chains between clay layers leading to clay exfoliation and to the formation of nanocomposites. Otherwise, high PBAT content in the blend can drive to form the clay intercalation. Detailed DSC thermograms showed that the degree of crystallinity of PLA was significantly affected when increasing the PBAT content in the blend. Moreover, pure PBAT was tested in similar conditions and it exhibited a crystallization peak at 72.3°C and a degree of crystallinity of 17.8%. This crystallization process can be therefore attributed to the PBAT phase in the blends and should probably reject PLA chains in the amorphous region. A great enhancement in viscoelastic and tensile properties was noticed for bionanocomposites when ROC or SOC was used as a toxicity-free nanofiller as compared to pristine polymer. PLA/PBAT nanocomposites based on rosin acid (ROC) exhibited strong antimicrobial activities against Gram-positive and Gram-negative bacteria, as well as fungi as compared to pristine PLA or other nanocomposites. This research depicts a new utilization for rosin



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nanocomposites in promising applications such as food packaging, bio-membranes and biomedical purposes. AUTHOR INFORMATION Corresponding Author

∗ Alain Dufresne, Email: [email protected]. Address: The International School of Paper, Print Media and Biomaterials (Pagora), CS10065, 38402 Saint Martin d'Hères Cedex, France. Tel.: +33 476826995; fax: +33 476826933. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †,‡,§,∥ These authors contributed equally. ACKNOWLEDGMENTS The authors would like to gratefully acknowledge the Higher Ministry of Education & Scientific research, Cairo, Egypt and Laboratoire de Génie des Procédés Papetiers (LGP2), Grenoble, France, for the financial support and for the postdoctoral fellowship of H. Moustafa. LGP2 and LRP are part of the LabEx Tec 21 (Investissements d’Avenir - grant agreement n°ANR-11-LABX-0030) and the PolyNat Carnot Institut (Investissements d’Avenir - grant agreement n°ANR-16-CARN-025-01). The authors also thank Mr. Loïc Vidal for TEM experiments. REFERENCES (1) Meinander, K.; Niemi, M.; Hakola, J. S.; Selin, J. F. Polylactides-degradable Polymers for Fibres and Films. Macromol. Symp. 1997, 123, 147-153. (2) Moustafa, H.; Guizani, C.; Dupont, C.; Martin, V.; Jeguirim, M.; Dufresne, A. Utilization of Torrefied Coffee Grounds as Reinforcing Agent to Produce High-quality


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Table of contents

PLA/PBAT Bionanocomposites for Green Packaging: Morphological, Mechanical, Thermal and Antimicrobial Properties Hesham Moustafa,†,‡ Nadia El Kissi,§ Ahmed I. Abou-Kandil, † Mohamed S. Abdel-Aziz,∥ Alain Dufresne‡* †

Polymer Metrology & Technology Department, National Institute of Standards (NIS),

Tersa Street, El Haram, El-Giza, P.O Box 136, Giza 12211, Egypt. ‡

Univ. Grenoble Alpes, CNRS, Grenoble INP, LGP2, F-38000 Grenoble, France

§

Univ. Grenoble Alpes, CNRS, Grenoble INP, LRP, UJF, F-38000 Grenoble, France

∥ Microbial

Chemistry Department, National Research Centre El-Behoos St.33, Dokki-Giza

12622, Egypt.



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PBAT Bionanocomposites with Antimicrobial Natural Rosin for

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