Influence of Different Treated Cellulose Fibers on the Mechanical and

Research Article pubs.acs.org/journal/ascecg

Influence of Different Treated Cellulose Fibers on the Mechanical and Thermal Properties of Poly(lactic acid) Atul P. Johari, Smita Mohanty,* Surendra K. Kurmvanshi, and Sanjay K. Nayak Laboratory for Advanced Research in Polymeric Materials (LARPM), CIPET, B-25, C.N.I. Complex, Patia, Bhubaneswar, 751 024, India

ABSTRACT: Cellulose microfibrils (CMFs) were prepared from sisal fiber (SF) and characterized using Fourier transform infrared and X-ray diffraction analysis. Further, modification of CMF has been carried out using alkali and silane. Thereafter, modified as well as unmodified CMF reinforced poly(lactic acid) (PLA) biocomposites were fabricated using melt blending technique followed by injection molding. PLA has also been grafted with maleic anhydride and fabricated with unmodified CMF. Differential scanning calorimetry measurements confirmed that the addition of CMF accelerates the crystallization process of the PLA matrix. The NaOH treated SF, i.e. NCMF, reinforced biocomposites exhibited optimum mechanical strength which increased by 21.4% over that of CMF reinforced PLA biocomposites. A silane treated PLA biocomposite showed a maximum impact strength which was 24% higher than that of virgin PLA. The thermal stability of PLA/CMF biocomposites has been evaluated using thermogravimetric analysis. The scanning electron micrographs also confirmed the uniform dispersion of CMF within the PLA matrix. KEYWORDS: CMF, Sisal, DSC, Crystallinity, Modulus

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INTRODUCTION In the recent years, natural fiber reinforced biodegradable biocomposites have attracted a great deal of attention in the development of ecofriendly materials in various sectors from automobiles, aerospace, building, and construction to packaging. Among biodegradable polymers, poly(lactic acid) (PLA) is a front runner in the field of biodegradable biocomposites due to its renewability; it is an excellent substitute for petrochemical-based polymers. However, its inherent brittleness, limited thermal stability during processing, and low heat deflection temperature (HDT) have been the major impediment which has restricted its use for various widespread applications. Use of natural fibers as reinforcement in the polymer matrix has been the subject of research in the current years because of its low cost, abundant availability, low density, high specific properties, and lack of residues upon incineration.1 Various types of natural fibers such as flax, sisal, banana, jute, © 2016 American Chemical Society

hemp, kenaf, and bamboo are used as reinforcement for the PLA matrix.2,3 The cellulose fibers derived from natural fibers have been used as reinforcement for the PLA matrix for the production of the biocomposites with improved performance characteristics.4−8 Despite its many advantages, cellulose is accompanied with a great number of hydroxyl groups, which leads to strong polarity and high water absorption and hinders their applicability as a reinforcer for biocomposites.9 Therefore, modification of cellulose has been necessary to overcome this problem of high water absorption. Various forms of cellulose such as microcrystalline cellulose or nanodimensional cellulose, cellulose nanowhiskers (CNWs), and cellulose microfibrils (CMFs) Received: November 23, 2015 Revised: January 9, 2016 Published: January 27, 2016 1619

DOI: 10.1021/acssuschemeng.5b01563 ACS Sustainable Chem. Eng. 2016, 4, 1619−1629

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ACS Sustainable Chemistry & Engineering Table 1. Physical Properties of Poly(D,L-lactic acid)

a

material

grade

% D- isomer

% L- isomer

density (g/cm3)

Mwa (kg/mol)

MFIb (g/10 min)

Tgc (deg C)

PLA

4042D

5

95

1.24

1.65

1.5−5.0

57

Weight-average molecular weight. bMelt flow index at 190 °C, 2.16 kg load. cGlass transition temperature.

have been explored as a reinforcing agent.10 Among various forms of cellulose, CMF found its suitability as an effective reinforcement for the biopolymers.11 Oksman et al. and Mofokeng et al. have reported CMF reinforced PLA biocomposites and an enhancement of mechanical properties as compared to the traditional CMF/ polypropylene (CMF/PP) composites.12,13 The poor dispersion of CMFs in the PP matrix might be the determinant in the mechanical properties.14,15 Also the CMF containing polar hydroxyl groups on the surface form poor interfaces with the nonpolar PP matrix, resulting in inefficient stress transfer under load leading to the deterioration of the mechanical strength and stiffness.16 Huda et al. have reported CMF/Talc/PLA and CMF/Talc/PP hybrid biocomposites and found an increase in the mechanical properties. They have also modified fillers through silane treatment and incorporated these into PLA and PP. The properties of PLA biocomposites increased significantly in terms of stiffness. Generally, organosilane coupling agents can maximize the compatibility between a polymer matrix and filler by improving interaction at the polymer/ mineral interface and dispersion of particles.17 Therefore, an improvement of the interfacial adhesion between natural fibrils and biodegradable polymers is essentially important. Various studies have been carried out to enhance the interfacial strength of the CMF/PLA through various treatment processes as silation, acetylation, carboxymethylation, and grafting of the PLA matrix.18 In this study, CMF has been extracted from sisal fiber and evaluated for its reinforcing ability within the PLA matrix. A low concentration of CMF was preferred to modify the brittle nature of PLA biocomposite. Chemical grafting onto the CMF has been carried out to improve the compatibility with the PLA matrix. The morphology was studied using Fourier transform infrared (FTIR) spectroscopy, wide angle X-ray diffraction (WAXD), and scanning electron microscopy (SEM). Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and heat deflection temperature (HDT) analysis were carried out in order to investigate the thermal properties of the prepared biocomposites. The mechanical properties in terms of tensile strength, tensile modulus, percentage elongation, and impact strength of the PLA and its biocomposites have also been evaluated.

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Subsequently, 70 g of SF was taken and washed with distilled water, dried in air for 2 days, and cut into 2−5 mm length using electronic fiber cutting machine. The isolation of CMF from SF was carried out using a combination of chemical treatment as depicted in Figure 1.

Figure 1. Extraction of cellulose microfibrils. Initially the detergent treated SF, taken as untreated SF (UTS) were mercerized by immersing the fibers in 0.5 M NaOH (1:10) solution under mechanical stirring at 30 °C for 18 h. Then, the insoluble residue obtained was bleached with 3 wt % H2O2 solution at 45 °C for 14 h. In the second stage, the obtained residue was again treated with 2 M NaOH solution at 55 °C for 2 h to remove the mineral traces. The mercerization process was carried out to solubilize the pectin and the hemicelluloses.20 Whereas the bleaching treatment was performed to breakdown phenolic compounds or molecules having chromophoric groups present in lignin and to remove the byproducts as well as to whiten the pulp.21 During bleaching, lignin was oxidized and became soluble in the alkaline medium. At each stage of the different treatments, the insoluble residue was extensively washed with distilled water for several hours, until pH was neutral22 and was then centrifuged to obtain mercerized SF (NCMF). Subsequently, the centrifuged residue was subjected to controlled acid hydrolysis by immersing the fibers in 50 wt % sulfuric acid (H2SO4) solution under mechanical stirring at 45 °C for 3 h. After hydrolysis, the mixture was diluted 10-fold with distilled water and neutralized with a 0.5 M NaOH solution, followed by extraction of precipitate by centrifugation. The aqueous suspension of the residue was dialyzed against deionized water and sonicated for 30 min in an ultrasonic bath. This suspension was then freeze-dried using the process reported in ref 23. Further, the freeze-dried residue obtained from the chemical process was dried at 60 °C for 24 h, followed by ball milling in a 50 mL cap (Cryomill, M/s Retch, Germany) to obtain (28 g, 40%) yield of CMF.24 For silane treatment (GPS), the above fibrils were then immersed in 3 wt % of GPS solution for 3 h at room temperature and were then washed and dried at 80 °C in an oven. The obtained silane treated CMF has been designated as GCMF. Fabrication of PLA/CMF Biocomposite. PLA was melt blended with 5, 10, and 15 wt % of CMF using microcompounder (DSM

EXPERIMENTAL SECTION

Materials. Poly(lactic acid) (PLA) was purchased from Cargill Dow, USA. As per the manufacturer specification, the physical properties of poly(D,L-lactic acid) are given in Table 1. Sisal fibers (Agave sisalana) having a density of 1.5 g/cm3 were obtained from Sheeba fibers and handicrafts, Poovancode, Tamilnadu, India. Common chemicals such as sodium hydroxide (NaOH), sodium hypochlorite (NaClO), and acetone of AR grade were procured from Merck Specialties Pvt. Ltd., Mumbai, India. Other chemicals such as glycidoxypropyltrimethoxysilane (GPS) and maleic anhydride (MAH) were obtained from Merck Specialties Pvt. Ltd., Mumbai, India. Preparation of Cellulose Microfibrils. Sisal fibers (SFs) are bonded into bundles by lignin, pectin, and other natural substances.19 SF, in the form of bundles, was scoured in mild detergent solution at 30 °C for about 24 h to remove dust and other impurities. 1620


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ACS Sustainable Chemistry & Engineering

Figure 2. Wide angle X-ray diffraction analysis of sisal fiber (UTS, NCMF, CMF, and GCMF). Xplore 15 mL, Netherlands). Prior to compounding, CMF and PLA were pre dried at 60 and 80 °C in a vacuum oven for 12 h. The compounding was carried out at 180 °C with a rotor speed of 50 rpm for 4 min. The melt mixes obtained were used for the preparation of specimens by employing a mini injection jet (model DSM, Netherlands). The dicumyl peroxide (DCP) was set to be very low (0.2%) to avoid the side reactions and the degradation of the PLA. Based on mechanical observations 5 wt % of CMF loading was optimized and biocomposites of PLA/5 wt % NCMF, PLA/5 wt % GCMF and MAH grafted PLA with CMF was prepared using the above-mentioned process parameters. The grafting yield was determined by titration method. The grafted sample (1.15 ± 0.01 g) of fabricated biocomposite was dissolved in 150 mL of chloroform: methanol (3:2 v/v), and 8−10 drops of 1% phenolphthalein in ethanol were added. The titration was carried out with 0.03 N of potassium hydroxide (KOH) in methanol, and the pH of the samples at end point was determined.25 The percentage of GPS grafting was calculated using eqs 1−3, and ultimate grafting ratio was about 1.89%.

normality of KOH =

g KHP × Purity of KHP (%fraction) 204.22 × L KOH

⎛ KOH ⎞ mL KOH × N KOH × 56.1 acid number⎜mg ⎟= g ⎠ g polymer ⎝ % GPS grafting =

236.34 × acid number 2 × 561

percentage of crystallinity (% Cr) was calculated using the following formula:26 %Cr =

I22.5 × 100 I22.5 + I16

(4)

Where, I22.5 and I16 are the crystalline and amorphous intensities at 2θ scale close to 22.5° and 16°, respectively. Fourier Transform Infrared Spectroscopy. All the samples were subjected to FTIR analysis to determine the interfacial bonds in CMF, modified CMF, PLA, and its biocomposites. Spectra were obtained at 4 cm−1 resolution and 64 consecutive scans within the standard wavenumber range from 400 to 4000 cm−1. All the samples were dried under vacuum at 60 °C for 12 h before testing. Scanning Electron Microscopy. The morphological analysis of fibers (UTS, CMF, NCMF, and GCMF) and impact fractured specimens of PLA biocomposites was carried out using SEM analysis (model EVO MA 15, Carl Zeiss, SMT, Germany). Prior to imaging, the samples were sputter coated with platinum and dried for half an hour at 70 °C in vacuum. Transmission Electron Microscopy. The particle size of the CMF was assessed using a TEM instrument (model JEOL 1400) operating at 80 kV. A drop of a diluted suspension of CMF was placed step by step on a copper grid coated with a thin carbon film and allowed to dry at room temperature and was subjected to TEM measurement. Mechanical Analysis. Tensile properties of PLA and its biocomposites were measured as per ASTM D 638 test standard using a Universal Testing Machine (Model Instron 3382, UK) with a solid fixture at a tensile speed of 5 mm/min. The tensile gauge length was fixed at 100 mm. Dumbbell-shape specimens of dimensions 150 mm × 12 mm × 3 mm were subjected to tensile strength measurement with the help of

(1)

(2)

(3)

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CHARACTERIZATION Wide Angle X-ray Diffraction Analysis. The WAXD analysis of fibers and extracted CMF has been characterized using a copper target of X-ray diffractometer model Schimadzu 7000L, Japan, (Cu Kα, radiation with λ = 0.15406 nm) at a scanning rate of 0.5°/min and diffraction angle of 5−40°. The 1621


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ACS Sustainable Chemistry & Engineering extensometer to get more precise results. An average of five samples has been taken for each analysis and the data reported are from the average of five. The impact strength of the PLA and its biocomposites has been performed using an impact strength tester (model 104, IT 504 plastic impact, 899 (Notch cutter) Tinius Olsen, USA). The specimens of size 65 mm × 12 mm × 3 mm, and 1.54 mm deep standard V-notch was engraved on each sample. The test was performed at room temperature, and the data reported are from the average of five. Differential Scanning Calorimetry Analysis. Thermal transitions of PLA and its biocomposites were studied using DSC (Q20, TA Instruments USA). Samples of ≤7 mg were heated from room temperature to 200 °C, then held for 5 min to remove the previous thermal history and cooled to 40 °C at a rate 10 °C/min, and reheated again to 200 °C with the same rate under N2 atmosphere. Corresponding glass transition (Tg), melting (Tm), cold crystallization (Tcc), heat of fusion (ΔHm), and degree of crystallinity of the materials were noted. The Tg was determined using a tangent method; whereas, the values of Tm, Tcc, and ΔHm were calculated from the curves of DSC thermogram. The degree of crystallinity Xc % of PLA and its biocomposites was calculated from the following eq 5. Xc(%) =

ΔHm × 100 wΔHm0

corresponds to the crystallites obtained as a result of disruption of CMF during acid hydrolysis process. Hence, a highly crystallized CMF has been obtained. The peaks of GCMF were observed around 2θ = 15° and 24°. The percentage crystallinity of UTS, NCMF, CMF, and GCMF was calculated using eq 4 and found to be 71%, 74%, 77%, and 82%, respectively. The increase in the crystallinity of GCMF may be attributed to the removal of amorphous phases of UTS by GPS modifications.30 FTIR Analysis of UTS, NCMF, CMF, and GCMF. The FTIR analysis has been utilized to elucidate the chemical structure of UTS, NCMF, CMF, and GCMF, and the results are shown in Figure 3. It is inferred from the figure that a broad

(5)

Where, ΔHm and ΔHmo are the melting enthalpies for PLA biocomposites and 100% crystalline PLA, respectively; w is the mass fraction of PLA in the biocomposite. The melting enthalpy of 100% crystalline PLA (ΔHmo) was considered to be 93 J/g. An overall accuracy of ±0.5 °C in temperature and ±1% in enthalpy was estimated. Thermogravimetric Analysis. Thermal stability of all the samples was evaluated using TGA (model Q50, TA Instruments USA). Samples of about 7 mg were heated from 30 to 600 °C at a rate of 10 °C/min under constant flow of nitrogen atmosphere. Corresponding weight loss vs temperature was noted. Heat Deflection Temperature. The HDT analysis of PLA and its biocomposites was conducted using a HDT analyzer (HV-2000A-C3, M/s GoTech, Taiwan), according to ASTM D 648. The specimens were analyzed in flexural mode using rectangular specimens of dimension 127 mm × 12.7 mm × 3 mm with a load of 0.455 MPa and a heating rate of 2 °C/min.

Figure 3. FTIR Analysis of sisal fiber (UTS, NCMF, CMF, and GCMF).

peak is observed at 3316 cm−1 in UTS, which corresponds to the O−H stretch (H bonded). After NaOH and silane treatment, the intensity of broad peak decreased, which revealed the removal of hydrophilic phases such as lignin, pectin, hemicellulose, etc. Further, the peak at 1611, 1724, and 1237 cm−1 assigned for C−C stretch (aromatic) and the CO stretch of unsaturated esters present in UTS. The intensities of these peaks almost disappeared in all the cases, i.e. NCMF, CMF, and GCMF, which further indicates the removal of lignin and wax content from the UTS.31−37 FTIR Analysis of PLA and its Biocomposites. FTIR spectroscopy was used to investigate the effect of reinforcement of CMF, GCMF, NCMF, and maleic anhydride onto PLA. The FTIR spectra of PLA and its biocomposites are shown in Figure 4. For the PLA, the strong absorption band at 1756 cm−1 appears in the spectrogram, which is assigned for CO group. The lower intensity peak at 2922 and 2351 cm−1 correspond to the symmetric stretching vibration of C−H group and O−H stretching of carboxyl acid group. The intense peak at 1368 and 1448 cm−1 are assigned for the CH3 bending vibration and CH2 bending, respectively. The peaks in the range of 1000−1200 cm−1 corresponds to the C−O stretch C in combination with C−C group. It is noteworthy to see that the intensity of peaks at 2922 and 2351 cm−1 decreased in the case of PLA/5 wt % CMF/5 wt % MAH, PLA/5 wt % NCMF, and PLA/5 wt %

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RESULTS AND DISCUSSION WAXD Analysis of UTS, NCMF, CMF, and GCMF. The WAXD patterns of UTS, NCMF, CMF, and GCMF are given in Figure 2. The Bragg’s angle was scanned from 5 to 40°. Sharp peaks were observed at 2θ = 22.5°, and a shoulder was observed around the region 2θ = 12° to 20° in all the samples which corresponds to the cellulose structure.27 The XRD peaks of UTS were observed at 2θ values of 16° and 22.25° whereas the shoulder peaks of CMF, NCMF, and GCMF were observed at 2θ values of 16°, 16°, and 15° respectively. Mercerization of UTS leads to the swelling of the fiber and subsequent increase in the absorption of moisture. Treatment with alkali leads to the removal of cementing materials like lignin, hemicellulose, and pectin which results in an increase in the percentage crystallinity of the NCMF.28 Also, NCMF shows distinct diffraction peaks centered at 2θ = 16° and 22.5°, corresponding to a typical form of cellulose I.29 Another peak of medium intensity at 32° and 34° has also been observed which 1622


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SEM Analysis of UTS, NCMF, CMF, and GCMF. Figure 5 exhibited SEM micrographs of UTS, NCMF, CMF, and GCMF. SEM morphology study was carried out to determine the surface roughness and texture of fiber to adhere the surface of the matrix. Figure 5a showed a smooth surface morphology with compact packing of components such as lignin, pectin, hemicellulose, wax, etc. The surface smoothness has a negative effect on the interfacial properties between the fiber and a matrix since it precludes physical bonding. In the case of NCMF, a rough surface topology was detected as compared with UTS, Figure 5b. A clear demonstration of the defibrillation and depolymerization in NCMF could also be observed from the image, wherein each fiber is composed of several microfibrils with diameters in the range of 90−120 μm.42 However, the important modification done in CMF wherein NaOH treatment followed by acid hydrolysis lead to the disruption of hydrogen bonding in the fiber network structure takes place and further increases the surface roughness of CMF as compared to both UTS and NCMF depicted in Figure 5c. This treatment removes lignin, wax, oils, and pectin covering the external surface of the fiber cell wall, depolymerises cellulose, and exposes the short length crystallites.43,44 As a result, the adhesive characteristics of the fiber surface can be enhanced.45 The wrinkled surface of GPS treated fibers increased the surface area, which would provide more efficient contact with the matrix polymer. As evident from Figure 5d, a tiny thin film detached from GCMF and further improved the surface roughness as compared to the UTS, CMF, and NCMF. Surface Morphology of PLA and its Biocomposites. Figure 6 reveals the SEM micrographs of impact fractured specimens of the virgin PLA (VPLA), PLA/5 wt % NCMF, PLA/5 wt % CMF/5% MAH, and PLA/5 wt % GCMF biocomposites. The fractured surface of PLA Figure 6a could be categorized as smooth and brittle fracture in which molecules are detached neatly whereas all the biocomposites shows more

Figure 4. FTIR analysis of PLA and its biocomposites.

GCMF. This observation is an indication toward the strong interaction between the filler and PLA matrix.38−40 In the case of PLA/5 wt % GCMF the peaks at 763 cm−1, which are also assigned for CH3 bending, have been shifted indicating the good interaction between PLA and GCMF. Moreover, in PLA/ 5 wt % CMF/5 wt % MAH, the peak at 1266 cm−1, which is assigned for C−O−C disappeared.41 This indicates excellent grafting of MAH into the 5 wt % CMF containing PLA. Again, in case of PLA/5 wt % NCMF, the peaks at 2922 cm−1 (C−H stretching vibration) and at 2351 cm−1 (O−H stretching) almost disappeared. These results indicate the nucleating effect of NCMF which improves the compatibility between PLA and NCMF.

Figure 5. Surface morphology of sisal fiber: (a) UTS, (b) NCMF, (c) CMF, (d) GCMF. 1623


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Figure 6. Surface morphology of CMF reinforced PLA and its biocomposites: (a) VPLA, (b) PLA/NCMF, (c) PLA/CMF/MAH, (d) PLA/GCMF.

observations, we have chosen 5 wt % CMF composition as optimum loading criteria for the fabrication of PLA biocomposites with modified CMF, i.e., NCMF, GCMF, and MAH grafted PLA with CMF. After modification of CMF and MAH grafting with PLA significantly improved the mechanical properties of biocomposites as well as VPLA. The tensile modulus of PLA biocomposites increased from 2657 MPa to 2552, 3610, 3097, and 3500 MPa respectively, in the case of PLA/5 wt % CMF, PLA/5 wt % NCMF, PLA/5 wt % CMF/5 wt % MAH, and PLA/5 wt % GCMF. The increase in the tensile modulus of PLA reinforced with modified CMF is primarily attributed to the increased intermolecular H bonding between the OH groups of CMF and >CO groups of PLA matrix.50 As evident from the XRD section, the relative crystallinity index of CMF obtained was 82% of cellulose which might have also contributed to the increase in the tensile modulus of all biocomposites.51 Similarly in the case of MAH grafted PLA and GPS treated CMF incorporated PLA biocomposites, the tensile strength of VPLA increased from 59.58 MPa to 55.35, 67.22, 62.33, and 65.46 MPa, in PLA/5 wt % CMF, PLA/5 wt % NCMF, PLA/5 wt % CMF/5 wt % MAH, and PLA/5 wt % GCMF, respectively. The maximum tensile strength was observed in PLA/5 wt % NCMF. This may be due to the improved interfacial strength between PLA and NCMF. The NCMF provides improved surface interlocking with the PLA matrix due to removal of waxes, lignin and pectin from UTS. The same has already been confirmed from SEM micrographs. In case of PLA grafted with MAH marginal increment in tensile strength from 59.58 to 62.33 MPa was observed which may be due to the improved dispersion and H bonding between CMF and PLA matrix. The anhydride group grafted on the molecular chain of PLA matrix might have reacted with hydroxyl groups from CMF to form ester linkages. The carboxylic group, arising from the hydrolyzed anhydride, also forms hydrogen bonding with the hydroxyl group of CMF.

surface roughness with some aggregation. As observed from Figure 6b typical fractographic features of a brittle fracture and agglomerated CMF can be easily seen with the irregular surface and holes. This is a clear indication of poor dispersion of CMF within the PLA matrix. Compared with PLA/5 wt % CMF, a better wetability of the CMF within the PLA matrix was achieved after GPS modification as shown in Figure 6d. In the case of PLA/5 wt % GCMF, the fractured surface indicated that the fibers are completely coated with the matrix, and there were no indications of fiber pullout. However, in case of PLA/5 wt % CMF/5 wt % MAH, fiber−matrix interfacial failures followed by extensive fiber pullout from the matrix as compared to the PLA/5 wt % GCMF has been observed in Figure 6c.46 TEM: Fiber Size Distribution and Aspect Ratio. CMFs were examined by TEM to analyze the size of the fibrils and their aspect ratio Figure 7. From Figure 7a, rodlike cellulose microfibrils with a length of 200 nm, diameter of 15 nm, and an aspect ratio of 13 were observed. A statistical analysis of fiber size distribution is shown in Figure 7b. It is reported that an aspect ratio higher than 10 is considered as the acceptable value for good stress transfer from the matrix to the fibers which influences the properties of the composites.47 However, in the case of other treated fibers like GCMF, it is concluded that silane enveloped the CMF surface and does not alter the aspect ratio of the fibrils.48 Mechanical Properties of PLA and its Biocomposites. The mechanical properties of PLA and its biocomposites are depicted in Table 2. As observed from the results of mechanical analysis, PLA/10 wt % CMF and PLA/15 wt % CMF shows poor tensile strength and tensile modulus as compared to the VPLA and PLA/5 wt % CMF biocomposites. The maximum strain of all biocomposites was slightly lower than VPLA. The deterioration of the overall mechanical properties could be due to the entangling effect of CMF, which may result in incompatibility between PLA and CMF.49 Based on these 1624


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Figure 7. (a) TEM micrograph of CMF. (b) Statistical analysis of fiber length and size distribution.

Table 2. Mechanical Properties of PLA and its Biocomposites sample type PLA PLA/5 wt % CMF PLA/10 wt % CMF PLA/15 wt % CMF PLA/5 wt % NCMF PLA/5 wt % CMF/5 wt % MAH PLA/5 wt % GCMF

tensile strength (MPa) 59.58 55.35 49.12 50.65 67.22 62.33 65.46

± ± ± ± ± ± ±

tensile modulus (MPa)

3.2 2.1 2.8 1.9 4.6 3.3 2.0

2657 2552 2930 3126 3610 3097 3500

± ± ± ± ± ± ±

032 102 068 047 156 059 097

strain at break (%) 5.62 4.00 3.07 2.64 3.68 5.00 4.28

± ± ± ± ± ± ±

1.3 1.1 1.2 1.6 1.4 2.1 2.6

impact strength (J/m) 27.70 30.65 27.89 26.34 28.92 32.85 34.39

± ± ± ± ± ± ±

3 2 1 3 1 3 4

that 5 wt % MAH grafted PLA with 5 wt % CMF biocomposites achieved optimum toughness as compared with VPLA as well as other biocomposites. This improvement may be attributed to the plasticization effect of MAH. DSC Analysis of PLA and its Biocomposites. The DSC thermogram of PLA and its biocomposites is depicted in Figure 8, and its corresponding data is represented in Table 3, showing that there was no significant increment in the glass transition temperature (Tg) of PLA in the case of PLA/5 wt % CMF biocomposite. However, the Tg of modified PLA biocomposites viz. PLA/5 wt % NCMF, PLA/5 wt % CMF/5 wt % MAH, and PLA/5 wt % GCMF was found to increase with respect to PLA. This observed enhancement in the Tg suggests that the modified CMF favors the improvement of the thermal stability

However, in case of PLA/5 wt % GCMF biocomposites, the GPS hydrolyzes which bonds with the OH group in CMF. Further, the epoxy end group on GPS grafted onto the CMF react with the carboxyl group of PLA matrix, which significantly increases the adhesion at the interface. The percentage of elongation of VPLA was found to be 5.62% which decreased with the incorporation of CMF. The incorporation of CMF also decreases the ductility of PLA due to heterogeneous dispersion of CMF within the PLA matrix. More clearly in the PLA/5 wt % NCMF biocomposites in which the elongation was dropped by around 34% with respect to VPLA. This may be due to the formation of stronger bonds between the highly crystalline cellulose which inhibit the percentage of elongation.52 Furthermore, it could be observed 1625


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ature shows the reinforcing effect of CMF within PLA matrix in case of all biocomposites. Further, the degree of crystallinity (Xc %) calculated using eq 5 is higher for PLA biocomposites than PLA. The Xc % of PLA/ 5 wt % CMF/5 wt % MAH and PLA/5 wt % GCMF was observed to be higher side as compared to the PLA/5 wt % CMF and PLA/5 wt % NCMF. This observed increment in Xc % is attributed to the grafting of MAH and GPS leading to the ordering of molecular arrangement which promotes the nucleating effect induced by CMF. Also, both MAH and GPS plasticizes the PLA and increase the chain mobility and hence the crystallization. Therefore, the combination of nucleating and plasticization can be the detrimental factors for the enhancement of Xc % of PLA. Also, the transcrystallization morphology is formed by a dense heterogeneous nucleation of PLA crystals at CMF surface and contributed toward an increase in the Xc %.60−62 The interaction between GPS and MAH with PLA is shown in Schemes 1 and 2. Scheme 1. Reaction Mechanism of Glycidoxypropyltrimethoxysilane (GPS), CMF, and PLA Biocomposites

Figure 8. DSC analysis of CMF reinforced PLA biocomposites.

and HDT of the PLA biocomposites.53,54 Further, the exothermic peak observed from the thermograms of the biocomposite represent a cold crystallization (Tcc), obtained during heating cycle. This peak is due to the reorganization of amorphous domains into crystalline regions on account of the increased macromolecular flexibility and mobility upon increasing temperature. On the other hand, the lack of peak in the cooling curve indicates that the polymer chains are not able to form any crystalline structure at a cooling rate of 10 °C/ min. The Tcc value of all biocomposites was found to be lower side than that of PLA. The lower value of Tcc noticed in the heating run of PLA induced by modified CMF which acts as nucleating agent for PLA and favors controlled cold crystallization.55 The melting transition (Tm) of PLA was observed to be 153.41 °C, whereas its biocomposite showed double melting peaks, i.e. Tm1 and Tm2. This behavior is probably because of the loss of chain alignment and conformational purity of PLA matrix during melt mixing process with CMF. The highest value of Tm2 was found in PLA/5 wt % NCMF, which might be attributed to the formation of more perfect crystalline structure. On the other hand, shoulder peak at lower temperature Tm1 is assigned for less perfect or disordered crystalline structure which might be assigned for α′ form.56−59 The value of ΔHm and ΔHmc of modified CMF biocomposites were observed to be increased as compared to the PLA. These values showed that the modified CMF enhanced the overall crystallinity of the PLA biocomposites. Furthermore, the increase in melting temper-

TGA Analysis of PLA and its Biocomposites. The thermal stability of PLA and its biocomposites is depicted in Figure 9 and also summarized in Table 4. The data reported in table represents T10, T50, Tf, and % residual weight at 600 °C of

Table 3. DSC Data on PLA and Biocomposites Obtained by Heating and Cooling Scans sample type PLA PLA/5 PLA/5 PLA/5 PLA/5

wt wt wt wt

% % % %

CMF NCMF CMF/5 wt % MAH GCMF

Tga(deg C)

Tccb(deg C)

Tm1c (deg C)

Tm2d (deg C)

ΔHme (J/g)

ΔHmcf (J/g)

Xc % g

59.78 59.41 63.63 62.93 64.89

123.53 117.76 115.00 097.48 120.10

153.41 150.00 156.39 140.79 155.49

155.48 162.85 153.83 162.00

27.21 25.37 26.78 36.58 33.75

20.27 24.12 21.39 29.39 30.20

21.79 25.93 23.00 31.60 32.47

a

Glass transition temperature. bCrystallization temperature. cMelting enthalpy. dCrystallization enthalpy. eFirst melting temperature. fSecond melting temperature. gDegree of crystallinity. 1626


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ACS Sustainable Chemistry & Engineering Scheme 2. Reaction Mechanism of MAH, CMF, and PLA Biocomposites

Table 4. TGA Results of PLA and its Biocomposites sample type

T10a (deg C)

T50b (deg C)

Tfc (deg C)

residual wt % at 600 °C

311 319 320 306

339 349 351 347

359 368 374 365

0 2.37 2.65 1.25

318

354

375

2.86

VPLA PLA/5 wt % CMF PLA/5 wt % NCMF PLA/5 wt % CMF/ 5 wt % MAH PLA/5 wt % GCMF a

10% of weight loss. temperature.

b

50% of weight loss. cFinal decomposition

TGA/DTG thermogram, the VPLA and its biocomposites exhibit a single step degradation pattern. VPLA shows T10, T50, and Tf at 311, 339, and 359 °C. However, in case of all biocomposites the weight loss started at 100−110 °C, which is primarily attributed to the residual moisture present in CMF. Incorporation of 5 wt % of CMF within PLA matrix enhanced the thermal stability (T10) by 8 °C. PLA/5 wt % CMF and PLA/5 wt % NCMF biocomposite displayed the T10, T50, and Tf at 319, 349, and 368 and 320, 351, and 374 °C, respectively, which indicated intermolecular H bonding between the OH groups of CMF and >CO groups of PLA matrix. Similarly, PLA/5 wt % GCMF biocomposite displayed the T10, T50, and Tf at comparatively higher temperature, i.e. 318, 354, and 375 °C, which may be due to reactive compatibilization induced by the GPS. In case of PLA/5 wt % CMF biocomposites with 5 wt % MAH, T10, T50, and Tf were observed at comparatively lower temperature. The peak temperature of PLA/5 wt % CMF biocomposites decreased by 5 °C with the incorporation of MAH. Similarly, the T50 as well as Tf also decreases but it was considerably higher than the degradation temperature and nearer to the decomposition temperature of MAH. Therefore, it was assessed that there was practically no loss of MAH during processing, via volatilization.63 The CMF enhances the thermal stability of PLA biocomposites as well as assist in the formation of char after thermal decomposition. In all biocomposites, the percentage char was more than VPLA, which also indicated improved flame retardancy of CMF reinforced PLA matrix. The PLA/5 wt % GCMF biocomposites displayed highest char residue of 2.86% as compared to the other biocomposites. This behavior is probably due to the presence of GCMF.64 HDT Analysis of PLA and its Biocomposites. The HDT values of VPLA, PLA/5 wt % CMF, PLA/5 wt % CMF/5 wt % MAH, PLA/5 wt % GCMF, and PLA/5 wt % NCMF have been represented in Table 5. It is observed that the HDT of PLA increases with the incorporation of 5% CMF, 5% GCMF, and 5% NCMF fibers, suggesting that these fibers have a good effect on improving the thermal resistance of VPLA. The HDT value of VPLA with 5 wt % fiber increases to the tune of 5 wt % as compared to VPLA matrix. In case of 5 wt % MAH incorporation with 5 wt % CMF in PLA it seems to decreased Table 5. Heat Deflection Properties of PLA and its Biocomposites Figure 9. Thermal degradation behavior of PLA and its biocomposites. (a) TGA of PLA and its biocomposites. (b) DTG of PLA and its biocomposites.

sample type VPLA PLA/5 PLA/5 PLA/5 PLA/5

PLA and its biocomposites. T10 refers 10% of weight loss, while T50 refers 50% of weight loss decomposition temperature, and Tf is the final decomposition temperature. As observed from the 1627

wt wt wt wt

% % % %

temperature (deg C)

CMF NCMF CMF/5 wt % MAH GCMF

53.5 55.5 56.3 53.0 59.7


Research Article


(8) Zainudin, E. S.; Sapuan, S. M.; Abdan, K.; Mohamad, M. T. M. Dynamic Mechanical Behaviour of Banana-pseudostem-filled Unplasticised Polyvinyl Chloride Composites. Polym. Polym. Comp. 2009, 17, 55−61. (9) Li, S. Z.; Xiao, M. M.; Zheng, A.; Xiao, H. N. Cellulose microfibrils grafted with PBA via surface-initiated atom transfer radical polymerization for biocomposite reinforcement. Biomacromolecules 2011, 12, 3305−3312. (10) Samir, M. A. S. A.; Alloin, F.; Dufresne, A. Review of Recent Research into Cellulosic Whiskers, their Properties and Their Application in Nanocomposite Field. Biomacromolecules 2005, 6, 612−626. (11) Pan, M.; Zhou, X.; Chen, M. Cellulose Nanowhiskers Isolation and Properties from Acid Hydrolysis Combined with High Pressure Homogenizatio. BioResources 2012, 8, 933−943. (12) Oksman, K.; Skrifvars, M.; Selin, J. F. Natural fibres as reinforcement in polylactic acid (PLA) composites. Compos. Sci. Technol. 2003, 63, 1317−24. (13) Mofokeng, J. P.; Luyt, A. S.; Tabi, T.; Kovacs, J. Comparison of injection moulded, natural fibre-reinforced composites with PP and PLA as matrices. J. Thermoplast. Compos. Mater. 2012, 25, 927−948. (14) Mathew, A. P.; Oksman, K.; Sain, M. Mechanical properties of biodegradable composites from poly lactic acid (PLA) and microcrystalline cellulose (MCC). J. Appl. Polym. Sci. 2005, 97, 2014−2025. (15) Wang, T.; Drzal, L. T. Cellulose-nanofiber-reinforced poly(lactic acid) composites prepared by a water-based approach. ACS Appl. Mater. Interfaces 2012, 4, 5079−85. (16) Arrakhiz, F. Z.; Malha, M.; et al. Tensile, flexural and torsional properties of chemically treated alfa, coir and bagasse reinforced polypropylene. Composites, Part B 2013, 47, 35−41. (17) Huda, M. S.; Drzal, L. T.; Mohanty, A. K.; Misra, M. The effect of silane treated- and untreated-talc on the mechanical and physicomechanical properties of poly(lactic acid)/newspaper fibers/talc hybrid composites. Composites, Part B 2007, 38, 367−379. (18) Abdul Khalil, H. P. S.; Davoudpour, Y.; Islam, M. N.; Mustapha, A.; Sudesh, K.; Dungani, R.; Jawaid, M. Production and modification of nanofibrillated cellulose using various mechanical processes: A review. Carbohydr. Polym. 2014, 99, 649−665. (19) Herrera-Franco, P. J. H.; Valadez-Gonzalez, A. V. A study of the mechanical properties of short natural-fiber reinforced composites. Composites, Part B 2005, 36, 597−608. (20) Camano, S.; Behary, N.; Vroman, P.; Campagne, C. Comparison of Bio and Eco-technologies with Chemical Methods for Pre-treatment of Flax Fibers: Impact on Fiber Properties. J. Eng. Fibers Fabrics 2014, 9, 56−68. (21) Christov, L.; Van Driessel, B. Waste water bioremediation in the pulp and paper industry. Indian J. Biotechnol. 2003, 2, 444−450. (22) Zuluaga, R.; Putaux, J. L.; et al. Cellulose microfibrils from banana rachis: Effect of alkaline treatments on structural and morphological features. Carbohydr. Polym. 2009, 76, 51−59. (23) Elanthikkal, S.; Gopalakrishnapanicker, U.; et al. Cellulose microfibres produced from banana plant wastes: Isolation and characterization. Carbohydr. Polym. 2010, 80, 852−859. (24) Bhatnagar, A.; Sain, M. Processing of cellulose nanofiberreinforced composites. J. Reinf. Plast. Compos. 2005, 24, 1259−1268. (25) Hwang, S. W.; Lee, S. B.; et al. Grafting of maleic anhydride on poly (L-lactic acid): Effects on physical and mechanical properties. Polym. Test. 2012, 31, 333−344. (26) Kaith, B. S.; Singha, A. S.; Gupta, S. K. Graft copolymerization of Flax fibres with binary vinyl monomer mixtures and evaluation of swelling, moisture absorbance and thermal behaviour of the grafted fibres. J. Polym. Mater. 2003, 20, 195−199. (27) Mathew, A. P.; Oksman, K.; Sain, M. Mechanical properties of biodegradable composites from poly lactic acid (PLA) and microcrystalline cellulose (MCC). J. Appl. Polym. Sci. 2005, 97, 2014−2025. (28) Gardner, J. D.; Oporto, S. G. S.; Mills, R.; Samir, A. Adhesion and surface issues in cellulose and nanocellulose. J. Adhes. Sci. Technol. 2008, 22, 545−567.

some extent due to plasticization effect of MAH. The highest HDT value was obtained in 5 wt % GCMF reinforced PLA biocomposites. This observed enhancement of HDT may be attributed to the better dispersion of GCMF within PLA matrix. It is very difficult to achieve high HDT enhancement without strong interaction among the fillers and polymer matrix. These results suggest that the improvement in HDT in PLA originates from the better reinforcement by the dispersed phase of CMF, GCMF, and NCMF and their intercalation within the PLA matrix.

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CONCLUSIONS The aim of this research work was to extract CMF from sisal fiber and to investigate its potential as filler for the fabrication of PLA biocomposites. The CMF was treated with NaOH and silane (GPS) to improve its surface roughness as well as to interlock CMF and PLA. The highest value of tensile strength and modulus with respect to PLA was obtained for 5 wt % GCMF reinforced PLA biocomposite. Excellent interfacial strength and uniform dispersion of GCMF in PLA were the reason for the enhancement of the mechanical properties. The addition of CMF increases the crystallization process Tcc, Tm, ΔHm, and ΔHmc of PLA. This improvement may be attributed to the nucleating effect of CMF. The thermal stability and HDT of CMF reinforced PLA biocomposites was found to be increased as compared to PLA. A synergistic effect between CMF and PLA may be determinant for the enhancement of the thermal properties. Hence, to achieve good thermal and mechanical properties in biocomposites, strong interfacial strength between filler and polymer matrix is desired. Thus, CMF can be used as potential filler for enhancing the mechanical as well as thermal properties of PLA matrix for automobile applications.

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AUTHOR INFORMATION

Corresponding Author

*Phone no.: +91-674-2742853. Fax no.: +91-674-2740463. Email: [email protected] (S.M.). Notes

The authors declare no competing financial interest.

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REFERENCES

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Influence of Different Treated Cellulose Fibers on the Mechanical and

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