Article pubs.acs.org/IECR
Green Jute-Based Cross-Linked Soy Flour Nanocomposites Reinforced with Cellulose Whiskers and Nanoclay Murshid Iman, Kusum K. Bania, and Tarun K. Maji* Department of Chemical Sciences, Tezpur University, Assam -784028, India S Supporting Information *
ABSTRACT: A practical procedure for extracting cellulose whiskers (CWs) from ordinary filter paper by acid hydrolysis has been developed. The extracted CWs alone and in combination with nanoclay were used as reinforcing agents for the preparation of green nanocomposites based on jute fabric and glutaraldehyde (GA) cross-linked soy flour (SF). A solution-induced intercalation method was implemented for the successful fabrication of the nanocomposites. The formation of nanocomposites was confirmed by various physicochemical and spectroscopic techniques and by DFT calculations. The results of the various analyses suggest a strong interfacial interaction between the filler and the matrix. The properties of nanoparticle-filled SF/jute nanocomposites were compared with those of unfilled composites. Furthermore, the mechanical strength, thermal stability, flame retardancy, and dimensional stability of the filled composites were found to be much better than those of the unfilled composites, allowing the processing of high-performance green nanocomposites.
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bamboo, and palm-tree fibers are used as reinforcing agents at the micro- and macroscale levels to improve the thermal and mechanical properties of natural biodegradable polymers.15 Huang and Netravali used micro-/nanosized bamboo fibrils (MBFs) and a modified soy protein resin to fabricate environmentally friendly composites. They reported a significant increase in the fracture stress and Young’s modulus of the soy protein concentrate upon incorporation of MBFs. Similarly, palm tree and kenaf natural fibers were used as reinforcing agents for the development of self-bonded polymer biocomposites.16 Aside from the already mentioned natural fibers, jute fiber can also be used as a reinforcing agent and for the synthesis of polymer composites. Jute, mainly comprising cellulose and lignin, is an extensively used material in cloth and other such industries.17 The most attractive features of jute fibers are that they are inexpensive and widely available and, most importantly, that they can be modified into cost-effective and environmentally friendly green products with improved physical properties such as thermal and mechanical properties.18 The combination of SF polymer and jute fibers cross-linked with glutaraldehyde can lead to biobased polymer composites whose properties can be improved by dispersing nanofillers into their matrix.19−24 Among the various nanofillers, cellulose whiskers (CWs) and clays with surface areas in the ranges of 150−170 and 750 m2/g, respectively, are extensively used for the modification of conventional polymer composites.25 The immobilization of nanoclays and CWs results in a high interfacial area between the matrix and the filler. This, in turn, endows the polymer nanocomposites with high thermomechanical performance.26
INTRODUCTION Biopolymer-based materials containing immobilized nanoparticles have recently dominated the traditionally obtained petroleum-based polymer materials.1,2 Because of their outstanding mechanical properties and biodegradability, biobased polymer nanocomposites are currently considered to be promising materials. These materials have the potential to overcome the disadvantages of materials obtained commercially from the petrochemical supply chain. Moreover, the use of biobased polymer nanocomposites plays a vital role in sustainable development and the development of newer methods for the production of environmentally benign “greener” products.3−7 Of the different types of commercially available soy proteins, soy flour (SF), a natural biodegradable polymer, is inexpensive and readily available.8−10 The presence of various polar and reactive amino acids such as cystine, arginine, lysine, and hystidine in the structure of SF enable them to cross-link with a cross-linking agent such as glutaraldehyde, thereby improving the tensile and thermal properties of the polymer.11,12 However, the potential of such material has been less explored in the synthesis of biobased composites. In addition to having various advantages, SF-based materials also exhibit certain disadvantages. For example, soy-protein-based plastics have poor flexibility and water resistance. Although several plasticizers such as glycerol, polyethylene glycol, and sorbitol have been used to improve the flexibility of SF but have been found to yield lower mechanical strength and high water sensitivity.8−13 In this respect, biofibers have become a promising option for improving the mechanical strength of such materials.14 The salient features of natural fibers over their conventional counterparts include comparatively low cost, low weight, high specific modulus, comparative safety, and copious and renewable resources. The applications of natural fibers in sectors such as automobiles, furniture, packing materials, and construction are now highly demanding. Currently, kenaf, © XXXX American Chemical Society
Received: February 26, 2013 Revised: April 23, 2013 Accepted: April 29, 2013
A
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were dried in a vacuum oven at 50 °C to constant weight and stored at ambient temperature in a desiccator. Preparation of the Slurry. Soy flour (SF) powders were first mixed thoroughly with deionized water (1:10, w/w) at room temperature for 1 h to obtain a suitable resin for the fabrication of “green” composites. SF was found to be very brittle, weak, and difficult to process into useful films in the absence of a plasticizer. Therefore, 5% glycerol (w/w with respect to dry SF) was added as a plasticizer. The slurry containing SF and glycerol (5% w/w with respect to dry SF) was transferred to a 500 mL round-bottom flask and stirred with a mechanical stirrer while the temperature was maintained in the range of 55−60 °C for 4 h. The slurry was then cooled to 35 °C. The resultant mixture was stirred for an additional 5 min at that temperature with subsequent addition of GA. This stage is referred to as precured resin. To prepare CW-filled resin, the desired amount of CWs (1−5% w/w with respect to dry SF) was first placed in a beaker containing water, stirred for 8 h using a mechanical stirrer, sonicated for 30 min under 0.5 cycle and a wave amplitude of 140 μm using a Hielscher UP200S sonicator (Teltow, Germany), and finally added to the precured resin. Similarly, for the CW- and nanoclay-filled resin, the desired amounts of CWs (5% w/w with respect to dry SF) and nanoclay (1−5% w/w with respect to dry SF) were first placed in a beaker containing water. Then, the mixture was stirred for 8 h using a mechanical stirrer, after which it was sonicated for 30 min. The mixture was then poured into the precured resin system, and the whole resin mixture was stirred for an additional 1 h with a high-speed mechanical stirrer and then sonicated for 30 min so that the CWs and nanoclay particles were well-dispersed inside the resin matrix. Impregnation of Jute Fabrics and Fabrication of Composites. The prepared precured resin was first poured into a bowl, and then surface-modified jute fabric was dipped into the precured resin slurry. The resultant mixture was held for 24 h at 25 °C. The impregnated jute fabrics were placed in a Teflon-coated glass plate and dried over a heating plate at 30− 40 °C for 12 h. The weight of the jute fabrics before impregnation was recorded as W1. Composite laminates were prepared by the method of film stacking. Layers of impregnated jute fabric were placed on one another on 15 levels in a metal mold with a thickness of 3 mm. The laminates were then compression molded in a compression molding press (Santec, New Delhi, India) at 80 °C under a pressure of 100 MPa. The laminates were 3 mm in thickness. The final weight of the composite was recorded as W2. The calculations were performed according to the following equations
Keeping in mind the advantages of biobased polymers and the increasing demand for greener products, we report herein the preparation of biobased polymer nanocomposites consisting of soy flour, jute, glutaraldehyde, cellulose whiskers, and nanoclay with high thermomechanical properties.
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EXPERIMENTAL SECTION
Materials. A plain weave of raw and unbleached jute fabrics (Corchorus capsularis) with a weight of approximately 0.450 g cm−2 and soy flour (SF) containing approximately 55% protein and 32% carbohydrate content were purchased from a local market in Tezpur, Assam, India. Glutaraldehyde (GA, 25% w/ v), benzene (purity > 99%), glycerol (AR-grade), and NaOH pellets (purity > 97%) were obtained from Merck Pvt Ltd. (Mumbai, India). Cellulose whiskers used as one of the reinforcing agents were extracted from Whatman filter paper (No. 1) by acid hydrolysis. H2SO4 (purity > 97%) and dimethyl sulfoxide (DMSO, purity > 99%) were purchased from Rankem, New Delhi, India. Montmorillonite K-10 clay (purity 99.99%) was obtained from Sigma-Aldrich (St. Louis, MO). All of these chemicals were used as received without any further treatment. Preparation of Cellulose Whiskers (CWs). CWs were obtained by acid hydrolysis of commercial filter paper (15 g, Whatman No. 1, GE Healthcare, Chalfont St. Giles, Buckinghamshire, U.K.). The Whatman cellulose filter papers were processed through a test sieve (Scientific Engineering Corporation, New Delhi, India) equipped with a 100-mesh screen. The resulting cellulose powder was then transferred into a beaker containing 100 mL of 5 M NaOH solution. The mixture was stirred at 65 °C for 2 h. The resultant slurry was then filtered, thoroughly washed with distilled water until the wash water was of neutral pH, and finally air-dried. To the airdried sample was added 100 mL of DMSO, and the slurry was then kept in a water bath at 65 °C for an additional 2 h. The slurry was filtered, washed with 500 mL of distilled water, and air-dried. Then, 1 g of the obtained cellulose was dispersed in 25 mL of 30% (v/v) H2SO4 in a three-necked (250 mL) roundbottom flask equipped with a mechanical stirrer and a thermometer. The flask was then placed in a water bath, and its contents were stirred vigorously for 4 h while the temperature was maintained at 65 °C. The suspension was then diluted with 75 mL of distilled water and sonicated for 1 h. The dispersion was centrifuged at nearly 8000 rpm for 30 min and washed with distilled water until the pH of the supernatant liquid was nearly 7. The residues (cellulose whiskers) separated after high-speed centrifugation were collected and freeze-dried. These cellulose whiskers were further used for the preparation of the composites. Surface Modification of Jute. Jute fabrics were first treated with 2% liquid detergent (Rankleen, Rankem, Faridabad, India; pH 6.5−7.5) at 70 °C for 1 h, washed with distilled water, and finally dried in a vacuum oven at 70 °C until reaching constant weight. The washed fabrics were dewaxed by treatment with a mixture of alcohol and benzene (1:2) for 72 h at 50 °C. The dewaxed fabrics were washed with distilled water and dried until reaching constant weight. The fabrics were then treated with 5% (w/v) NaOH solution for 30 min at 30 °C and washed with distilled water several times to leach out the absorbed alkali. The fabrics were finally kept immersed in distilled water overnight and washed repeatedly to avoid the presence of any trace amount of alkali. The alkali-treated fabrics
percentage of jute in the composite = (W1/ W2) × 100
(1)
percentage of resin in the composite = [(W2 − W1)/W2] × 100
(2)
where W1 is the weight of jute fabrics before impregnation and W2 is the weight of the final composite. Physical Measurements. Fourier transform infrared (FTIR) spectra of the postcured composite samples were recorded on an Impact 410 FTIR spectrophotometer (Nicolet, Madison, WI). Small quantities of finely powdered composite samples were thoroughly ground with exhaustively dried KBr, and pellets were prepared by compression under a vacuum. 13C solid-state nuclear magnetic resonance (NMR) spectra of the B
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were measured with a model H100K-S Hounsfield universal testing machine (Tinius Olsen Ltd., Redhill, Surrey, U.K.) at room temperature. Ten samples were tested for each set, and the mean values are reported. The synthesized composite were cut into blocks of 3 mm × 10 mm × 25 mm (radial × tangential × longitudinal dimensions) for dimensional stability tests. All data are expressed as mean ± standard deviation. The results were analyzed statistically using ANOVA followed by a Tukey HSD (honestly significant difference) test (see the Supporting Information). The dimensions of oven-dried samples were measured, and the samples were conditioned at room temperature (30 °C) and 30% relative humidity. The samples were then kept in a humidity chamber for 72 h at room temperature while the relative humidity was maintained at 65%. The specimens were removed from the humidity chamber after the stipulated time period, and their volumes were measured with slide callipers. Swelling was considered to be a change in volume and is expressed as a percentage increase in volume compared to that of the oven-dried sample. The percentage swelling in water vapor was calculated according to the expression
synthesized composites were obtained using a Bruker DSX-300 solid-state NMR spectrometer. Electronic absorption spectra were recorded using a Shimadzu Corporation UV2450 spectrophotometer (Kyoto, Japan) with a diffuse-reflectance attachment equipped with an integrating sphere of 60-mm inner diameter. Monochromatic light was used throughout the whole spectral region to minimize the effects of fluorescence. To record the spectra of the synthesized green nanocomposites, the powdered samples were placed in a black absorbing hole (10 mm in diameter and 3 mm in depth) of a sample holder, and the surface was smoothed. XRD measurements were carried out with a Rigaku X-ray diffractometer (MiniFlex, Sevenoaks, Kent, U.K.) using Cu Kα (λ = 0.154 nm) radiation at a scanning rate of 2° min−1 with an angle ranging from 2° to 40° to determine the degrees of CW and clay intercalation in the synthesized composite. The dispersion of the silicate layers of montmorillonite clay nanoparticles in jute-reinforced SF composite and the morphology of the CWs were studied by high-resolution transmission electron microscopy (HRTEM) (JEM 2100, JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV. The surface morphologies of some of the prepared samples were studied by scanning electron microscopy (SEM) before being subjected to mechanical tests. The morphologies of the fracture surfaces of the samples obtained from flexural tests were also studied by SEM. SEM analysis was performed using a JEOL model JSM-6390LV scanning electron microscope at an accelerating voltage of 15 kV. All samples were Ptcoated before observation. Thermogravimetric analysis of the composite samples was performed using a Shimazdu TGA 50 thermal analyzer under a nitrogen flow rate of 30 mL min−1 at a heating rate of 10 °C min−1 from 30 to 600 °C. The sample weight was approximately 5 mg. Three samples of each set were tested to obtain a constant value. The limiting oxygen index (LOI) is defined as the minimum concentration of oxygen, expressed as a volume percentage, in a flowing mixture of oxygen and nitrogen that will support flaming combustion of a material initially at room temperature. The specimens were cut into 100 mm × 6 mm × 3 mm (longitudinal × tangential × radial dimensions) according to standard method ASTM-D 2863 and placed vertically in the flammability tester (S.C. Dey Co., Kolkata, India). The total volume of the gas mixture (N2 + O2) was kept fixed at 18 cm3. The volumes of nitrogen and oxygen gas were initially kept at maximum and minimum levels, respectively. Thereafter, the volume of nitrogen gas was decreased, and the volume of oxygen gas was increased gradually. However, the total volume of the gas mixture was kept fixed at 18 cm3 during the experiment. The ratio of nitrogen to oxygen at which the sample continued to burn for at least 30 s was recorded. The limiting oxygen index was calculated as
swelling (%) = [(Vt − V0)/V0] × 100
where Vt is the volume of the sample after time t and V0 is the initial volume of the composite sample before water vapor absorption.
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RESULTS AND DISCUSSION The IDs and compositions of the obtained nanocomposites (each approximately 3 mm thick) are reported in Table 1. The Table 1. Sample IDs and Filler Contents of Nanocomposites Based on Jute and Cross-Linked SF with CWs and Nanoclay (wt %) sample SF/J/G50 SF/J/G50/ C1 SF/J/G50/ C3 SF/J/G50/ C5 SF/J/G50/ C5/M1 SF/J/G50/ C5/M3 SF/J/G50/ C5/M5
soy flour (SF)
glycerol
glutaraldehyde (GA)
jute (J)
CWs (C)
nanoclay (M)
100 100
5 5
50 50
75 75
− 1
− −
100
5
50
75
3
−
100
5
50
75
5
−
100
5
50
75
5
1
100
5
50
75
5
3
100
5
50
75
5
5
samples as identified in Table 1 were prepared by keeping the weight percentages of the components SF, glycerol, glutaraldehyde, and jute fixed in all of the samples while varying the weight percentages of CWs and nanoclay. FTIR Study. To confirm the formation of CWs upon acid hydrolysis of cellulosic filter paper, FTIR analysis was performed. The composition changes of filter paper and the resultant CWs are shown in Figure 1. In the FTIR spectrum of the untreated filter paper (spectrum a), a strong broad band appears at 3432 cm−1 that is attributed to the stretching of Hbonded hydroxyl groups.27 The peak at 2902 cm−1 is due to symmetric CH vibrations.28 The intense band at 1642 cm−1 is assigned to the stretching vibration resulting from HOH
limiting oxygen index (LOI) = [volume of O2 /volume of (O2 + N2)] × 100
The tensile properties of the composites were determined in accordance with standard method ASTM D-638 at a crosshead speed of 2 mm min−1. The samples were cut into blocks of 100 mm × 3 mm × 20 mm (longitudinal × radial × tangential dimensions). For the flexural test, the specimens were cut into blocks of 3 mm × 10 mm × 100 mm (radial × tangential × longitudinal dimensions) and examined in accordance with standard method ASTM D-790 at a crosshead speed of 2 mm min−1. Both the tensile and flexural strengths of the samples C
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Figure 1. FTIR spectra of (a) filter paper and (b) cellulose whiskers.
intermolecular linkages. The bands at 1433 and 1328 cm−1 in the spectrum are assigned to symmetric CH2 bending and wagging, respectively, and CH bending occurs at 1369 and 1255 cm−1.29,30 The two bands at 1162 and 898 cm−1 correspond to COC stretching at the β-(1→4)-glycosidic linkages.31 The in-plane ring of the β-(1→4)-glycosidic linkages gives a shoulder at 1109 cm−1. The strong peak at 1054 cm−1 indicates the CO stretching at C-3.32 The vibrational frequencies of cellulose filter paper (as obtained from the FTIR spectrum) before and after acid hydrolysis treatment were found to be very similar. However, the intensities of some peaks were found to be different to some extent in the CW spectrum (spectrum b). For CWs, the intensities of the peaks at 898, 1109, and 1369 cm−1 increased slightly, whereas the intensities of the peaks at 1255, 1328, 1642, and 2900 cm−1 did not change during acid hydrolysis. This indicates that the acid hydrolysis affects the surface and amorphous zone of cellulose. In the spectra of untreated filter paper and CWs, the bands at 710 and 755 cm−1 are the characteristic absorptions of cellulose Iα and Iβ, respectively.33 The FTIR spectra of composites SF/J/G50, SF/J/G50/C1, SF/J/G50/C5, SF/J/G50/C5/M1, and SF/J/G50/C5/M5 are shown in Figure 2. The FTIR spectra of all of the samples show peaks in the range of 3400−3000 cm−1 that can be attributed to OH and NH stretching vibrations. Characteristic peaks of SF at 1648, 1544, and 1248 cm−1 for the CO stretching of amide I, NH bending of amide II, and CN stretching of amide III are present in all of the spectra. It is known that polypeptides and proteins of mainly α-helical structure exhibit absorption bands near 1650 cm−1 (amide I) and 1540 cm−1 (amide II). The amide III band in the spectrum is due to the presence of glutamic acid in soy flour. Similarly, the peaks
Figure 2. FTIR spectra of (a) SF/J/G50, (b) SF/J/G50/C1, (c) SF/J/ G50/C5, (d) SF/J/G50/C5/M1, and (e) SF/J/G50/C5/M5.
appearing in all of the spectra at 1254 and 1116−1057 cm−1 are for COC bonds in the cellulose chain and the CO stretching vibration, respectively, which are characteristic peaks for jute. The entire spectrum shows a peak around 900 cm−1 D
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Scheme 1. Probable Interactions of Glutaraldehyde with SF, Jute, Cellulose Whiskers, and Nanoclay
Figure 3. Optimized geometry of the cross-linked product formed by CN linkage.
due to the β-(1→4)-glycosidic linkages. The intensities of the OH and NH stretching vibrations were found to decrease with respect to those for the unfilled composite. From the IR spectrum, it was found that, with increasing CW concentration, the peak intensities decreased. This behavior can be ascribed to the interaction between the free OH and NH groups of jute and SF with the OH groups of CWs. Upon the addition of nanoclay, the peaks in the 3400−3000 cm−1 range corresponding to OH and NH stretching vibrations were reduced in intensity and sharpened. The peak intensities in the ranges of 1030−460 and 1620 cm−1 were found to be enhanced. These results confirm the interaction of the clay particles with GA and the SF/J composite.34 Glutaraldehyde can exist in different forms in solution and can cross-link proteins, leading to the formation of a broad range of conjugates. In the FTIR spectra of all of the composites, two common bands appear at 1648 and 1167 cm−1. The presence of these two peaks, corresponding to NCO and CN stretching vibrations, respectively, indicates the interaction of the aldehydic (CHO) group of GA with the OH and NH2 groups present in jute and SF, respectively. A plausible mechanism for the formation of NCO and CN linkages is presented schematically in Scheme 1. To further confirm the possibility of CN linkages between SF and GA, we performed DFT calculations at the
B3LYP/6-31G level of theory using the Gaussian 03 program.35 In these calculations, we considered a small unit of GA consisting of 12 C atoms including aldehydic (CHO) and hydroxyl (OH) functional groups and a small unit of SF including a NH2 group at the terminal position. The optimized geometry of the cross-linked product formed by a CN linkage is shown in Figure 3. In the optimized geometry, the CH, CO, CN, OH, NH, and CC bond lengths were found to be 1.071, 1.258, 1.467, 0.959, 0.999, and 1.540 Å, respectively. These bond lengths are within the ranges of those obtained by experiment and by theoretical calculation.36 It can be observed from Figure 3 that the CO bond in the NCO linkage is slightly longer than that in the free carbonyl moiety. Similarly, the N H bond distance in the amide moiety becomes slightly compressed after the formation of a NCO linkage by the release of a H 2O molecule. The elongation and compression of the CO and NH bonds, respectively, can be attributed to a kind of pπ → pπ back-bonding from the filled π orbital of N to the vacant π orbital of CO. It can also be observed from Figure 3 that the ∠NCO bond angle is 118.5°. This indicates that the NCO linkage does not take place in a linear fashion but rather is twisted from the molecular plane. Natural bond orbital (NBO) analysis further shows a E
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1162 and 1658 cm−1 indicating the formation a CN linkage are predicted both theoretically and experimentally. A comparison of the observed vibrational frequencies to the calculated values provides a clear indication of the possibility of a strong interaction between the carbonyl group of GA and the amide group of SF. 13 C Solid-State NMR Study. From the FTIR spectra, it is evident that the components are linked through a NCO linkage. To obtain more significant evidence for the proposed mechanism, we performed 13C NMR analysis of GA and the composites SF/J/G50, SF/J/G50/C5, and SF/J/G50/C5/M5. The 13C NMR spectrum of GA (Figure 5) shows characteristic chemical shift values above 186 ppm that are due to the carbon atom of the carboxyl group. The chemical shifts of 207 and 215 correspond to the carbon atom of the free aldehydic moiety of glutaraldehyde. After GA had been treated with SF, the intensity of the signal at δ = 215 ppm greatly diminished, indicating that the most of the free aldehydic group had participated in the formation of CN linkages with the free amino group of SF (CHO + NH2 → CN, Scheme 1). Moreover, a new signal appears at δ = 48.79 ppm corresponding to the CN linkage. This provides clear evidence that SF and GA form a composite that is linked through the formation of CN bonds, resulting from the aldehydic CHO group and the NH2 group of SF.37
sufficient amount of overlapping between the carbon and nitrogen atoms, leading to the formation of a CN σ bond. A comparison of the IR frequencies obtained from theoretical calculations with those obtained from experiment shows similar results (Figure 4). The corresponding vibrational frequencies at
Figure 4. Comparison of (a) experimental and (b) theoretical IR frequencies.
Figure 5. 13C solid-state NMR spectra of (a) GA, (b) SF/J/GA50, (c) SF/J/GA50/C5, and (d) SF/J/GA50/C5/M5. F
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Figure 6. UV−vis and DR spectra of (a) SF, (b) GA, (c) SF/GA50, (d) SF/GA50/J, (e) SF/J/GA50/C5, and (f) SF/J/GA50/C5/M5.
these might be taken as strong evidence for our proposed mechanism. UV−Visible Spectroscopy Study. The UV−visible spectra of SF, GA, and a mixture of SF and GA are shown in Figure 6. The UV−visible spectrum of SF shows three absorbance peaks at 217, 265, and 330 nm. The peak at 217 nm is due to the π → π* transition originated from RCONH2 linkages. The two weak intense bands at 265 and 330 nm are due to the n → π* transition of the CO group. In case of GA, two absorbance peaks are observed at 233 and 280 nm that are due to π → π* and n → π* transitions of the CHO group. In the mixture of SF + GA, three bands are observed. These bands are different from those of pure SF and GA. The first peak at 214 nm is again because of the π → π* transition of the CHO group. The peak at 276 nm is because of the n → π* transition. This indicates that, upon mixing of SF and GA, the two electronic transitions are blue-shifted and their intensities are diminished. This might be because of the participation of the CHO group of GA in the formation of the CN linkage with the RCONH2 group of SF. The presence of a weak band at 227 nm in the mixture of SF and GA is because of the π → π* transition of the CN group, which further reveals the formation of CN linkages between SF and GA. Diffuse-reflectance spectra of the SF/J/GA50, SF/J/GA/C5, and SF/J/GA/C5/M5 composites are shown in Figure 6, and their peak assignments are listed in Table 2. The patterns of the UV−visible spectra of the composites are more or less similar. In all of the spectra, we observe basically two electronic transitions due to π → π* and n → π* appearing at different wavelengths. The peak appearing at ∼215 nm indicates the presence of the RCONH2 group in all of the synthesized composite. The peak at ∼272 nm indicates the presence of unreacted CHO functional groups. More interestingly, in all of the composites, we observed peaks above 320 and 350 nm, and this provides further strong evidence that, because of an increase in the conjugation of the synthesized composite, a bathocromic (red) shift of the π → π* and n → π* transitions
The composite of GA and SF was then treated with jute fibers. To confirm the presence of jute and its combination with the GA and SF composite, we performed 13C solid-state NMR spectroscopy of the resultant composite. The 13C NMR spectrum of the composite shows chemical shift values at δ = 24−42, 55−65, 134, 173, and 214 ppm, corresponding to aliphatic C, CN, >CC, NCO, and CHO, respectively. More prominently, a doublet appears at δ = 74.89 and 72.49 ppm, corresponding to CO bond formation.34 This indicates that the OH group of jute fiber linked to the free CHO and NH2 groups of SF and GA composite through CO and CN bond formation. To the composite of SF + GA and jute, we added cellulose whiskers. The cellulose whisker also contain OH groups. Hence, we observed a similar pattern in the 13C NMR spectrum. However, there was a slight shift in the chemical shift value that might be due to the covalent interaction of the cellulose whiskers with the free functional groups in the SF + GA + jute composite. The shifting of the chemical shift values and the presence of the chemical shift values corresponding CO and CN further confirms the presence of cellulose whiskers and their linkage to the composite. Finally, to improve the thermal and mechanical properties of these composites, we also added nanoclay. Clay particles are basically layered structures in which octahedral and tetrahedral sheets are linked through exchangeable cations or OH groups. Upon addition of clay particles, some sort of interaction occurs between the clay particles and the synthesized composites. From the 13C NMR spectrum of the clay-added composite, it is evident that, except for a decrease of the peak intensity and a very slight change in the chemical shift, no significant change occurs. This fact provides strong evidence that the addition of the clay does not cause any structural modification of the composite. Rather, it is possible that some amount of the composite might remain in the interlayer spacing of the clay. Thus, from the 13C NMR spectrum, it is truly evident that green composites were formed through CN, NCO, and CO linkages. The addition of the clay nanoparticles did not cause any structural modification, and G
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X-ray diffraction was further used to detect the incorporation of CWs and nanoclay into the jute-based cross-linked SF composites prepared by solution-induced intercalation. Figure 8
Table 2. UV−Vis/DRS Spectral Data (λmax, nm) for Neat SF and GA and Their Composites electronic transition sample
π → π*
SF GA SF/GA50 SF/J/GA50 SF/J/GA50/C5 SF/J/GA50/C5/M5
217 233 214, 227 213 215 214
n → π* 265, 280 276 276, 272, 274,
330
326, 352 327, 356 330, 446
occurs.34 Furthermore, the clay-added nanocomposites give a similar electronic transition, in addition to a greater bathocromic shift of the n → π* transition to the 446-nm range. This also indicates the participation of clay nanoparticles in the conjugated system. Hence, as shown in the plausible mechanism (Scheme 1), it can be said that the green nanocomposites comprising SF, jute, GA, CWs, and nanoclay are cross-linked by NCO and CN linkages, among others. XRD Study. As an indirect but noninvasive technique, XRD analysis is capable of providing detailed information regarding the crystal structures of CWs and the synthesized composites. Figure 7 presents the diffractograms of ground filter paper and
Figure 8. XRD patterns of (a) soy flour, (b) jute, (c) SF/J/G50, (d) SF/J/G50/C1, (e) SF/J/G50/C3, (f) SF/J/G50/C5, (g) nanoclay, (h) SF/J/G50/C5/M1, (i) SF/J/G50/C5/M3, and (j) SF/J/G50/ C5/M5.
shows the XRD patterns of SF, jute, nanoclay, and SF/J/G50 composites with and without CWs and nanoclay. Curve (a) represents the diffractograms of SF. A broader hump representing the amorphous nature of SF is observed at 2θ = 20°.40 Jute (curve b) shows peaks at 2θ = 22° (002 plane of cellulose I) and 19° (101 plane of cellulose II).41 The peak due to the 002 plane of cellulose I was found to shift marginally toward lower angle in jute-reinforced cross-linked SF composite (curve c). The positions of the peaks remained unchanged irrespective of the variation of CWs in the composites (Figure 8d,e) This reveals that the CWs do not affect the crystallinity of the composite. However, in the SF/J/G50/C5 composite containing 5% CWs, an additional peak appeared at 2θ = 15°, suggesting the incorporation of CWs into the synthesized composite (Figure 8f). The absence or decrease in intensity of the peaks at 2θ = 23° and 16° for CWs was apparent in the Xray diffractograms of CW-loaded (1−5%) cross-linked jute/SF composites. CW was organized differently and lost its structure, such as the unit-cell parameters and chain conformation. Because of either vigorous mixing or the presence of SF macromolecules, CW cannot form structures within the composite as in the case of pure CW. A strong diffraction peak for nanoclay at 2θ = 9° and a small peak at 2θ = 24° were observed. The peak at 9° corresponds to a d spacing of about 1.2 nm in pure clay.42 The diffraction peak of the nanoclay tactoids is absent from the X-ray diffractograms for 1% and 3% clay-incorporated composites (Figure 8h,i). The d001 peak of the clay in the nanocomposites completely disappeared,
Figure 7. XRD patterns of (a) Whatman No. 1 filter paper and (b) cellulose whiskers.
prepared CWs. The diffraction profiles were obtained with peak maxima at 2θ angles of 15.3°, 17.1°, 23°, and 34.8° corresponding to the (1−10), (110), (002), and (004) crystallographic planes, respectively. These values were compared with those reported in the literature, confirming that the extracted CWs were typical of cellulose I.38 The peak for the (002) plane (2θ = 23°) in the intensity profile of the treated cellulose is much sharper than that of the untreated sample. The intensity of the peak for the (110) plane (2θ = 17.1°) in the case of CWs is greater than that in the case of the filter paper.39 It can be noticed from the diffractograms that the peaks for treated cellulose are slightly shifted toward higher 2θ value, indicating a decrease in the d spacing of the CWs. On the other hand, the crystallinity increased, as the peak intensity of the CWs was greater than that of the untreated filter paper. H
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Figure 9. TEM micrographs of (a) SF/J/G50, (b) SF/J/G50/C5, and (c) SF/J/G50/C5/M5.
Figure 10. SEM images of (a,b) Whatman No. 1 filter paper and (c,d) cellulose whiskers.
intersections of the clay layers, and the bright areas denote the jute-based cross-linked SF composite matrix. The appearance of the threadlike lines suggests that the nanoclay was not homogeneously distributed within the composite, indicating the existence of a partial compatibility between the polymer, CWs, and nanoclay surface.44 Both individual silicate layers along with double or triple layers can be observed in the morphology, and very few tactoids of clay were present in jutebased cross-linked SF composite matrix. The TEM image clearly reveals that most of the clay layers lost their stacking structure and dispersed in a disorderly fashion in the jute-based cross-linked SF composite matrix. The results obtained from HRTEM analysis are in accordance with our XRD analysis. Both sets of results suggest the formation of partially exfoliated nanocomposites. SEM Study. Scanning electron microscopy is considered as a strong tool for determining the homogeneity of CWs, the microscale dispersion level of the whiskers, and presence of aggregates of CWs in the composite. SEM images of cellulose powder before and after acid treatment are shown in Figure 10. It can be observed from Figure 10c,d that, after acid hydrolysis, the cellulose microfibrils were swollen and separated into much smaller crystalline cellulose products in forms such as rods and spheres. These two different forms of cellulose nanocrystals
indicating the formation of a delaminated structure in the nanocomposites. It can be said that either the full expansion of the nanoclay gallery occurred, which is not possible to detect by XRD, or the nanoclay layers were delaminated and no crystal diffraction peak appeared. In SF/J/G50/C5/M5 composite, the position of the characteristic peak of nanoclay at 2θ = 9° was found to shift to lower 2θ values, indicating the greater intercalation of resin and CWs due to an increase in interlayer spacing. TEM Study. The morphological features of cellulose layered-silicate nanocomposites were examined by HRTEM. Figure 9 presents micrographs of samples SF/J/G50, SF/J/ G50/C5, and SF/J/G50/C5/M5. TEM images show that the nanowhiskers appeared as a network structure (shown by arrow marks). The typical size of the cellulose nanowhiskers as determined by TEM was found to be around 15−40 nm in diameter within the composite. The HRTEM image of SF/J/ G50/C5 reveals that the cellulose chains are bundled together randomly to form a network structure. The formation of a network structure results from the formation of hydrogen bonds between cellulose nanoparticles during nanocomposite preparation.43 The image corresponding to sample SF/J/G50/ C5/M5 shows the dispersion of nanoclay in the composite. The threadlike dark lines (indicated by arrows) represent the I
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Figure 11. SEM micrographs of (a) SF/J/G50, (b) SF/J/G50/C1, (c) SF/J/G50/C5, (d) SF/J/G50/C5/M1, and (e) SF/J/G50/C5/M5.
Figure 12. TGA thermograms of (a) SF/J/G50, (b) SF/J/G50/C1, (c) SF/J/G50/C3, (d) SF/J/G50/C5, (e) SF/J/G50/C5/M1, (f) SF/J/G50/ C5/M3, and (g) SF/J/G50/C5/M5.
cannot be separated by the common methods of filtration and centrifugation. The bundles of rodlike CWs observed in the SEM image had a width of ∼1 μm. It seems that some aggregation of CW rods took place to form larger bundles during freeze-drying. However, these bundles of CW rods were loosely packed and could easily be separated.45 Figure 11 shows SEM images of the fracture surfaces of SF/ J/G50, SF/J/G50/C1, SF/J/G50/C5, SF/J/G50/C5/M1, and
SF/J/G50/C5/M5 composites. The SF/J/G50 sheet shows a relatively smooth surface (Figure 11a). It is noted that the fracture surface of SF/J/G50/C1 (Figure 11b) displays a homogeneous structure, suggesting a relatively uniform distribution of the CWs in the SF and jute composite. However, as the cellulose content increased, the fracture surface of SF/J/G50/C5 (Figure 11c) exhibited a relatively rough structure, suggesting relatively high interfacial adhesion among J
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Table 3. Thermal Properties of SF/J/G50, SF/J/G50/C1, SF/J/G50/C3, SF/J/G50/C5, SF/J/G50/C5/M1, SF/J/G50/C5/M3, and SF/J/G50/C5/M5 Tm (°C)
Td (°C) at different weight loss percentages
sample
Ti (°C)
first step
second step
20%
30%
40%
50%
60%
RW at 600 °C (%)
SF/J/G50 SF/J/G50/C1 SF/J/G50/C3 SF/J/G50/C5 SF/J/G50/C5/M1 SF/J/G50/C5/M3 SF/J/G50/C5/M5
172 177 183 191 202 209 215
212 219 225 234 243 249 253
298 304 309 317 328 334 339
224 235 243 252 258 266 271
265 268 270 272 279 281 286
285 290 298 305 313 314 318
304 321 327 331 337 341 346
337 363 376 395 430 441 448
3 5 7 9 10 12 14
Table 4. Comparison of the Tensile, Flexural, and Limiting Oxygen Index (LOI) Values of Unfilled and Filled Jute-Based CrossLinked SF Nanocompositesa,b flexural properties
tensile properties
a
composite system
strength (MPa)
SF/J/G50 SF/J/G50/C1 SF/J/G50/C3 SF/J/G50/C5 SF/J/G50/C5/M1 SF/J/G50/C5/M3 SF/J/G50/C5/M5
9.488 14.654 23.420 30.780 32.685 52.591 47.429
(±0.826) (±0.931) (±1.699) (±1.252) (±1.179) (±0.746) (±1.384)
modulus (MPa) 937.453 1122.644 1791.879 1877.287 1956.118 2363.098 2113.082
strength (MPa)
(±11.500) (±10.714) (±8.118) (±12.848) (±8.401) (±11.041) (±10.487)
14.516 27.473 37.688 46.697 68.334 82.216 76.933
(±1.361) (±1.206) (±1.430) (±0.950) (±1.212) (±0.745) (±1.004)
modulus (MPa) 1702.369 2156.261 2874.533 3535.505 4129.661 6232.557 5789.353
(±7.349) (±7.141) (±9.596) (±8.912) (±8.890) (±8.361) (±7.933)
LOI (%) 39 44 48 51 57 62 64
(±1.0) (±1.0) (±3.0) (±1.0) (±2.0) (±1.0) (±2.0)
For tensile and flexural properties, each value represents an average of 10 samples. bValues in the parentheses represent standard deviations.
retention were obtained for a range of polymer nanocomposites using nanoclay. This is generally attributed to the ability of nanoclay to function as a mass-transport barrier, hindering the out-diffusion of decomposition products and restricting the thermal motion of the polymer chain at the nanoclay surface.48 In the CWs incorporated nanocomposites, the CWs might restrict the thermal motion of the polymer chain and retard the diffusion of degradable decomposed products through the nanocomposites. Hence, an improvement in thermal stability was observed that is evident from the TGA thermograms shown in Figure 12. The increased thermal resistance of the nanoclay-incorporated CW nanocomposites compared to the CW nanocomposites; that is, SF/J/G50/C5/M1, SF/J/G50/ C5/M3, and SF/J/G50/C5/M5 can be attributed to the retarded out-diffusion of decomposition products due to the labyrinth morphology of the exfoliated nanoclay in the matrix.49 LOI Study. Limiting oxygen index (LOI) values for SF/J composites with different percentages of CWs and nanoclay are given in Table 4. From Table 4, it can be observed that all of the the samples produced small localized flames and that nanoclay-filled samples generated higher char than those without nanoclay. The LOI test assumes that inherently less flammable materials require greater oxygen concentrations to produce the heat necessary for the continued production of flammable volatiles and flame propagation. It is also observed in Table 4 that the LOI value increases with increasing percentage of CWs in the composites. CWs form cross-links with SF, jute, and GA and thus restrict the accessibility of oxygen for the production of degradable components from the composites, hence resulting in higher LOI values.50 Further, the higher the CW concentration, the higher the LOI value. The enhancement in properties might be due to the increase in interactions among SF, jute, GA, and CWs. However, upon addition of nanoclay into the CW-incorporated composites, the LOI value is further enhanced. From Table 4, it can be seen that the LOI values of the nanoclay-filled composites are greater than those
SF, jute, and CWs. Compared with the SF/J/G50 sheet, the presence of the CWs in the SF and jute composites can be observed in the synthesized composites. Nevertheless, it is difficult to distinguish the individual filler dispersion because of its small size. Some filler appears as white domains on the fracture surface of the samples. This corresponds to the fillers in the perpendicular plane of the composite film. Moreover, upon addition of nanoclay into the SF/J/G50/C5 composite, the roughness was found to increase (Figure 11d,e). This might be due to the fact that the clay particles increased the interaction with the SF matrix, CWs, and jute surface. Further analysis was done through energy-dispersive X-ray analysis of the nanoparticles observed in the fracture of the clay-loaded composite, as shown in Figure 11f. Elements such as Al, Na, and Si, which are mainly from the silicate nanoclay, were detected, indicating that the nanoclay had been successfully incorporated into the composite.46 TGA Study. Thermogravimetric analysis (TGA) was used to study the degradation pattern and thermal stability of the synthesized green nanocomposites. Figure 12 displays the weight percentage thermograms for some samples of the SF/J/ G50, SF/J/G50/C1, SF/J/G50/C3, SF/J/G50/C5, SF/J/G50/ C5/M1, SF/J/G50/C5/M3, and SF/J/G50/C5/M5 composites. Table 3 (derived from Figure 12) lists the initial decomposition temperatures (Ti), maximum pyrolysis temperatures (Tm), decomposition temperatures (Td) at different weight loss percentages, and residual weights (RWs) of the composites at 600 °C. All of the thermograms showed an initial degradation around 100 °C due to the loss of water molecules. It is clearly seen from the thermograms that the values of Ti, Tm, Td at different weight loss percentages, and residual weight (RW, %) were improved upon the addition of CWs. The Ti values of the composites were found to be enhanced with increasing CW concentration. On the other hand, nanoclay is often used to enhance the thermal properties of composite materials.47 Significant increases in the decomposition and char K
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a transfer of stress is only possible with suitable adhesion of the polymer matrix on the nanoclay, which allows the the platelets of the nanoclay to be load bearing. At higher concentrations of nanoclay, agglomeration of the clay platelets can take place, and therefore, the interaction between the clay−polymer is reduced. This leads to a reduction in the extent of enhancement of reinforcement efficiency with the addition of higher amounts of nanoclay.53 Dimensional Stability Study. Water vapor absorption onto the composite samples at room temperature (∼30° C) and 65% relative humidity for different time periods is shown in Figure 13. In all cases, the percentage swelling was found to
of the composites without nanoclay. The higher the percentage of nanoclay, the higher the LOI value. Jute and SF are primarily composed of plant materials that require a very small amount of oxygen for the production of flammable volatiles and the propagation of a flame; hence, they show very low LOI values. The incorporation of nanoclay into the synthesized composite produces a silicate char on the surface and, hence, improve their flame resistance properties. The silicate-rich surface has better barrier properties to heat and oxygen transport, resulting in a delay of the ignition of the composite delays.51 Therefore, with an increase in the concentration of clay, the resistance to flame propagation is improved, and hence, higher LOI values are observed. Mechanical Property Study. The nonlinear mechanical properties of jute-based cross-linked SF composites reinforced with CWs and nanoclay were studied at room temperature. From these measurements, the tensile strength, tensile modulus, flexural strength, and flexural modulus were determined. Experimental data are reported in Table 4. A significant reinforcing effect was observed upon filler addition, as evidenced by the increase in both the modulus and the strength for the filled composites compared to the unfilled composites. A similar effect of reinforcing agents (micro-/ nanosized bamboo fibrils) on the fracture stress and Young’s modulus of soy protein was also observed by Huang and Netravali.16 It was observed that both the flexural and tensile modulus and strength increased with the increase in the concentration of CWs. The macroscopic behavior of CW-based composites depends, as for any heterogeneous materials, on the specific behavior of each phase, composition (volume fraction of each phase), morphology (spatial arrangement of phases), and interfacial properties. The mechanical properties of such materials mainly depends on three parameters, namely, the geometrical aspect, the processing method, and the matrix structure and the resulting competition between matrix/filler and filler/filler interactions. The mechanical behavior of CWbased composites suggests the formation of a rigid network of whiskers, which should be responsible for the observed reinforcing effect.52 Considering the 5% concentration of CWs as the optimum, we modified the composites SF/J/ G50/C5/M1, SF/J/G50/C5/M3, and SF/J/G50/C5/M5 with different percentages of nanoclay ranging from 1% to 5% (w/ w) with respect to SF. However, the nanoclay-incorporated composites showed significant improvements in the mechanical properties with respect to those without nanoclay. This enhancement in mechanical properties of clay-loaded composites is because of their intrinsic high modulus. In other words, it can be said that clays act as rigid reinforcement agent to the polymer matrix. However, the incorporation of a material with high modulus cannot be the only reason for such improvement in mechanical properties of the composites. The other reason might be the high aspect ratio of the nanoclay, which generates a large surface area for polymer adsorption. Apart from this, the intercalation of the polymers into the galleries of the nanoclay restricts their mobility. However, such improvement in mechanical properties with the incorporation of nanoclay can be realized only up to a definite nanoclay loading. In this present work, an enhancement in mechanical properties was found up to clay loading of 3%, and beyond that, these properties declined upon further addition of nanoclay. It is also noted that the improvement in reinforcement efficiency due to the incorporation of nanoclay occurred when there was a proper transfer of applied stress to the nanoclay platelets. Such
Figure 13. Swelling behaviors of (a) SF/J/G50, (b) SF/J/G50/C1, (c) SF/J/G50/C3, (d) SF/J/G50/C5, (e) SF/J/G50/C5/M1, (f) SF/ J/G50/C5/M3, and (g) SF/J/G50/C5/M5.
increase with increasing time. Sample SF/J/G50 is hydrophilic in nature, and hence, it absorbed a high amount of water vapor. The water vapor absorption decreased in the presence of the dispersed phase of CWs in the composite. The CW particles act as a barrier for water vapor, resulting in a decrease of water vapor transmission through the jute-based SF cross-linked composites. CWs might elongate the path of water particles in the nanocomposites.54 Water vapor absorption of the nanocomposites is decreased with increasing CW content. The addition of CWs provides an elongated path for the water molecules to traverse. Further, it was observed that nanoclaytreated composites swelled less than those of samples that were not treated with nanoclay. The higher the amount of nanoclay, the lower the swelling. This might be due to the fact that the silicate layers of nanoclay provide a tortuous path that hinders the diffusivity of water particles through the nanocomposite.55
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CONCLUSIONS Natural-resource-based green nanocomposites comprising SF, jute, GA, CWs, and nanoclay were synthesized by a solutioninduced intercalation method. The synthesized nanocomposites were successfully characterized by FTIR, solid-state 13C NMR, and diffuse-reflectance spectroscopies; XRD; TEM; and SEM. The 13C NMR and DRS study revealed the formation of longchain composite through the cross-linking of SF, jute, GA, CWs, and clay. A plausible mechanism for the cross-linking process is proposed to understand the type of interaction that might be possible among the additives. The formation of CN L
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Polylactide-layered silicate nanocomposite: A novel biodegradable material. Nano Lett. 2002, 2, 1093. (5) Nam, J. Y.; Ray, S. S.; Okamoto, M. Crystallization behavior and morphology of biodegradable polylactide/layered silicate nanocomposite. Macromolecules 2003, 36, 7126. (6) Tang, X.; Alavi, S. Structure and physical properties of starch/ poly vinyl alcohol/laponite RD nanocomposite films. J. Agric. Food Chem. 2012, 60, 1954. (7) Lim, S. T.; Hyun, Y. H.; Choi, H. J. Synthetic biodegradable aliphatic polyester/montmorillonite nanocomposites. Chem. Mater. 2002, 14, 1839. (8) Chabba, S.; Matthews, G. F.; Netravali, A. N. ‘Green’ composites using cross-linked soy flour and flax yarns. Green. Chem. 2005, 7, 576. (9) Wang, Y.; Cao, X.; Zhang, L. Effects of cellulose whiskers on properties of soy protein thermoplastics. Macromol. Biosci. 2006, 6, 524. (10) (a) Johnson, L. A.; Myers, D. J. Industrial uses for soybeans. In Practical Handbook of Soybean Processing and Utilization; AOCS Press: Champaign, IL, 1995. (b) Huang, J.; Li, K. A new soy flour-based adhesive for making interior type II plywood. J. Am. Oil Chem. Soc. 2008, 85, 63. (c) Nanda, P. K.; Rao, K. K.; Nayak, P. L. Biodegradable polymers. XI. Spectral, thermal, morphological, and biodegradability properties of environment-friendly green plastics of soy protein modified with thiosemicarbazide. J. Appl. Polym. Sci. 2007, 103, 3134. (d) Liu, Y.; Li, K. Chemical modification of soy protein for wood adhesives. Macromol. Rapid Commun. 2002, 23, 739. (11) (a) Lin, Q.; Chen, N.; Bian, L.; Fan, M. Development and mechanism characterization of high performance soy-based bioadhesives. Int. J. Adhes. Adhes. 2012, 34, 11. (b) Liu, D.; Chen, H.; Chang, P. R.; Wub, Q.; Li, K.; Guan, L. Biomimetic soy protein nanocomposites with calcium carbonate crystalline arrays for use as wood adhesive. Bioresour. Technol. 2010, 101, 6235. (c) Prasittisopin, L.; Li, K. A new method of making particleboard with a formaldehydefree soy-based adhesive. Composites A 2010, 41, 1447. (d) AmaralLabat, G. A.; Pizzi, A.; Gonçalves, A. R.; Celzard, A.; Rigolet, S.; G. Rocha, J. M. Environment-friendly soy flour-based resins without formaldehyde. J. Appl. Polym. Sci. 2008, 108, 624. (e) Li, K.; Peshkova, S.; Geng, X. Investigation of soy protein-Kymene® adhesive systems for wood composites. J. Am. Oil Chem. Soc. 2004, 81, 487. (f) Wang, Y.; Mo, X.; Sun, X. S.; Wang, D. Soy protein adhesion enhanced by glutaraldehyde crosslink. J. Appl. Polym. Sci. 2007, 104, 130. (g) Kim, J. T.; Netravali, A. N. Mechanical, thermal, and interfacial properties of green composites with ramie fiber and soy resins. J. Agric. Food Chem. 2010, 58, 5400. (12) Song, F.; Tang, D. L.; Wang, X. L.; Wang, Y. Z. Biodegradable soy protein isolate-based materials: A review. Biomacromolecules 2011, 12, 3369. (13) Li, H.; Huneault, M. A. Comparison of sorbitol and glycerol as plasticizers for thermoplastic starch in TPS/PLA blends. J. Appl. Polym. Sci. 2011, 119, 2439. (14) (a) Bledzki, A. K.; Gassan, J. Composites reinforced with cellulose based fibres. Prog. Polym. Sci. 1999, 24, 221. (b) Kengkhetkit, N.; Amornsakchai, T. Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber. Ind. Crops Prod. 2012, 40, 55. (15) (a) Islam, M. R.; Beg, M. D. H.; Gupta, A.; Mina, M. F. Optimal performances of ultrasound treated kenaf fiber reinforced recycled polypropylene composites as demonstrated by response surface method. J. Appl. Polym. Sci. 2013, 128, 2847. (b) Liang, K.; Gao, Q.; Shi, S. Q. Kenaf fiber/soy protein based biocomposites modified with poly(carboxylic acid) resin. J. Appl. Polym. Sci. 2013, 128, 1213. (c) Phong, N. T.; Gabr, M. H.; Okubo, K.; Chuong, B.; Fujii, T. Enhancement of mechanical properties of carbon fabric/epoxy composites using micro/nano-sized bamboo fibrils. Mater. Des. 2013, 47, 624. (d) Zainuddin, S. Y. Z.; Ahmad, I.; Kargarzadeh, H.; Abdullah, I.; Dufresne, A. Potential of using multiscale kenaf fibers as reinforcing filler in cassava starch−kenaf biocomposites. Carbohydr. Polym. 2013, 92, 2299. (e) Saadaoui, N.; Rouilly, A.; Fares, K.; Rigal, L. Characterization of date palm lignocellulosic by-products and self-
linkages caused by the interaction between the aldehydic group of GA and the hydroxyl and amide groups of jute and SF was confirmed by NMR spectroscopy, DRS, and density functional theory. The composite with 5% CWs was found to show better physical properties than those of either unfilled composites or containing lower percentages of CWs. The mechanical and thermal properties, LOI, and dimensional stability of the composites were found to be enhanced with increasing interfacial interaction between the filler and matrix. The incorporation of nanoclay resulted in further improvement of the physicochemical properties of the synthesized nanocomposites. Thus, CWs in combination with nanoclay can significantly improve the characteristic properties of naturalresource-based composites. Hence, such types of CW- and nanoclay-loaded jute-fabric-based cross-linked SF composites can add a new dimension in the field of renewable-resourcebased industries.
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ASSOCIATED CONTENT
* Supporting Information S
Mean comparison of different formulations used in this study as analyzed by one-way ANOVA followed by Tukey HSD test. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +91 3712 267007 ext. 5053. Fax: +91 3712 267005. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Ranjit Thakuria (Tezpur University) and Gaurango Chakraborty (IISc, Bangalore, India) for providing the solidstate 13C NMR data. Rizwin Khanam (Tezpur University) is acknowledged for her help on the DRS study. The authors thank the Department of Science & Technology, India, for funding this research under Grant SR/S5/GC-13/2008.
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REFERENCES
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