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Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell
Accepted Manuscript Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell Baoxia Wang, Dagang Li PII: DOI: Reference:
S1359-835X(15)00300-0 http://dx.doi.org/10.1016/j.compositesa.2015.08.029 JCOMA 4036
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
7 July 2015 24 August 2015 26 August 2015
Please cite this article as: Wang, B., Li, D., Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell, Composites: Part A (2015), doi: http://dx.doi.org/10.1016/j.compositesa. 2015.08.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell Baoxia Wanga,b, Dagang Lia,* a
College of Material Science and Engineering, Nanjing Forestry University, Nanjing
210037, Jiangsu, People’s Republic of China b
College of Lighttextile Engineering and Art, Anhui Agricultral University, Hefei 230031,
Anhui, People’s Republic of China Corresponding author: Dagang Li, Tel: +86 25 13912981251. Email: [email protected] ABSTRACT: Cellulose nanofibers-reinforced PVA biocomposites were prepared from peanut shell by chemical-mechanical treatments and impregnation method. The composite films were optically transparent and flexible, showed high mechanical and thermal properties. FE-SEM images showed that the isolated fibrous fragments had highly uniform diameters in the range of 15-50 nm and formed fine network structure, which is a guarantee of the transparency of biocomposites. Compared to that of pure PVA resin, the modulus and tensile strength of prepared nanocomposites increased from 0.6 GPa to 6.0 GPa and from 31 MPa to 125 MPa respectively with the fiber content as high as 80 wt%, while the light transmission of the composite only decreased 7% at a 600 nm wavelength. Furthermore, the composites exhibited excellent thermal properties with CTE as low as 19.1 ppm/K. These favorable properties indicated the high reinforcing efficiency of the cellulose nanofibers isolated from peanut shell in PVA composites. Keywords: A. Nano-structures; B. Optical properties; D. Mechanical testing; D. Thermal analysis. 1
1. Introduction Recently, the fabrication of nanocomposites reinforced by natural fibrils has attracted much attention due to the environmental concerns. Among these natural fibrils, cellulose nanofibers with complete biological degradability and renewability are extensively researched. Their nanostructures which have a width ranging from 5 to 30 nm [1], are highly crystalline materials stabilized laterally by long cellulose chains with hydrogen bonding. Thus, they exhibit outstanding structural strength and stiffness, such as a high Young’s modulus of 138 GPa and a specific tensile strength of 3 GPa [2]. Cellulose nanofibers also have very unique optical and thermal properties [3-4]. Because of these characteristics, cellulose nanofibers have generated a great deal of interest as reinforcement for developing functionalized optical and lightweight nanocomposites. Currently, the most important industrial source of cellulose nanofibers is certainly wood. So far, many investigations have focused on the isolation of cellulose nanofibers from wood [5-8]. Abe et al. [9] have successfully obtained cellulose nanofibers with a uniform width of 15 nm from pine wood by grinding just one time. However, competition from different sectors such as the furniture industries and the pulp and paper industries makes it challenging to supply all users with the quantities of wood at a reasonable cost. For this reason, the isolation of nano-sized cellulose whiskers and fibers from annual fiber crops such as wheat straw [10], sugar beet [11], potato tube [12], banana pseudostem [13] and others, especially from by-products of these different plants, is of increasing interest to utilize the nano elements. Peanut shell is an important annual crop by-products, which consists of cellulose, lignin, hemicelluloses and tannins [14]. In cell walls of peanut shells, 2
cellulose nanofibers could be extracted by the removal of matrix substances as well as the fibrillation process. However, except a few works dealing with peanut shell nanocrystals [15], any work with regard to the cellulose nanofibers isolated from peanut shell has been not recorded in literature. Each year at least 4.5 megton of peanut shells were generated in China [16]. Only a small percentage is being used in applications such as feedstock and adsorbent material, while most are discarded as waste. In this regard, obtaining cellulose nanofibers based on peanut shell seems to be a prospective commercial application at lower cost. As described in some reports, cellulose nanofibers have been shown to be an ideal reinforcement for resins due to their exceptional mechanical properties and their highly uniform network structure [4]. Poly(vinyl alcohol) (PVA) is a kind of water soluble, biocompatible and biodegradable synthetic polymer. Although PVA has many unique properties such as high elasticity, hydrophilic characteristics and the ability to form good films, its poor durability and high thermal expansion restrict it from further applications. One approach is to create a PVA-based nanocomposite [17-19]. Roohani et al. [20] reported the hydroxyl groups on PVA are expected to interact with the hydrophilic surfaces of cellulose wiskers or nanofibers due to strong hydrogen bonding. In this study, we have isolated cellulose nanofibers from peanut shell with a combination of chemical and mechanical treatments, and then the morphology and structure of the isolated cellulose nanofibers were studied. Moreover, cellulose nanofibers were incorporated into PVA by impregnation method. We prepared the cellulose nanofibers-reinforced PVA biocomposites with high fiber content up to 80 wt%. At last, mechanical properties and optical transparency 3
of the composites were measured and analyzed respectively. 2. Material and methods 2.1. Materials Peanut shell powders (Jiangsu Yancheng, China) passed through a 70-mesh stainless steel sieve were used as a cellulose raw material. The dried peanut shell is characterized and its chemical composition is shown in Tab. 1. PVA (Mw=74800-79200), Sodium chlorite (NaClO2), potassium hydroxide (KOH), and acetic acid (CH3COOH) were purchased from Nanjing Chemical Regent Co., Ltd., China. Table 1
2.2. Isolation of cellulose nanofibers from peanut shell The production of cellulose nanofibers from peanut shell required a series of chemical purification and mechanical defibrillation. First, peanut shell powders were treated with acidified sodium chlorite (NaClO2) solution for 5 h at 75 ℃ to remove lignin after Soxhlet extraction. Then the delignified samples were stirred in 2 wt% potassium hydroxide (KOH) solution at 90 ℃ for 2 h. In order to completely remove the hemicelluloses and residual proteins, the alkaline treatment was repeated two times. Finally samples were further purified with an acidified sodium chlorite solution for 1 h at 75 ℃. Following chemical treatments, samples were filtered and rinsed with distilled water until the solution was neutral. The chemically treated fiber with 1 wt% undried sample was passed four times through a grinder (MKCA6-2, Masuko Sangyo Co., Ltd., Japan) at 1500 rpm with a clearance gauge of -1.5 from zero position (corresponding to a 0.15 mm shift). Subsequently the slurry was 4
diluted to 0.1 wt% with water, and then passed through a high pressure homogenizer (EmulsiFlex-C3, Avestin Co., Ltd., Canada) with a pressure of 1000 bar for additional 5 passes. As a result, a stable and translucent suspension with no sediment and flocculation based on peanut shell was prepared. The concentration of suspension was adjusted to 0.08 wt% and part of them was freeze-dried for FE-SEM. 2.3. Fabrication of the PVA nanocomposites Composite films of PVA and cellulose nanofibers were prepared by impregnation method. Firstly, 400 ml cellulose nanofibers suspension was slowly vacuum filtered using a polytetrafluoroethylene (PTFE) membrane to form a wet sheet which was sandwiched between two glass plates and dried in oven at 65 ℃ for 48 h with a slight pressure. The dry cellulose nanofibers film was approximately 60 μm thick with a diameter of 9 cm in circle shape and with the density of 1.39 g/cm3. PVA powder was dissolved in distilled water and stirred at 80 ℃ for 2 h to produce a 5 wt% solution. Then the dried cellulose nanofibers film was impregnated into PVA solution and maintained at room temperature for 12 h. The impregnated samples were subsequently taken out of the resin and dried at 40 ℃ overnight. Finally, cellulose nanofibers-reinforced PVA composites were obtained. In this process, the thickness of the resulting samples was about 80 μm. By consulting the sketch of the preparation process of clay nanopaper [21], we draw up the corresponding process flow chart shown in Fig. 1 of PVA nanocomposites. According to the weights of cellulose nanofibers sheet and nanocomposites, fiber content was calculated to be about 80 wt%. Figure 1
2.4. Characterization and testing 5
Field emission scanning electron microscope (FE-SEM) was used to investigate the morphology of the peanut shell, cellulose nanofibers and biocomposites. Samples were freeze-dried at -55 ℃ for 48 h. The sample was firstly sputter-coated with 2-nm layer of platinum to prevent the buildup of an electrostatic charge and then observed using a FE-SEM (S-4800, Hitachi High-Tech. Corp., Japan) operating at 5 kV. Fourier transform infrared (FTIR) spectra (transmission) were measured on a Nicolet iS10 FTIR spectrometer (Thermo Scientific Inc., America) equipped with a single reflection attenuated total reflectance (ATR) system. All samples were tested in the range of 4000-500 cm-1 at a resolution of 2 cm-1. The crystal structure and crystallinity of samples were examined by an Ultima IV X-ray diffractometer (Rigaku Corp., Japan) with CuKα radiation operating at 40 kV and 30 mA. The raw peanut shell and purified samples were made into pallets by pressing after dried at 105 ℃. For the isolated nanofibers, ten-layered dry cellulose nanofibers films were used for the measurement. All samples were scanned at a speed of 5 s per step over a scanning range from 5° to 40°. Five samples for each were subjected to the measurement. The relative degree of cellulose crystallinity was calculated according to the Segal method [22]. Tensile tests of the films were carried out with a universal material-testing machine equipped with a 100 N load cell (SANS, Shenzhen, China) at room temperature. Specimens of 40 mm length and 60-90 μm thickness and 5 mm width were tested with the crosshead speed of 1 mm/min. Prior to testing, the specimens were conditioned for at least 48h in the environment of relative humidity at 50% and temperature at 23 ℃. Light transmittance spectroscopy of the film was collected using a UV-Visible near infrared 6
spectrometer (U-4100, HITACHI Inc., Japan) at wavelengths from 200 to 1000 nm and the scan speed was set as 300 nm/min. The coefficient of thermal expansion (CTE) of films was investigated using a thermo mechanical analyzer (TMA-402-F1, NETZSCH, Germany). The samples of 20 mm length and 3 mm width with a 16 mm span were tested twice with a heating rate of 5 ℃/min in a nitrogen atmosphere at a load of 0.2 N. The values were identified as the mean values from 20 ℃ to 140 ℃ in the second run. 3. Results and discussion 3.1. Characterization of cellulose nanofibers (CNFs)
Figure 2
Dried peanut shell was used as a starting material. Fig. 2 shows the structure and appearance of peanut shell fibers in macro- to micro-scale by SEM. As seen in Fig. 2a, the raw peanut shell has porous morphology of the surface particles. The matrix components such as lignins and hemicelluloses are located in the outer waxy layer, so single fibers are difficult to be observed. After purification, the twisted rod-like structure of the peanut shell fibers about 10 μm wide was observed (Fig. 2b) and the individual microfibril bundles approximately 15 nm wide could be clearly seen from the fiber surfaces (Fig. 2c). In order to obtain nanoscalar cellulose fibers from thick bundles, we applied a series of mechanical treatment including grinding and high-pressure homogenization. The treated peanut shell fibers were observed in Fig. d-f. Compared to the original aggregates, the fibers were found to have been disintegrated into micro- to nano-meter-sized fragments (Fig. 2d, e). Higher magnification (Fig. 2f) revealed that the fibrous fragments extracted in this study had uniform diameters in 7
the range of 15-50 nm along with the formation of high aspect ratio CNFs, confirming that high-quality nanofibers were successfully obtained from peanut shell as reported from wood. The individual CNFs within the bundles self-aggregated along the longitudinal direction and formed a very fine network. Although identifying an exact length of the nanofibers was difficult, it was evident that their lengths were longer than 1μm. This suggested that their higher aspect ratio would guarantee a greater reinforcing efficiency corresponding to the nanofibers reported from other agricultural wastes such as soy hulls and banana rachis [23-24]. Fig. 3 represents three FTIR spectra of raw peanut shell, purified peanut shell, and obtained cellulose nanofibers, respectively. The OH, CH2 group stretching vibrations at 3334 and 2893 cm-1, C-O stretching vibration at 1092 cm-1 and C-H rocking vibration at 895 cm-1, observing in all the samples, were the special characteristics of lignocellulosic materials. The absorption bands at 1460 and 1508 cm-1 existed in Fig. 3a completely disappeared in Fig. 3b, indicating that all the lignin had been removed sufficiently. On the other hand, the sharp peak at 1731 cm-1 corresponding to the unconjugated carbonyl from acetyl group almost disappeared after chemical treatment, which suggested a strong reduction of hemicelluloses. However, a small peak at 1640 cm-1 existed in Fig. 3b, c showed some residual hemicelluloses, which were insoluble in alkali might have been present in the fibers. Iwamoto et al. [25] reported the presence of hemicelluloses was beneficial to nanofibrillation. The sample before and after nanofibrillation processes had almost identical FTIR spectra, showing that the chemical composition of cellulose was not affected by mechanical treatment. 8
Figure 3
Fig. 4 shows XRD patterns of the peanut shell before and after chemical-mechanical treatment. It’s obvious that the patterns of all the samples were fairly close to typical cellulose I type with diffraction peaks at 2θ = 22.7° and 16.5°. Although the details of the diffraction patterns appeared small differences in the half-width and in the position of the diffraction peaks, we can also considered the chemical-mechanical process in this study caused little effect on the crystalline character of cellulose. After purification the relative degree of cellulose crystallinity was calculated to be 65%, much higher than that of the raw peanut shell whose relative degree of crystallinity was 45%. It’s owed to the removal of the matrix materials such as lignin and hemicelluloses. While after nanofibrillation the sample also exhibited slightly higher crystallinity increased to 69% than the purified sample. As reported [26], the degree of polymerization of the cellulose nanofibers decreased along with the increasing mechanical process. From the XRD profiles, it can be observed that the intensities of the amorphous and crystalline areas all decreased and the proportion of crystalline areas has a slower decline. Therefore, this increment may be explained as two reasons, one is by filtration the amorphous cellulose and residual hemicelluloses could be further removed [27], the other reason is likely that the degree of mechanical degradation in this study was minimal. Figure 4
3.2. Nanocomposite structure In this study, we prepared PVA composite films reinforced with cellulose nanofibers by impregnation method. SEM images of cross sections conducted in liquid nitrogen (Fig. 5a) 9
showed the original layered structure of CNFs film was well retained, while the PVA resin was just tightly attached on the surface of film, forming a characteristic sandwich structure. And even though the interface was magnified further, there was still no obvious delamination observed between PVA and cellulose nanofibers, revealing that the successful bonding formed during impregnation. Due to its layer structure, the force must breakthrough every layer of the material when it was stretched, which can ensure the strong interlaminar fracture toughness of the composites (Fig. 5b). Figure 5
3.3. Optical properties To substantiate the fineness and uniformity of the nanofibers, Fig. 6 compares the regular light transmittance spectra of different films at a 600 nm wavelength as well as the appearance of CNFs film and CNFs/PVA composites film. Recently, Yano et al. [28] reported that nanocomposite using the nanofibers as reinforcement was optically transparent even with high fiber content due to the size effect. Obviously visual appearance of the half-folded composite film in Fig. 6c demonstrated the excellent optical transparency in all colors. Combining with the transmission spectrum in Fig. 6a, the regular transmittance of the nanocomposite film was increased from 66% to 84% at a 600 nm wavelength by impregnating into PVA resin. This confirms that PVA was effective in improving the transparency of the material. The reason is that light scattering can be suppressed owed to the smooth surfaces of the composites which can be obtained by the lamination of optically transparent PVA resin [4]. When comparing the light transmittance against that of pure PVA resin, we found that the degradation of light transmission of the reinforcing peanut shell 10
fibers was only 7% at a fiber content as high as 80 wt%. This indicated that the uniformly nano-sized fibrillation of peanut shell fibers in this study was good for enhancing the light transmittance of the fiber composites. Therefore, the CNFs-reinforced PVA films are sufficiently transparent and flexible. Figure 6
3.4. Mechanical properties Generally, mechanical properties are the key factors determining the reinforcing effect of cellulose nanofibers isolated from peanut shell for PVA. Thus we compared the stress-strain curves, Young’s modulus and tensile strengths of sheets prepared from the CNFs, PVA and CNFs/PVA composite in Fig. 7. These comparisons showed significant difference among samples. CNFs film, which was based on nano-sized fiber units, exhibited excellent mechanical properties with Young’s modulus of 7.1 GPa and tensile strength of 182 MPa, respectively. These high values are due to the high aspect ratio and high crystallinity of CNFs. Furthermore, mechanical properties of the PVA film were significantly improved by using CNFs as reinforcing element. Particularly the average values of modulus and tensile strength of the PVA increased from 0.6 GPa to 6.0 GPa and 31 MPa to 123 MPa, respectively. This magnitude of the increments was due to the rigid three-dimensional nano-structural network of CNFs film, restricting the growth of PVA matrix and resulting better mechanical properties [29]. Besides, as investigated from SEM images, the structural characteristic of CNFs film was not affected by resin during the preparation process, so the outstanding mechanical properties of CNFs film were retained. Figure 7 11
As was stated above, the improvement could be contributed to the non disintegrated CNFs sheet in this work. Actually, the nanocomposites could not only show great improvements in both modulus and tensile strength, but also maintain certain toughness upon the continuity of original web-like structure [30]. In Fig. 7a, the fracture strain of CNFs/PVA composites dropped to 8.6% due to the stiff nanofibers, however, the toughness of the composites is still excellent. Therefore, as for CNFs/PVA composites, the additional fibers improved the stiffness of PVA and PVA improved the flexibility of fibers simultaneously. In comparison with the cellulose nanofibers reported from a series of agricultural residues [12, 20], the samples in the present study exhibited better mechanical properties, which demonstrated the strong potential utilization of peanut shell cellulose nanofibers for biocomposite applications such as membranes, tissue scaffolds, and so on. 3.5. Thermal properties The reinforcement effect of cellulose nanofibers can also be characterized to reduce the thermal expansion of plastics examined using the TMA. In Fig. 8 the coefficient of thermal expansion (CTE) showed significant differences between PVA resin sheet and nanocomposites. As shown in the figure, the CTE of the resin decreased from 124 to 19.1 ppm/K from 20 to 140 ℃ by adding 80 wt% cellulose nanofibers. As reported, thermal stability has an inverse relationship with Young’s modulus [31]. Consequently the effect can be owed to the function of celloluse nanofiber sheet, whose Young’s modulus was up to 7.1 GPa and CTE was as low as 12.2 ppm/K. Therefore, cellulose nanofibers with low thermal expansion and high modulus are the perfect reinforcement for enhancing the thermal stability of PVA resin. 12
Figure 8
4. Conclusions In this study, cellulose nanofibers were isolated from peanut shell and were used as a reinforcing material for PVA to produce biocomposites by impregnation method. It is clear that the mechanical separation after the chemical purification steps of peanut shell enabled the isolation of cellulose nanofibers with fine uniform network. FE-SEM observations revealed that the fibrous fragments had highly uniform diameters in the range of 15-50 nm, ensuring the transparency of PVA resin. The composite films were optically transparent and flexible and showed high mechanical and thermal properties after incorporation with CNFs. Compared to pure PVA resin, the prepared nanocomposites whose modulus and tensile strength increased from 0.6 GPa to 6.0 GPa and from 31 MPa to 125 MPa respectively, exhibited excellent mechanical properties when the fiber content was up to 80 wt%. While the light transmission of the composite only decreased 7%. It was also a kind of composites with CTE as low as 19.1 ppm/K. These results imply the high reinforcing efficiency of the cellulose nanofibers in PVA composites applied in the areas of electro-optical device and medical device applications. Moreover, the choice of non-wood resource does not limit the range of application of the isolated nanofibers and could effectively encourage the efficient exploitation of cellulose. Acknowledgements This work was financially supported by National Natural Science Foundation of China (NSFC 31170514, 31370557), The Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Graduate Cultivation Innovative Project of 13
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9. Abe K, Iwamoto S, Yano H. Obtaining cellulose nanofibers with a uniform width of 15 nm from wood. Biomacromolecules 2007; 8(10): 3276-3278. 10. Alemdar A, Sain M. Biocomposites from wheat straw nanofibers: Morphology, thermal and mechanical properties. Compos Sci Technol 2008; 68(2): 557-565. 11. Leitner J, Hinterstoisser B, Wastyn M, Keckes J, Gindl W. Sugar beet cellulose nanofibril-reinforced composites. Cellulose 2007; 14(5): 419-425. 12. Dufresne A, Dupeyre D, Vignon MR. Cellulose microfibrils from potato tuber cells: processing and characterization of starch-cellulose microfibril composites. J Appl Polym Sci 2000; 76(14): 2080-2092. 13. Pereira ALS, Nascimento DM, Filho MS, Morais JPS, Vasconcelos NF, Feitosa JPA. Improvement of polyvinyl alcohol properties by adding nanocrystalline cellulose isolated from banana pseudostems. Carbohydr Polym 2014; 112: 165-172. 14. Bilir MH, Sakalar N, Acemioglu B, Baran E, Alma MH. Sorption of remazol brilliant blue R onto Polyurethane-type foam prepared from peanut shell. J Appl Polym Sci 2013; 127(6): 4340-4351. 15. Liu X, Dong HZ, Hou HX. Optimization of preparation of cellulose nanocrystals from peanut shells using response surface methodology. Adv J Food Sci Technol 2015; 7(6): 466-473. 16. Zhang CS, Zhang RQ, Li XP, Li YW, Shi W, Ren XT, Xu XM. Bench-scale fluidized-bed fast pyrolysis of peanut shell for bio-oil production. Environ Prog Sustain 2011; 30(1): 11-18. 17. Li W, Zhao X, Huang Z, Liu S. Nanocellulose fibrils isolated from BHKP using 15
ultrasonication and their reinforcing properties in transparent poly(vinyl alcohol) films. J Polym Res 2013; 20(8): 1-7. 18. Lu J, Wang T, Drzal LT. Preparation and properties of microfibrillated cellulose polyvinyl alcohol composite materials. Composites Part A 2008; 39(5): 738-746. 19. Cheng Q, Wang S, Rials TG. Poly(vinyl alcohol) nanocomposites reinforced with cellulose fibrils isolated by high intensity ultrasonication. Composites Part A 2009; 40(2): 218-224. 20. Roohani M, Habibi Y, Belgacem NM, Ebrahim G, Karimi AN, Dufresne A. Cellulose whiskers reinforced polyvinyl alcohol copolymers nanocomposites. Eur Polym J 2008; 44(8): 2489-2498. 21. Liu AD, Walther A, Ikkala O, Belova L, Berglund LA. Clay nanopaper with tough cellulose nanofiber matrix for fire retardancy and gas barrier functions. Biomacromolecules 2011; 12:633-641. 22. Segal L, Creely JJ, Martin AE, Conrad CM. An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer. Text Res J 1959; 29: 786-794. 23. Alemdar A, Sain M. Isolation and characterization of nanofibers from agricultural residues-wheat straw and soy hulls. Bioresour Technol 2008; 99(6): 1664-1671. 24. Zuluaga R, Putaux JL, Restrepo A, Mondragon I, Gañán P. Cellulose microfibrils from banana farming residues: isolation and characterization. Cellulose 2007; 14(6): 585-592. 25. Iwamoto S, Abe K, Yano H. The effect of hemicelluloses on wood pulp nanofibrillation and nanofiber network characteristics. Biomacromolecules 2008; 9(3): 1022-1026. 16
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17
Table Caption Tab. 1 Chemical composition of peanut shell Components
wt %
Cellulose
49.4±1.9
Lignin
33.1±1.5
Hemicellulose
8.1±0.6
Ash
9.4±0.9
18
Figure Caption
Fig. 1 Flow chart of the fabrication process of cellulose nanofiber sheet and PVA nanocomposites
19
Fig. 2 FE-SEM images of peanut shell fibers: (a) raw peanut shell fibers (1000×); (b-c) purified peanut shell fibers (500×, 20000×); (d-f) peanut shell fibers after mechanical treatments (10000×, 20000×, 40000×)
20
Fig. 3 FTIR spectra of samples: (a) raw peanut shell, (b) purified peanut shell, (c) cellulose nanofibers
Fig. 4 XRD patterns for the peanut shell and its treated samples
21
Fig. 5 FE-SEM images of fracture surface of prepared nanocomposites: (a) 1000×; (b) 10000×
Fig. 6 Regular light transmittance of PVA nanocomposites. (a) Light transmittances from 200 to 1000 nm wavelength. (b-c) The half-folded CNFs and PVA-CNFs films. Thickness of the films were 60 μm and 81 μm respectively 22
Fig. 7 Stress-strain curves in tension (a) and tensile strength and modulus (b) of PVA/CNFs composites
Fig. 8 Thermal expansion behavior of different sheets
23
S1359-835X(15)00300-0 http://dx.doi.org/10.1016/j.compositesa.2015.08.029 JCOMA 4036
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
7 July 2015 24 August 2015 26 August 2015
Please cite this article as: Wang, B., Li, D., Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell, Composites: Part A (2015), doi: http://dx.doi.org/10.1016/j.compositesa. 2015.08.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell Baoxia Wanga,b, Dagang Lia,* a
College of Material Science and Engineering, Nanjing Forestry University, Nanjing
210037, Jiangsu, People’s Republic of China b
College of Lighttextile Engineering and Art, Anhui Agricultral University, Hefei 230031,
Anhui, People’s Republic of China Corresponding author: Dagang Li, Tel: +86 25 13912981251. Email: [email protected] ABSTRACT: Cellulose nanofibers-reinforced PVA biocomposites were prepared from peanut shell by chemical-mechanical treatments and impregnation method. The composite films were optically transparent and flexible, showed high mechanical and thermal properties. FE-SEM images showed that the isolated fibrous fragments had highly uniform diameters in the range of 15-50 nm and formed fine network structure, which is a guarantee of the transparency of biocomposites. Compared to that of pure PVA resin, the modulus and tensile strength of prepared nanocomposites increased from 0.6 GPa to 6.0 GPa and from 31 MPa to 125 MPa respectively with the fiber content as high as 80 wt%, while the light transmission of the composite only decreased 7% at a 600 nm wavelength. Furthermore, the composites exhibited excellent thermal properties with CTE as low as 19.1 ppm/K. These favorable properties indicated the high reinforcing efficiency of the cellulose nanofibers isolated from peanut shell in PVA composites. Keywords: A. Nano-structures; B. Optical properties; D. Mechanical testing; D. Thermal analysis. 1
1. Introduction Recently, the fabrication of nanocomposites reinforced by natural fibrils has attracted much attention due to the environmental concerns. Among these natural fibrils, cellulose nanofibers with complete biological degradability and renewability are extensively researched. Their nanostructures which have a width ranging from 5 to 30 nm [1], are highly crystalline materials stabilized laterally by long cellulose chains with hydrogen bonding. Thus, they exhibit outstanding structural strength and stiffness, such as a high Young’s modulus of 138 GPa and a specific tensile strength of 3 GPa [2]. Cellulose nanofibers also have very unique optical and thermal properties [3-4]. Because of these characteristics, cellulose nanofibers have generated a great deal of interest as reinforcement for developing functionalized optical and lightweight nanocomposites. Currently, the most important industrial source of cellulose nanofibers is certainly wood. So far, many investigations have focused on the isolation of cellulose nanofibers from wood [5-8]. Abe et al. [9] have successfully obtained cellulose nanofibers with a uniform width of 15 nm from pine wood by grinding just one time. However, competition from different sectors such as the furniture industries and the pulp and paper industries makes it challenging to supply all users with the quantities of wood at a reasonable cost. For this reason, the isolation of nano-sized cellulose whiskers and fibers from annual fiber crops such as wheat straw [10], sugar beet [11], potato tube [12], banana pseudostem [13] and others, especially from by-products of these different plants, is of increasing interest to utilize the nano elements. Peanut shell is an important annual crop by-products, which consists of cellulose, lignin, hemicelluloses and tannins [14]. In cell walls of peanut shells, 2
cellulose nanofibers could be extracted by the removal of matrix substances as well as the fibrillation process. However, except a few works dealing with peanut shell nanocrystals [15], any work with regard to the cellulose nanofibers isolated from peanut shell has been not recorded in literature. Each year at least 4.5 megton of peanut shells were generated in China [16]. Only a small percentage is being used in applications such as feedstock and adsorbent material, while most are discarded as waste. In this regard, obtaining cellulose nanofibers based on peanut shell seems to be a prospective commercial application at lower cost. As described in some reports, cellulose nanofibers have been shown to be an ideal reinforcement for resins due to their exceptional mechanical properties and their highly uniform network structure [4]. Poly(vinyl alcohol) (PVA) is a kind of water soluble, biocompatible and biodegradable synthetic polymer. Although PVA has many unique properties such as high elasticity, hydrophilic characteristics and the ability to form good films, its poor durability and high thermal expansion restrict it from further applications. One approach is to create a PVA-based nanocomposite [17-19]. Roohani et al. [20] reported the hydroxyl groups on PVA are expected to interact with the hydrophilic surfaces of cellulose wiskers or nanofibers due to strong hydrogen bonding. In this study, we have isolated cellulose nanofibers from peanut shell with a combination of chemical and mechanical treatments, and then the morphology and structure of the isolated cellulose nanofibers were studied. Moreover, cellulose nanofibers were incorporated into PVA by impregnation method. We prepared the cellulose nanofibers-reinforced PVA biocomposites with high fiber content up to 80 wt%. At last, mechanical properties and optical transparency 3
of the composites were measured and analyzed respectively. 2. Material and methods 2.1. Materials Peanut shell powders (Jiangsu Yancheng, China) passed through a 70-mesh stainless steel sieve were used as a cellulose raw material. The dried peanut shell is characterized and its chemical composition is shown in Tab. 1. PVA (Mw=74800-79200), Sodium chlorite (NaClO2), potassium hydroxide (KOH), and acetic acid (CH3COOH) were purchased from Nanjing Chemical Regent Co., Ltd., China. Table 1
2.2. Isolation of cellulose nanofibers from peanut shell The production of cellulose nanofibers from peanut shell required a series of chemical purification and mechanical defibrillation. First, peanut shell powders were treated with acidified sodium chlorite (NaClO2) solution for 5 h at 75 ℃ to remove lignin after Soxhlet extraction. Then the delignified samples were stirred in 2 wt% potassium hydroxide (KOH) solution at 90 ℃ for 2 h. In order to completely remove the hemicelluloses and residual proteins, the alkaline treatment was repeated two times. Finally samples were further purified with an acidified sodium chlorite solution for 1 h at 75 ℃. Following chemical treatments, samples were filtered and rinsed with distilled water until the solution was neutral. The chemically treated fiber with 1 wt% undried sample was passed four times through a grinder (MKCA6-2, Masuko Sangyo Co., Ltd., Japan) at 1500 rpm with a clearance gauge of -1.5 from zero position (corresponding to a 0.15 mm shift). Subsequently the slurry was 4
diluted to 0.1 wt% with water, and then passed through a high pressure homogenizer (EmulsiFlex-C3, Avestin Co., Ltd., Canada) with a pressure of 1000 bar for additional 5 passes. As a result, a stable and translucent suspension with no sediment and flocculation based on peanut shell was prepared. The concentration of suspension was adjusted to 0.08 wt% and part of them was freeze-dried for FE-SEM. 2.3. Fabrication of the PVA nanocomposites Composite films of PVA and cellulose nanofibers were prepared by impregnation method. Firstly, 400 ml cellulose nanofibers suspension was slowly vacuum filtered using a polytetrafluoroethylene (PTFE) membrane to form a wet sheet which was sandwiched between two glass plates and dried in oven at 65 ℃ for 48 h with a slight pressure. The dry cellulose nanofibers film was approximately 60 μm thick with a diameter of 9 cm in circle shape and with the density of 1.39 g/cm3. PVA powder was dissolved in distilled water and stirred at 80 ℃ for 2 h to produce a 5 wt% solution. Then the dried cellulose nanofibers film was impregnated into PVA solution and maintained at room temperature for 12 h. The impregnated samples were subsequently taken out of the resin and dried at 40 ℃ overnight. Finally, cellulose nanofibers-reinforced PVA composites were obtained. In this process, the thickness of the resulting samples was about 80 μm. By consulting the sketch of the preparation process of clay nanopaper [21], we draw up the corresponding process flow chart shown in Fig. 1 of PVA nanocomposites. According to the weights of cellulose nanofibers sheet and nanocomposites, fiber content was calculated to be about 80 wt%. Figure 1
2.4. Characterization and testing 5
Field emission scanning electron microscope (FE-SEM) was used to investigate the morphology of the peanut shell, cellulose nanofibers and biocomposites. Samples were freeze-dried at -55 ℃ for 48 h. The sample was firstly sputter-coated with 2-nm layer of platinum to prevent the buildup of an electrostatic charge and then observed using a FE-SEM (S-4800, Hitachi High-Tech. Corp., Japan) operating at 5 kV. Fourier transform infrared (FTIR) spectra (transmission) were measured on a Nicolet iS10 FTIR spectrometer (Thermo Scientific Inc., America) equipped with a single reflection attenuated total reflectance (ATR) system. All samples were tested in the range of 4000-500 cm-1 at a resolution of 2 cm-1. The crystal structure and crystallinity of samples were examined by an Ultima IV X-ray diffractometer (Rigaku Corp., Japan) with CuKα radiation operating at 40 kV and 30 mA. The raw peanut shell and purified samples were made into pallets by pressing after dried at 105 ℃. For the isolated nanofibers, ten-layered dry cellulose nanofibers films were used for the measurement. All samples were scanned at a speed of 5 s per step over a scanning range from 5° to 40°. Five samples for each were subjected to the measurement. The relative degree of cellulose crystallinity was calculated according to the Segal method [22]. Tensile tests of the films were carried out with a universal material-testing machine equipped with a 100 N load cell (SANS, Shenzhen, China) at room temperature. Specimens of 40 mm length and 60-90 μm thickness and 5 mm width were tested with the crosshead speed of 1 mm/min. Prior to testing, the specimens were conditioned for at least 48h in the environment of relative humidity at 50% and temperature at 23 ℃. Light transmittance spectroscopy of the film was collected using a UV-Visible near infrared 6
spectrometer (U-4100, HITACHI Inc., Japan) at wavelengths from 200 to 1000 nm and the scan speed was set as 300 nm/min. The coefficient of thermal expansion (CTE) of films was investigated using a thermo mechanical analyzer (TMA-402-F1, NETZSCH, Germany). The samples of 20 mm length and 3 mm width with a 16 mm span were tested twice with a heating rate of 5 ℃/min in a nitrogen atmosphere at a load of 0.2 N. The values were identified as the mean values from 20 ℃ to 140 ℃ in the second run. 3. Results and discussion 3.1. Characterization of cellulose nanofibers (CNFs)
Figure 2
Dried peanut shell was used as a starting material. Fig. 2 shows the structure and appearance of peanut shell fibers in macro- to micro-scale by SEM. As seen in Fig. 2a, the raw peanut shell has porous morphology of the surface particles. The matrix components such as lignins and hemicelluloses are located in the outer waxy layer, so single fibers are difficult to be observed. After purification, the twisted rod-like structure of the peanut shell fibers about 10 μm wide was observed (Fig. 2b) and the individual microfibril bundles approximately 15 nm wide could be clearly seen from the fiber surfaces (Fig. 2c). In order to obtain nanoscalar cellulose fibers from thick bundles, we applied a series of mechanical treatment including grinding and high-pressure homogenization. The treated peanut shell fibers were observed in Fig. d-f. Compared to the original aggregates, the fibers were found to have been disintegrated into micro- to nano-meter-sized fragments (Fig. 2d, e). Higher magnification (Fig. 2f) revealed that the fibrous fragments extracted in this study had uniform diameters in 7
the range of 15-50 nm along with the formation of high aspect ratio CNFs, confirming that high-quality nanofibers were successfully obtained from peanut shell as reported from wood. The individual CNFs within the bundles self-aggregated along the longitudinal direction and formed a very fine network. Although identifying an exact length of the nanofibers was difficult, it was evident that their lengths were longer than 1μm. This suggested that their higher aspect ratio would guarantee a greater reinforcing efficiency corresponding to the nanofibers reported from other agricultural wastes such as soy hulls and banana rachis [23-24]. Fig. 3 represents three FTIR spectra of raw peanut shell, purified peanut shell, and obtained cellulose nanofibers, respectively. The OH, CH2 group stretching vibrations at 3334 and 2893 cm-1, C-O stretching vibration at 1092 cm-1 and C-H rocking vibration at 895 cm-1, observing in all the samples, were the special characteristics of lignocellulosic materials. The absorption bands at 1460 and 1508 cm-1 existed in Fig. 3a completely disappeared in Fig. 3b, indicating that all the lignin had been removed sufficiently. On the other hand, the sharp peak at 1731 cm-1 corresponding to the unconjugated carbonyl from acetyl group almost disappeared after chemical treatment, which suggested a strong reduction of hemicelluloses. However, a small peak at 1640 cm-1 existed in Fig. 3b, c showed some residual hemicelluloses, which were insoluble in alkali might have been present in the fibers. Iwamoto et al. [25] reported the presence of hemicelluloses was beneficial to nanofibrillation. The sample before and after nanofibrillation processes had almost identical FTIR spectra, showing that the chemical composition of cellulose was not affected by mechanical treatment. 8
Figure 3
Fig. 4 shows XRD patterns of the peanut shell before and after chemical-mechanical treatment. It’s obvious that the patterns of all the samples were fairly close to typical cellulose I type with diffraction peaks at 2θ = 22.7° and 16.5°. Although the details of the diffraction patterns appeared small differences in the half-width and in the position of the diffraction peaks, we can also considered the chemical-mechanical process in this study caused little effect on the crystalline character of cellulose. After purification the relative degree of cellulose crystallinity was calculated to be 65%, much higher than that of the raw peanut shell whose relative degree of crystallinity was 45%. It’s owed to the removal of the matrix materials such as lignin and hemicelluloses. While after nanofibrillation the sample also exhibited slightly higher crystallinity increased to 69% than the purified sample. As reported [26], the degree of polymerization of the cellulose nanofibers decreased along with the increasing mechanical process. From the XRD profiles, it can be observed that the intensities of the amorphous and crystalline areas all decreased and the proportion of crystalline areas has a slower decline. Therefore, this increment may be explained as two reasons, one is by filtration the amorphous cellulose and residual hemicelluloses could be further removed [27], the other reason is likely that the degree of mechanical degradation in this study was minimal. Figure 4
3.2. Nanocomposite structure In this study, we prepared PVA composite films reinforced with cellulose nanofibers by impregnation method. SEM images of cross sections conducted in liquid nitrogen (Fig. 5a) 9
showed the original layered structure of CNFs film was well retained, while the PVA resin was just tightly attached on the surface of film, forming a characteristic sandwich structure. And even though the interface was magnified further, there was still no obvious delamination observed between PVA and cellulose nanofibers, revealing that the successful bonding formed during impregnation. Due to its layer structure, the force must breakthrough every layer of the material when it was stretched, which can ensure the strong interlaminar fracture toughness of the composites (Fig. 5b). Figure 5
3.3. Optical properties To substantiate the fineness and uniformity of the nanofibers, Fig. 6 compares the regular light transmittance spectra of different films at a 600 nm wavelength as well as the appearance of CNFs film and CNFs/PVA composites film. Recently, Yano et al. [28] reported that nanocomposite using the nanofibers as reinforcement was optically transparent even with high fiber content due to the size effect. Obviously visual appearance of the half-folded composite film in Fig. 6c demonstrated the excellent optical transparency in all colors. Combining with the transmission spectrum in Fig. 6a, the regular transmittance of the nanocomposite film was increased from 66% to 84% at a 600 nm wavelength by impregnating into PVA resin. This confirms that PVA was effective in improving the transparency of the material. The reason is that light scattering can be suppressed owed to the smooth surfaces of the composites which can be obtained by the lamination of optically transparent PVA resin [4]. When comparing the light transmittance against that of pure PVA resin, we found that the degradation of light transmission of the reinforcing peanut shell 10
fibers was only 7% at a fiber content as high as 80 wt%. This indicated that the uniformly nano-sized fibrillation of peanut shell fibers in this study was good for enhancing the light transmittance of the fiber composites. Therefore, the CNFs-reinforced PVA films are sufficiently transparent and flexible. Figure 6
3.4. Mechanical properties Generally, mechanical properties are the key factors determining the reinforcing effect of cellulose nanofibers isolated from peanut shell for PVA. Thus we compared the stress-strain curves, Young’s modulus and tensile strengths of sheets prepared from the CNFs, PVA and CNFs/PVA composite in Fig. 7. These comparisons showed significant difference among samples. CNFs film, which was based on nano-sized fiber units, exhibited excellent mechanical properties with Young’s modulus of 7.1 GPa and tensile strength of 182 MPa, respectively. These high values are due to the high aspect ratio and high crystallinity of CNFs. Furthermore, mechanical properties of the PVA film were significantly improved by using CNFs as reinforcing element. Particularly the average values of modulus and tensile strength of the PVA increased from 0.6 GPa to 6.0 GPa and 31 MPa to 123 MPa, respectively. This magnitude of the increments was due to the rigid three-dimensional nano-structural network of CNFs film, restricting the growth of PVA matrix and resulting better mechanical properties [29]. Besides, as investigated from SEM images, the structural characteristic of CNFs film was not affected by resin during the preparation process, so the outstanding mechanical properties of CNFs film were retained. Figure 7 11
As was stated above, the improvement could be contributed to the non disintegrated CNFs sheet in this work. Actually, the nanocomposites could not only show great improvements in both modulus and tensile strength, but also maintain certain toughness upon the continuity of original web-like structure [30]. In Fig. 7a, the fracture strain of CNFs/PVA composites dropped to 8.6% due to the stiff nanofibers, however, the toughness of the composites is still excellent. Therefore, as for CNFs/PVA composites, the additional fibers improved the stiffness of PVA and PVA improved the flexibility of fibers simultaneously. In comparison with the cellulose nanofibers reported from a series of agricultural residues [12, 20], the samples in the present study exhibited better mechanical properties, which demonstrated the strong potential utilization of peanut shell cellulose nanofibers for biocomposite applications such as membranes, tissue scaffolds, and so on. 3.5. Thermal properties The reinforcement effect of cellulose nanofibers can also be characterized to reduce the thermal expansion of plastics examined using the TMA. In Fig. 8 the coefficient of thermal expansion (CTE) showed significant differences between PVA resin sheet and nanocomposites. As shown in the figure, the CTE of the resin decreased from 124 to 19.1 ppm/K from 20 to 140 ℃ by adding 80 wt% cellulose nanofibers. As reported, thermal stability has an inverse relationship with Young’s modulus [31]. Consequently the effect can be owed to the function of celloluse nanofiber sheet, whose Young’s modulus was up to 7.1 GPa and CTE was as low as 12.2 ppm/K. Therefore, cellulose nanofibers with low thermal expansion and high modulus are the perfect reinforcement for enhancing the thermal stability of PVA resin. 12
Figure 8
4. Conclusions In this study, cellulose nanofibers were isolated from peanut shell and were used as a reinforcing material for PVA to produce biocomposites by impregnation method. It is clear that the mechanical separation after the chemical purification steps of peanut shell enabled the isolation of cellulose nanofibers with fine uniform network. FE-SEM observations revealed that the fibrous fragments had highly uniform diameters in the range of 15-50 nm, ensuring the transparency of PVA resin. The composite films were optically transparent and flexible and showed high mechanical and thermal properties after incorporation with CNFs. Compared to pure PVA resin, the prepared nanocomposites whose modulus and tensile strength increased from 0.6 GPa to 6.0 GPa and from 31 MPa to 125 MPa respectively, exhibited excellent mechanical properties when the fiber content was up to 80 wt%. While the light transmission of the composite only decreased 7%. It was also a kind of composites with CTE as low as 19.1 ppm/K. These results imply the high reinforcing efficiency of the cellulose nanofibers in PVA composites applied in the areas of electro-optical device and medical device applications. Moreover, the choice of non-wood resource does not limit the range of application of the isolated nanofibers and could effectively encourage the efficient exploitation of cellulose. Acknowledgements This work was financially supported by National Natural Science Foundation of China (NSFC 31170514, 31370557), The Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Graduate Cultivation Innovative Project of 13
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Table Caption Tab. 1 Chemical composition of peanut shell Components
wt %
Cellulose
49.4±1.9
Lignin
33.1±1.5
Hemicellulose
8.1±0.6
Ash
9.4±0.9
18
Figure Caption
Fig. 1 Flow chart of the fabrication process of cellulose nanofiber sheet and PVA nanocomposites
19
Fig. 2 FE-SEM images of peanut shell fibers: (a) raw peanut shell fibers (1000×); (b-c) purified peanut shell fibers (500×, 20000×); (d-f) peanut shell fibers after mechanical treatments (10000×, 20000×, 40000×)
20
Fig. 3 FTIR spectra of samples: (a) raw peanut shell, (b) purified peanut shell, (c) cellulose nanofibers
Fig. 4 XRD patterns for the peanut shell and its treated samples
21
Fig. 5 FE-SEM images of fracture surface of prepared nanocomposites: (a) 1000×; (b) 10000×
Fig. 6 Regular light transmittance of PVA nanocomposites. (a) Light transmittances from 200 to 1000 nm wavelength. (b-c) The half-folded CNFs and PVA-CNFs films. Thickness of the films were 60 μm and 81 μm respectively 22
Fig. 7 Stress-strain curves in tension (a) and tensile strength and modulus (b) of PVA/CNFs composites
Fig. 8 Thermal expansion behavior of different sheets
23