Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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A Chemical Process for Preparing Cellulose

Nanowhiskers (CNW) from Kenaf Bast Fibers for Polymer Composites

Jinshu Shi, Sheldon Shi,

H. Michael Barnes

Department of Forest Products, Mississippi State University,

Starkville, MS, USA

Charles U. Pittman, Jr. Department of Chemistry,

Mississippi State University, Starkville, MS, USA

Abstract

The objective of this research was to develop a chemical process to prepare cellulose nanowhiskers (CNWs) from kenaf bast fibers. With this process, the CNWs were obtained without using specialized equipment, such as high-pressure homogenizers or cryo-crushing equipment. The procedures used in this all-chemical process include alkaline retting (to obtain single cellulosic fibers), bleaching (to obtain bleached fibers) and acidic hydrolysis (to obtain cellulose microfibrils and CNWs). At each step of this chemical process, the resultant fibers were characterized for crystallinity using X-ray diffraction (XRD), for surface functional groups using the Fourier Transform Infrared spectroscopy (FTIR), and for surface morphology using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The fiber yields were 26.3% for the cellulose microfibrils and 10.4% for the CNWs. The crystallinity of the fibers increased at each treatment step from 49.9% (retted fibers) 83.9% (CNWs). The CNWs had fiber lengths of 100 nm to 1,400 nm, and diameters of 7 to 84 nm. Aspect ratios were in the range of 10-50. The use of the CNWs in polyvinyl alcohol (PVA) composites enhanced the tensile properties compared with the neat PVA. Keywords Cellulose nanowhisker, kenaf fiber, chemical processes, CNW/PVA composites

Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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Introduction The natural fibers used to reinforce the polymer composites are mainly in a form of single cellulosic fiber or fiber bundles obtained from wood or agricultural plants through retting process, such as chemical, mechanical, bio-retting, and the retting processes. Kenaf bast fiber is a promising reinforcement element for polymer composites because of its high cellulose content and fast growing rate. The cellulosic fibers are composed of amorphous regions and crystalline regions in cellulose chains, together with some lignin, pectin and hemicelluloses. The crystalline regions are highly ordered with a high strength, while the amorphous regions are disordered, which are susceptible to water, chemicals and enzyme modifications, so that the resulting products exhibit poor dimensional stability and less resistance to chemicals and enzymes. Removing the hemicelluloses and lignin, and reducing the amorphous regions can effectively increase cellulose content and percentage of crystalline regions n cellulosic fibrils, so that fibers will have a much higher strength. Zadorecki and Anthony (1989) reported that the elastic moduli of solid wood, single pulp fiber, microfibrils and crystallites were 10 GPa, 40 GPa, 70 GPa and 250 GPa, respectively. Thus, by breaking down the cellulosic fiber into micro or nano scale, the strength of the resulting fibers can be improved significantly. Several technologies to prepare cellulose nanofibers have been reported, including an enzymic method (Henriksson et al. 2007), bacterial method (Tsuchida and Yoshinaga 1997), cryocrushing (Chakraborty et al. 2005), a grinding treatment (Iwamoto et al. 2005), an ultrasonic technique (Wang 2006) and an electrospinning technique (Kulpinski 2005; Sui et al. 2008). All these methods require using a combination of processes including chemical, mechanical, and others. These cellulose nanofibers were mainly long flexible cellulose strings consisting of alternating crystalline and amorphous regions. Another type of cellulose “nanofiber” is a rod-like “nanoparticle”. This could be obtained through further acid hydrolysis of the cellulosic fibers. Different terminologies are used to designate these rod-like “nanoparticles” or “nanofibers”, e.g. nanowhiskers, monocrystals, nanocrystals and etc. (Siqueira et al. 2009). In this study, we use the term "cellulose nanowhisker (CNW) ".

Objectives

The objective of this study is to use an all chemical method to obtain the CNWs by extracting the cellulosic fibers from kenaf bast using alkaline retting, followed by bleaching and then, acid hydrolysis, step by step. The first two chemical treatments mainly remove lignin and hemicelluloses. The acid hydrolysis results in the cleavage of the cellulose molecular chains at the amorphous regions. The retted fibers, bleached fibers and the microfibers and CNWs were characterized respectively. The CNWs reinforced polyvinyl alcohol (PVA) composites were fabricated. The tensile strength of these CNW/PVA composites was evaluated.

Materials

Kenaf bast fibers obtained from Mississippi State University’s (MSU) North Farm were used as the raw material. Sodium hydroxide beads (laboratory grade, from Fisher

Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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Scientific.) were used to prepare the 5% aqueous solution. Glacial acetic acid was used to neutralize the pH value of the alkaline retting system. Hydrogen peroxide (37%) solution and sulfuric acid 98% solution were diluted to 10% and 30%, respectively. The original solutions were technical grade from Fisher Scientific. Polyvinyl alcohol (MW=100,000) powder (Fisher Scientific) was used to fabricate the composites.

Methods

Chemical Treatments of Kenaf Bast Fibers The kenaf bast fibers with a moisture content of 11% were retted in a 5% NaOH solution at 160°C for 1 hour. A sealed reactor was used in which the alkaline liquid reached its autogeneous vapor pressure. After the pH value of the retting liquid and the retted fibers was adjusted to 7.0 using acetic acid, the retted fibers were washed with water to remove the chemicals from the fibers. The retted fibers were then bleached with 10% H2O2 at 70 °C for 1 hour in order to remove the remaining lignin. Acid hydrolysis of the bleached fibers was then conducted with 30% H2SO4 at 80°C under the mechanical stirring for 4 hours. A suspension of microfibers and CNWs was obtained from the acid hydrolysis. The acid was removed by Eppendorf Centrifuge (Model 5810) at a rotating speed of 6,500 rpm for 5 minutes and subsequent dialysis against distilled water until the suspension became neutral. Then, a higher centrifugation speed of 7,600 rpm was applied to separate the CNWs from the microfibers. The CNW suspension was sonicated to disrupt the of nanowhiskers aggregation for 10 minutes using Cole-Parmer ultrasonic processor with CV33 converter and 13mm probe (750 watts, 20 kHz, 40% amplitude of vibration). The samples of retted fibers, bleached fibers, microfibers and CNWs were freeze-dried before characterization. The yields of retted fibers, bleached fibers, microfibers and CNWs were calculated based on the oven-dry weight comparing to the original weight of raw kenaf bast fiber.

CNW/PVA Composites Fabrication PVA aqueous solutions were mixed with CNW aqueous suspensions followed by ultrasonic treatment for 5 min (750 watts, 20 kHz, 40% amplitude of vibration) in order to homogenize the distribution of CNWs in the mixtures. The weight ratios of CNW to PVA were controlled at 1:99, 3:97, 6:94 and 9:91, respectively. CNW/PVA composite films, with the CNWs loading of 1%, 3%, 6% and 9%, were fabricated after the evaporation of the water at room temperature and atmospheric pressure. CNW/PVA composites were dried at 50°C for 12 hours and stored in vacuum bags before testing and analyzing. Fiber and composites Characterization Surface Analysis The samples of retted fibers, bleached fibers and microfibers were coated with 15nm-thick gold layer to provide electrical conductivity. Scanning electron microscopy (SEM, Zeiss Supra TM 40) was applied to analyze the fibers morphology using an accelerating voltage of 15 kV. The CNW samples for morphology analysis were obtained by placing a drop of the CNW suspension onto a grid without any staining, and drying it in air at ambient temperature. The dried samples were examined in a JEOL JEM-2000 EX-II transmission electron microscopy (TEM) at an accelerating voltage of 100 kV. Fourier Transform

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October 11-14, 2010, Geneva, Switzerland

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Infrared (FTIR) spectra were recorded to analyze the functional groups of the fibers on a Thermo Scientific Nicolet 6700 spectrophotometer. Fiber Dimension Analysis A Zeiss Axiovert 200M microscope was used to examine the retted fibers and bleached fibers. Over one hundred retted fibers and bleached fibers were measured using the software AxioVision Rel. 4.8 to obtain the length and diameter statistics. The statistics on the dimensions of microfibers and CNWs were obtained from their SEM and TEM images. Crystallinity Analysis The crystallinities of the four types of fibers were measured using a Rigaku SmartLab X-ray Diffraction System with an operation voltage of 40 kV and a current of 44 mA. The crystallinities of the fibers (χCR) were calculated by the Segal method (Segal et al. 1959) as shown in Equation (1). The height of the 200 peak (I200, 2θ = 22.7˚) and the minimum between the 200 and 110 peaks (200 peak is at 2θ = 22.7˚; 110 peak is at 2θ = 16˚; IAM is at 2θ = 18˚) were used for the calculation. I200 represents both the crystalline and amorphous regions, while IAM represents the amorphous regions only. χCR = (I200-IAM)/I200 Eq.(1) Tensile Testing Composites with different CNW contents were tested using Instron 5869 universal testing machine in accordance with ASTM D638-08. A minimum of three replicates was used.

Results and Discussion Yields The yield results are shown in Table 1.

Table 1: Yields of chemically treated fibers based

on the weight of the raw kenaf bast fiber.

Types of fibers Retted fiber Bleached fiber Microfiber CNW

Yields 44.6% 41.4% 26.3% 10.4%

Alkaline retting aimed to remove most of the lignin and hemicelluloses in the kenaf bast fibers. The fiber yield of 44.6% (by mass) was obtained after the alkaline retting. The α-cellulose content of the raw kenaf bast fibers was determined as 45.95%, which was close to the yield of the retted fibers, indicating that the components remaining in the fiber after retting were mainly α-cellulose. Bleaching treatment removed the remaining lignin in the cellulosic fibers. Cellulose molecular chains were also cleaved during bleaching. A 41.4%

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October 11-14, 2010, Geneva, Switzerland

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yield was obtained after bleaching. The cellulosic fibers were hydrolyzed by the sulfuric acid within the amorphous regions of cellulose molecular chains. The percentage of the crystalline region increased, while the fiber size was reduced by the acid hydrolysis. Some cellulosic fibers were degraded into individual nanowhiskers, while others were still bound in the cell wall. The yield of CNWs was 10.4%, while the yield of microfiber was 26.3%. Considering the 45.95% α-cellulose content in kenaf bast fiber, about 22.6% of α-cellulose had been converted into CNWs by the process used here. Morphology of the Fibers The length and diameter statistics of the fibers are shown in Table 2.

Table 2: Fiber length and diameter statistics.

Retted fiber

(µm)

Bleached fiber

(µm)

Microfiber

(µm)

CNW

(nm)

Length Mean 468.32 215.32 82.52 628.38

Stdev

.

605.94 141.62 27.99 360.05

Diameter Mean 18.19 18.09 12.43 34.75

Stdev

.

6.37 5.37 2.36 21.43

A SEM image of the microfibers and TEM images of CNWs are shown in Figures 1 and 2.

Figure 1: SEM image of cellulosic microfibers.

Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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(a) (b)

Figure 2: TEM images of cellulose nanowhiskers (CNWs). (a) magnification = 50,000X, accelerating voltage = 100 kV; (b) magnification= 370,000X, accelerating voltage = 100

kV.

The cell walls of kenaf bast fibers were not disintegrated completely in the microfibers. Further acidic hydrolysis may be required to break the cellulosic fiber from the micro scale to the nano scale. Alternatively, a mechanical treatment should be applied in order to liberate the individual microfibrils from the cell walls. A high variation of the microfiber lengths, ranging from 10 um to 100 um, was observed. The diameters of the microfibers were about 10 um. The lengths of the CNWs were in a range from100 to 1,400 nm, and the diameters of the CNWs were ranged from 7 to 84 nm. The CNW aspect ratios ranged from 10 to 50. An individual CNW was composed of several parallel aligned crystallites which could be observed from Fig. 2 between the two arrows. The width of a crystallite was measured about 5.4 nm. The dimensions of the monoclinic unit cell of the regenerated cellulose were reported (Nugmanov 1987) as a = 8.14 Å, b = 9.19 Å, c = 10.3 Å (fiber axis). The widths of the CNWs were obtained in a range from 7 to 84 nm, while the lengths were from 100 to 1,400 nm. Therefore, an individual CNW was composed of 100 to 1,300 unit cells aligned in the direction of the fiber axis (c). A cross section of an individual CNW consisted of 8 to103 unit cells, or, 1 to 15 crystallites. Fourier Transform Infrared (FT-IR) Spectroscopy In Fig. 3, the FT-IR spectra show the functional groups in the fiber surfaces and within the regions where the infrared signals could detected.

Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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Figure 3: FT-IR spectra of the fibers

The hydroxyl stretching bonds are found between 3,000 cm-1 to 3,500 cm-1. Clearly, a highly hydrogen bonded network exists, as indicated by the lower frequencies, with some free hydroxyls at high frequencies. The intensity of this peak envelope increased gradually from untreated fiber, retted fiber, bleached fiber, microfiber to nanofiber because the specific surface area of the fibers increased and more hydroxyl groups in the surface and in the regions below the surface are exposed as the fibers size reduced from the macro scales to the nano scales. The peaks at 2,896.6 cm-1, 1,718.3 cm-1, 1,307.5cm-1 and 1,020 cm-1 represent the C-H, C=O, C-O and C-C stretching, respectively. The peak at 1,648.8 cm-1 (C=C stretching) found in untreated kenaf bast fibers disappeared in all the treated fibers. This may be due to the removal of carbon-carbon unsaturation presented in lignin components and extractives.

Crystallinity The X-ray diffraction spectra of the fibers are shown in Fig. 4. The calculated degrees of fiber crystallinity of the fibers are showed in Table 3.

Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe – Timber Committee

October 11-14, 2010, Geneva, Switzerland

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Figure 4: X-ray diffraction spectrums of the fibers.

Table 3: Crystallinities of the raw kenaf bast fiber and the chemcially treated fibers.

Types of fibers Raw kenaf

bast fiber

Retted

fiber

Bleached

fiber

Microfiber CNW

Crystallinities 49.9% 63.8% 68.9% 83.5% 83.9%

The fiber crystallinities gradually increased at each stage of the process. Alkaline retting removes lignin and hemicelluloses, so that the percentage of the crystalline regions in cellulose increased. Hydrogen peroxide bleaching accelerated the cleavage of the cellulose molecular chains within the amorphous regions, resulting in the further increase of the crystallinity of the bleached fibers. In addition, the remaining lignin was degraded by hydrogen peroxide and removed during bleaching. Acid hydrolysis improved the crystallinity of the fibers significantly by the cleavage of glycosidic bonds in cellulose molecular chains within amorphous regions. Therefore, the amorphous regions were greatly diminished. However, the crystalline regions were highly resistant to chemical attack including acid hydrolysis. Tensile Strengths of CNW/PVA Composites The tensile strengths of the CNW/PVA composites were shown in Fig. 5.

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October 11-14, 2010, Geneva, Switzerland

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Figure 5: Tensile strength of the CNW/PVA composites

The tensile strength of the composites increased as the CNW content was raised from 0% to 9%. The tensile strength of the PVA composites was improved by 46.2% when 9% CNWs was incorporated. High interlaminar shear strength between CNW and PVA resulted in high energy absorption that caused crack growth during failure. The high interlaminar shear strength may be due to the hydrogen bonds between CNW and PVA.

Conclusions CNWs have been successfully prepared through an all chemical process. Approximately 22.6% of the α-cellulose in the raw kenaf bast fibers could be converted into CNWs. The chemical treatments reduced the average length and diameter of the fibers. The fiber crystallinity increased at each stage of the chemical processes with the removal of lignin and hemicelluloses and with the reduction of amorphous regions of cellulose molecular chain. A high CNW crystallinity was obtained at 83.9%. The CNWs endowed the CNW/PVA composites with a significantly improved tensile strength of 46.2% when only 9% CNWs was incorporated.

References

ASTM D 638-08. 2008. Standard Test Method for Tensile Properties of Plastics.

Chakraborty, A. Sain, M. Kortschot, M. 2005. Cellulose microfibrils: A novel method of preparation using high shear refining and cryocrushing. Holzforschung. 59:102-107. Henriksson, M. Henriksson,G. Berglund,L.A. Lindström, T.2007. An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers. European Polymer Journal. 43:3434-3441. Iwamoto, S. Nakagaito, A.N. Yano,H. Nogi, M. 2005. Optically transparent composites reinforced with plant fiber-based nanofibers. Applied Physic A. 81:1109-1112.

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Kulpinski, P. 2005. Cellulose nanofibers prepared by the N-methylmorpholine-N-oxide method. Journal of Applied Polymer Science. 98:1855-1859.

Nugmanov, O.K. Pertsin, A.I. Zabelin, L.V. Marchenko, G.N. 1987. The molecular-crystal structure of cellulose. Russian Chemical Reviews. 56 (8):764-776. Segal, L. 1959. An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer. Textile Research Journal. 29: 786-794. Siqueira, G. Bras, J. Dufresne, A. 2009. Cellulose whiskers versus microfibrils: influence of the nature of the nanoparticle and its surface functionalization on the thermal and mechanical properties of nanocomposites. Biomacromolecules. 10: 425-432. Sui, X. Yuan, J. Yuan, W, Zhou, M. 2008. Preparation of cellulose nanofibers/nanoparticles via electrospray. Chemistry Letters. 37(1):114-115. Tsuchida, T. and Yoshinaga, F. 1997. Production of bacterial cellulose by agitation culture systems. Pure and Applied Chemistry. 69 (11): 2453-2458. Wang, S. Cheng, Q. Rials, T.G. Lee, S.H. 2006. Cellulose microfibril/nanofibril and its nanocompsites. Proceedings of the 8th Pacific Rim Bio-based Composites Symposium. Zadorecki, P. J. and Anthony, M. 1989. Future prospects for wood cellulose as reinforcement in organic polymer composites. Polymer Composites. 10 (2): 69-77.