Abstract
Cellulose nanocrystals were extracted from Agave angustifolia fibres by alkali and bleaching treatments followed by acid hydrolysis. The chemical composition of the Agave fibres was determined at different stages of chemical treatment. The structural analysis was carried out by a Fourier Transform Infrared spectroscopy and X-ray diffraction. The morphology and thermal stability of the Agave fibres at different stages of chemical treatment were investigated by field emission scanning electron microscopy and thermogravimetric analysis, respectively. The results indicated that the hemicellulose and lignin were removed extensively from the extracted cellulose. The two peaks at 1735 cm-1 and 1247 cm-1, which were attributed to the C=O stretching and C-O out of plane stretching vibration of the hemicellulose and lignin in raw Agave, completely disappeared in the spectra of chemically treated fibres. The X-ray diffraction data showed enrichment in the portion of crystalline cellulose from 59% to 82% in the raw and cellulose nanocrystals, respectively. Thermogravimetric analysis showed that the thermal stability improved significantly by various chemical stages. The size reduction of the Agave cellulose into nano-sized particles from 7 µm to 8 nm in diameter by acid hydrolysis was confirmed with transmission electron microscopy images.
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Isolation and Characterization of Cellulose Nanocrystals from Agave angustifolia Fibre
Noor Afizah Rosli, Ishak Ahmad,* and Ibrahim Abdullah
Cellulose nanocrystals were extracted from Agave angustifolia fibres by alkali and bleaching treatments followed by acid hydrolysis. The chemical composition of the Agave fibres was determined at different stages of chemical treatment. The structural analysis was carried out by a Fourier Transform Infrared spectroscopy and X-ray diffraction. The morphology and thermal stability of the Agave fibres at different stages of chemical treatment were investigated by field emission scanning electron microscopy and thermogravimetric analysis, respectively. The results indicated that the hemicellulose and lignin were removed extensively from the extracted cellulose. The two peaks at 1735 cm-1 and 1247 cm-1, which were attributed to the C=O stretching and C-O out of plane stretching vibration of the hemicellulose and lignin in raw Agave, completely disappeared in the spectra of chemically treated fibres. The X-ray diffraction data showed enrichment in the portion of crystalline cellulose from 59% to 82% in the raw and cellulose nanocrystals, respectively. Thermogravimetric analysis showed that the thermal stability improved significantly by various chemical stages. The size reduction of the Agave cellulose into nano-sized particles from 7 µm to 8 nm in diameter by acid hydrolysis was confirmed with transmission electron microscopy images.
Keywords: Acid hydrolysis; Cellulose; Nanocrystals; Natural fibres
Contact information: Polymer Research Centre (PORCE), School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi Selangor, Malaysia; *Corresponding author: gading@ukm.my
INTRODUCTION
Biocomposites processing using natural fibres as reinforcement has increased dramatically in recent years (Jiang and Hinrichsen 1999; Luo and Netravali 1999; Takagi and Asano 2008; Takagi and Ichihara 2004). In addition, natural fibres have found extensive applications in textiles, paper manufacturing, building, and civil engineering fields (Kalia et al. 2009).
Natural fibres of lignocellulosic materials can be classified according to their plant origin i.e.: (i) bast or stem, (ii) leaf, (iii) seed or fruit, (iv) grass, and (v) straw fibres. Natural fibres have many advantages over man-made fibres, such as low density, low cost, availability, renewability, biodegradability, and low abrasivity (Bledzki et al.1996). A better understanding of the chemical, mechanical, and physical properties of natural fibres is necessary for processing natural fibre-reinforced composites. Natural fibres consist of cellulose, hemicelluloses, lignin, pectin, waxes, and water soluble substances. Cellulose is a semi-crystalline polysaccharide consisting of D-glucopyranosyl units linked together by β-(1-4)-glucosidic bonds (Bledzki and Gassan 1999). Three hydroxyl groups, at the C2 and C3 positions of secondary hydroxyl groups, and the C6 position of primary hydroxyl groups, can form intra- and intermolecular hydrogen bonds. These hydrogen bonds allow the creation of highly ordered, three dimensional crystal structures (Abdul Khalil et al.2011). Lignin is a polymer of phenylpropane units which are highly complex, mainly aromatic and amorphous, but have less water sorption than other natural fibre components (Rowell et al. 1997). Hemicellulose is branched, fully amorphous, and has a lower molecular weight than cellulose; as a consequence it is partly soluble in water. Hemicellulose is hygroscopic due to its open structure containing hydroxyl and acetyl groups (Frederick and Norman 2004).
The purpose of this study was to produce cellulose nanocrystals (CNCs) from Agave angustifolia as a substitute for synthetic fibres to make a good economic and ecologic composite material. The term “cellulose nanocrystal” is used to designate elongated crystalline rod-like cellulose nanoparticles. Agave angustifolia belongs to the Agavaceae family and is one of 140 species of the Agave genus. Agave angustifolia is known as the “Century Plant” or locally named as “Kelumpang Telur”. The leaf of A. angustifolia is about 1½ inches wide with creamy yellow stripes along the spiny margins as shown in Fig. 1. The plant grows into a spherical clump of 3 to 4 feet in diameter. This species can tolerate full sun, part shade, and reflected heat. It can also handle more water than most Agave species. It has been utilized by people for domestic purposes such as making rope, soap, and other products. Although the leaves are widely used, no studies on the extraction or properties of the cellulose fibres from these leaves have been conducted to date.
Fig. 1. Photograph of Agave angustifolia plants
CNCs have great potential as reinforcing agents in nanocomposites due to their size and the possibility of chemically modifying their surface. It is known that the conventional CNC exhibits high stiffness and modulus, which can be as high as 134 GPa (Oksmann and Sain 2006). It has been stated that the crystal structure exhibits a tensile strength from 0.8 up to 10 GPa (Nishino et al. 1995).
A well known process for the isolation of CNCs is strong acid hydrolysis. It allows the removal of the form crystalline nanoparticles. Many researchers have recently used this method to prepare CNCs, from kenaf (Kargarzadeh et al. 2012), banana plant (Elanthikkal et al. 2010), coconut husk (Rosa et al. 2010), rice husk (Johar et al. 2012), and sugarcane bagasse (Mandal and Chakrabarty 2011).
In this study, cellulose and CNCs were extracted from Agave fibres by chemical methods of alkali and bleaching treatments followed by acid hydrolysis. The next study aims for the application of cellulose and CNCs of Agave fibre in biocomposites. Thus, investigations on the chemical and physical properties were carried out to analyse their suitability as reinforcing agent in biocomposites.
EXPERIMENTAL
Materials
Agave leaves used in this study were harvested in Kajang, Selangor (Malaysia). The chemical reagents used were sodium chlorite (NaClO2, Sigma-Aldrich), acetic acid glacial (99%, Systerm), sodium hydroxide (NaOH, Systerm), sulphuric acid (H2SO4, 98%, Univar), methanol (R&M Chemicals), toluene (Systerm), and acetone (Systerm).
Extraction of Cellulose
The thorns on the mature Agave leaf margin and tips were removed. The leaves were dried for three days to remove excess moisture. Agave fibres were extracted from the leaves using fibre extracting machine and were retted for one week. Then the fibres were combed and shadow-dried for 4 h. The long fibres were cut to 3 to 5 cm and ground using a mill. The ground fibres were treated with 4% NaOH at 70 to 80 ºC for 2 h, after which bleaching treatment was carried out using 1.7 w/v% NaClO2 at 70 to 80 ºC for 4 h. The ratio of the fibres to liquor was 1:25 (g/mL). Each fibre treatment was done twice, and the fibres were washed with distilled water after each treatment.
Isolation of Cellulose Nanocrystals
Scheme 1 depicts the methodology routes to obtained CNCs from Agave fibres. The hydrolysis was carried out using 60 wt% H2SO4solutions at 45 ºC for 45 min. The ratio of the obtained cellulose to liquor was 1:20 (wt%).
Scheme 1. Scheme for CNCs production from Agave fibre
The hydrolyzed cellulose was washed five times by centrifugation (10,000 rpm, 10 min, and 10 ºC). After washing, the products were neutralized with 2 N NaOH to a pH of 7. The neutralized products were further washed three more times. The suspension was then dialyzed against distilled water to a constant pH.
Determination of Chemical Composition
The chemical composition of the Agave fibres before and after the chemical treatment was determined. Dewaxing was carried out by boiling the fibres in a mixture of toluene/methanol (1:1 v/v) in a sachet for 6 h. The de-waxed fibres were then filtered, washed with methanol, and dried. The percentage of holocellulose was calculated according to the method described previously (Wise et al. 1946). In this method, the extracted residue of de-waxed fibres was boiled in a mixture of NaClO2 and acetic acid for 4 h. The suspension was cooled in an ice bath for 30 min, filtered, and washed with cold distilled water. Finally, it was washed with acetone and dried. The α-cellulose and the acid-insoluble lignin content was determined according to TAPPI standard methods T203 and T222, respectively.
Field Emission Scanning Electron Microscopy
Field emission scanning electron (FESEM) micrographs of the treated and untreated fibres were recorded on a Zeiss Supra 55VP microscope at a voltage of 3 kV. The micrographs of the longitudinal and cross section of the Agave leaves were also recorded. The samples were coated with gold to avoid charging.
Transmission Electron Microscopy
In order to determine the dimensions of the CNCs, the hydrolyzed suspension were analyzed by a Philips CM12 transmission electron microscope (TEM) operating at 80 kV. A drop of a highly diluted suspension of CNCs was placed on a copper grid coated with a thin carbon film and allowed to dry at room temperature. The copper grid was then stained with a 2 wt% solution of uranyl acetate for one minute and air dried.
Fourier Transform Infrared Spectroscopy
The Fourier transform infrared (FTIR) spectra of the Agave fibres were recorded on a Perkin-Elmer spectrometer (SpectrumGX) in the range 500 to 4000 cm-1 with a scanning resolution of 8 cm-1. Before analysis, all of the samples were ground into a fine powder and dried for 24 h at 60 ºC in an oven. The ground fibres was mixed with KBr, and pressed into an ultra-thin pellet.
X-ray Diffraction
X-ray diffraction (XRD) analysis was performed with a Bruker AXS D8 Advance diffractometer at 40 kV, 40 mA with Cu-Kα radiation (λ=0.1541 nm). Before analysis, the samples were ground into a fine powder using a mill and pressed into a sample holder. The data was acquired in a 2θ range from 5 to 60º. The crystallinity index (lc) of Agave fibres at different chemical stages was calculated using the following formula (Segal et al. 1959).
(1)
where l002 is maximum intensity of diffraction of the (002) lattice peak (22º to 23º), and lam is that of the amorphous material between 18º to 19º where the intensity is minimum.
Thermogravimetric Analysis
A Mettler Toledo model TGA/SDTA851e thermogravimetric analyzer was used to characterize the thermal stability of the Agave fibres. Approximately 2 mg of each sample was placed in an aluminium pan and heated from 30 to 600 ºC at a heating rate of 10 ºC/min. All of the measurements were performed under a nitrogen atmosphere.
RESULT AND DISCUSSION
Chemical Composition
The chemical compositions of raw, alkali treated, and bleached Agavefibres are presented in Table 1. The raw Agave fibres are composed of 67.0% cellulose, 25.2% hemicelluloses, 6.3% lignin, and 2.5% extractives. The amount of hemicelluloses and lignin are higher in raw fibre as compared to the treated fibres. NaOH was found to be efficient in removing the hemicellulose from the fibre, as hemicellulose content was decreased from 25.2% to 3.9%. Based on the chemical composition analysis, most of the lignin content was removed by the bleaching treatment, in which it reacts with NaClO2to dissolve as a lignin chloride. Bleaching does not only remove lignin, but also some of the hemicellulose. The final fibre obtained after the bleaching treatment was found to have the highest cellulose content (97.3%).
Table 1. Chemical Composition of Agave Fibres at Different Stages of Chemical Treatment
Morphological Analysis
Figure 2 shows the Agave angustifolia fibres at different stages of the chemical treatment. The colour of the fibres changed from cream to light brown after the alkali treatment, and white after bleaching.
Fig. 2. Photographs of (a) raw, (b) alkali-treated, and (c) bleached Agave fibres
Figure 3 shows FESEM micrographs of Agave leaves and fibres. Microscopic examinations on the longitudinal and cross section of the leaves are depicted in Figs. 3a and 3b. By examining the longitudinal section of the Agave leaves, one can see a ‘composite’-like structure in which the fibre bundles are held together by non-cellulosic substances.
Figure 3b shows the cross section of the arch fibres, which are usually found in the middle of the leaf. The arch fibres show an irregular section with a wide variety of sizes of the lumen, and growth in association with the conducting tissue of the plant (Nutman 1936). Xylem fibres are connected to the arch fibres through the conducting tissue and grow opposite to the arch fibres. These fibres run from base to the tip of the plant and give good mechanical strength (Nutman 1936). Nutman (1936) stated that they are composed of thin-walled cells which are broken up during the fibre extraction process. Every fibre contains numerous elongated individual fibres, or fibre cells, and each of them is made up of four main parts (primary wall, the thick secondary wall, tertiary wall, and lumen) as shown in Fig. 3b (Murherjee and Satyanarayana 1984). The fibre cells are linked together by the middle lamellae which consist of hemicelluloses and lignin.
Figures 3c-e presents FESEM micrographs of the raw and chemically treated fibres. From these three micrographs it is clear that the morphology of the fibres changed with the chemical treatment. The changes in the morphology are important to predict the fibres interaction with the polymer matrix in the composites. As shown in Fig. 3c, the raw Agave is composed of bundles of continuous individual cells that are bound together by cemented components. The surface of the raw Agave was irregular and covered with impurities such as hemicelluloses, lignin, pectin, and waxy substances. The diameter of the raw Agave fibres ranged from 60 to 230 µm, and about 59% of them were found to have diameters between 110 to 190 µm.
The results portrayed in Fig. 3d illustrate the surface of the Agavefibres after the alkali treatment. It can be seen that the fibre began to look smoother than the raw one as the impurities were removed from the fibre surface. Hemicellulose is hydrolyzed and becomes water-soluble upon the alkali treatment. This phenomenon helps defibrillation and the opening of the fibre bundles as shown in Fig. 3d, with the diameter of the fibrils being reduced to a great extent. The diameter of the Agave fibres after the alkali treatment was found to be between 9 to 110 µm.
As seen in Fig. 3e, the bleaching resulted in further defibrillation. The defibrillation already occurred upon the alkali treatment, and this trend increased along with the bleaching treatment due to the removal of the lignin. The bleaching treatment effectively modified the surface of the microfibrils, which appeared smoother than the case of the untreated fibre. After bleaching, the fibre bundles were disintegrated into individual cells with diameters in the range of 7 to 12 µm.
Figures 4(a) and 4(b) show the TEM micrograph and distribution of the diameter, and the aspect ratios of the Agave CNCs, respectively. The TEM micrographs demon-strated the efficiency of the acid hydrolysis treatment, which showed the CNCs’ needle-like structure consisting mostly of individual fibrils and some aggregates. The CNCs ranged from 8 to 15 nm in diameter and 170 to 500 nm in length, with an average of 10 nm in diameter and 310 nm in length. The calculated aspect ratios of the CNCs were in the range of 10 to 45 with 70% in the range of long CNC; this indicates great potential for them to be used as a reinforcing agent in nanocomposites. The aspect ratios reported here are similar to those reported on CNCs from other cellulosic sources, such as coconut husk (Rosa et al.2010).
Fig. 3a. FESEM micrographs of Agave leaves and fibres: (a) longitudinal section, (b) cross section of the arch (Murherjee and Satyanarayana,1984), (c) raw, (d) alkali-treated, and (e) bleached
Fig. 3b. FESEM micrographs of Agave leaves and fibres: (a) longitudinal section, (b) cross section of the arch (Murherjee and Satyanarayana,1984), (c) raw, (d) alkali-treated, and (e) bleached
Fig. 3c. FESEM micrographs of Agave leaves and fibres: (a) longitudinal section, (b) cross section of the arch (Murherjee and Satyanarayana,1984), (c) raw, (d) alkali-treated, and (e) bleached
Fig. 3d. FESEM micrographs of Agave leaves and fibres: (a) longitudinal section, (b) cross section of the arch (Murherjee and Satyanarayana,1984), (c) raw, (d) alkali-treated, and (e) bleached
Fig. 3e. FESEM micrographs of Agave leaves and fibres: (a) longitudinal section, (b) cross section of the arch (Murherjee and Satyanarayana,1984), (c) raw, (d) alkali-treated, and (e) bleached
Fig. 4. (a) TEM micrograph CNCs extracted from Agave cellulose
b
Fig. 4. (b) Diameter and aspect ratio histograms of CNCs extracted from Agave cellulose
FTIR Spectroscopy Analysis
The FTIR spectroscopic analysis of the raw and chemically-treated Agave fibres is presented in Fig. 5. The changes in infrared absorption indicate that the composition of the fibre had undergone changes during the chemical treatments. The FTIR peak at 1735 cm-1for the raw Agave is attributed to the C=O stretching vibration of the acetyl and uronic ester groups, from pectin, hemicelluloses, or the ester linkage of the carboxylic group of ferulic and p-coumaric acids of lignin and/or hemicellulose (Sain and Panthapulakkal 2006; Sun et al. 2005). The absorption peak at 1247 cm-1 as present in the raw fibre spectra is attributed to the C-O out of plane stretching vibration of the aryl group in the lignin (Troedec et al. 2008). These two peaks had completely disappeared in the spectra of the chemically treated fibres. The spectra of cellulose and CNCs showed similar peaks in all wave numbers, the only difference concerning a slight intensity change in the peaks. All of the spectra exhibited a broad band in the region of 3400 to 3300 cm-1, which indicates the free O-H stretching vibration of the OH group in cellulose molecules. In addition, all of the spectra showed the characteristic of C-H stretching vibration around 2900 cm-1 (Abdul Khalil et al. 2001). Moreover, the vibration peak detected at 1360 to 1375 cm-1 in all of the fibre samples is related to the bending vibration of the C-H and C-O bond in the polysaccharide aromatic rings (Nacos et al. 2006). Note that the absorbance peaks that are apparent in the spectra in the region of 1627 to 1638 cm-1 can be attributed to the O-H bending of the absorbed water (Troedec et al. 2008). The peak observed in all of the spectra at 1057 cm-1 is due to the C-O-C pyranose ring skeletal vibration. The increase in the intensity of this band showed an increase in crystallinity of the samples (Elanthikkal et al. 2010). The most significant absorption band, which continually increases with alkali and bleaching treatment, is 898 cm-1 which corresponds to glycosidic –C-H- deformation, with a ring vibration contribution and –O-H bending. These features are characteristic of the β-glycosidic linkage between the anhydroglucose units in cellulose (Alemdar and Sain 2008).
Fig. 5. FTIR spectra of (a) raw, (b) alkali-treated, (c) bleached, and (d) acid-hydrolysed Agave fibres
X-ray Diffraction Measurements
Figure 6 shows an X-ray diffraction pattern of the Agave fibre and those treated at different stages of the chemical treatments. All of the diffraction patterns showed peaks around 2θ = 16º, 22.6º, and 35º, indicating the typical cellulose I structure. The only difference concerns a slight intensity change in the peaks, representing some changes in the fibres crystallinity. For fibres with high cellulose content, like cotton and flax, two peaks around 16º were observed; however, for the raw Agave fibres, only one broad peak was observed due to the presence of amorphous material which covered the two peaks (Subramanian et al. 2005; Tserki et al. 2005). It can be seen in Fig. 6 that the peaks at 16º and 22.6º were more defined for the alkali-treated fibre, and further intensified in the bleached and acid hydrolysed fibre. During the chemical treatment, the cementing materials such as hemicelluloses and lignin were dissolved, and the remaining pure crystalline particles were isolated. These particles gave rise to more intense and narrower crystalline peaks for the chemical treated fibres. Acid hydrolysed cellulose, which shows two peaks at 14º and 16º, like cotton and flax, with a sharp peak at 22.4º, exhibited higher crystallinity due to the more efficient removal of the amorphous parts.
The crystallinity index for the raw, alkali-treated, bleached, and acid-hydrolysed fibres was found to be 59%, 69%, 74%, and 82%, respectively. The results in Table 2 clearly demonstrate the increase in the degree of crystallinity after the chemical treat-ment. Note that the crystallinity index is used to indicate the order of crystallinity rather than the crystallinity of crystalline regions (Mwaikambo and Ansell 2002). The increase in the crystallinity index is attributed to two effects: (a) the removal of some of the amorphous constituents and (b) the rearrangement of the crystalline regions into a more ordered structure.
Fig. 6. X-ray diffraction patterns for (a) raw, (b) alkali-treated fibres, (c) bleached, and (d) acid- hydrolysed Agave fibres
Table 2. Crystallinity Index of Agave Fibres at Different Stages of Chemical Treatment
Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was performed to investigate the suitability of the raw and chemically treated Agave fibres for biocomposites processing. The thermal behaviour of the lignocellulosic materials depends on their chemical composition, structure, and degree of crystallinity (Fisher et al. 2002). The thermal decomposition parameters were determined from the TGA and DTG curves as shown in Figs. 7(a) and 7(b), respectively. The TGA curve of the raw Agave fibre showed four degradation steps related to: (i) moisture evaporation, (ii) hemicellulose, (iii) cellulose, and (iv) lignin degradation. The initial weight losses started at 34 ºC for all the samples, and were attributed to the evaporation of the moisture in the fibres. The DTG analysis of the raw Agave fibre showed that the peaks at 210 ºC and 312 ºC were caused by hemicelluloses and α-cellulose degradation, respectively (Basak et al. 1993). The degradation of the alkali-treated fibre occurred from 250 to 386 ºC with a maximum rate at 356 ºC, while the bleached fibre degraded in the range of 260 to 398 ºC with a maximum rate at 367 ºC; both are attributed to cellulose degradation.
Fig. 7. (a) TGA and (b) DTG curves of raw, alkali-treated, bleached, and acid-hydrolysed Agave fibres
The acid-hydrolysed cellulose also showed the same trend as the alkali-treated and bleached fibres with the degradation occurring from 250 to 390 ºC with a maximum rate at 361 ºC. Based on the thermal stability results of the Agave fibres, the results clearly indicated that the degradation temperature increased by chemical treatment, which could be attributed to the removal of hemicelluloses and lignin, as well as a higher degree of crystallinity in the treated fibre samples. The higher crystallinity led to a higher heat resistance, and improved the thermal degradation. The reduction in the Tmax of acid-hydrolysed fibre in comparison to the bleached fibre may correspond to the introduction of sulfate groups into the crystals of cellulose in sulfuric acid hydrolysis (Julien et al. 1993). The residue from the degradation step at this onset temperature of degradation was 16, 15, 13, and 12 wt% for raw, alkali-treated, bleached, and acid-hydrolysed cellulose, respectively. The higher amount of residues in the raw Agave fibre is due to the presence of lignin as well as ash, which both have a slow degradation rate (Ashori et al. 2006). On the other hand, the small amount of residues in the acid hydrolysed cellulose may be the results of the lignin removal during the acid hydrolysis.
CONCLUSIONS
- Cellulose was extracted from Agave fibres by an effective two-step process: the removal of hemicellulose by 4% NaOH and delignification by 1.7% NaClO2.
- The isolation of nanocellulose was successfully carried out by acid hydrolysis.
- The determination of the chemical composition demonstrated the effects of chemical treatments, based on morphological, XRD, FTIR, and TGA analysis.
- Upon the two-step chemical treatment, the diameter of the fibre was reduced from 230 µm to 7 µm, and thermal stability improved.
- The cellulose content and crystallinity index increased from 67% and 59% to 97% and 82%, respectively.
- TEM micrographs suggested that the Agave fibres are successfully disintegrated from 7 µm to the range of 8 to 15 nm in diameter, which may have great potential to be used as a reinforcing agent in biocomposite applications.
ACKNOWLEDGEMENT
This work was supported by research grants from the Ministry of Higher Education (MOHE) and Universiti Kebangsaan Malaysia under the research grant GUP-2012-003 and ERGS/1/2012/STG01/UKM/02/4. The authors would also like to thank Dr. Md. Akhir Hamid from Malaysian Agriculture Research & Development Institute (MARDI) for the processing of raw Agavefibres.
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Article submitted: Dec. 16, 2012; Peer review completed: Jan. 25, 2013; Revised version received and accepted: Feb. 18, 2013; Published: Feb. 21, 2013.