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Effect of fiber extraction methods on some properties of kenaf bast fiber
Industrial Crops and Products 46 (2013) 117–123
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Effect of fiber extraction methods on some properties of kenaf bast fiber B. Ahmed Amel a,∗∗ , M. Tahir Paridah a,∗ , R. Sudin b , U.M.K. Anwar b , Ahmed S. Hussein c a
Laboratory Biocomposite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Darul Ehsan, Malaysia Forest Product Division, Forest Research Institute Malaysia (FRIM), 52109 Kepong Selangor, Daurl Ehsan, Malaysia c Advanced Materials and Nanotechnology Lab., Institute of Advanced Technology, Universiti Putra Malaysia,43400 UPM Serdang, Selangor, Malaysia b
a r t i c l e
i n f o
Article history: Received 6 September 2012 Received in revised form 5 December 2012 Accepted 16 December 2012 Keywords: Kenaf Decortications Density Fiber morphology Chemical constituent Tensile strength
a b s t r a c t The objective of this study was to investigate the effect of different extraction methods on the fiber morphology, density, chemical composition and tensile strength of kenaf bast fiber. Kena fbast fibers were extracted using different methods (crude, decorticated, water retted, 5% sodium hydroxide retted and 5% benzoate retted) and their physicochemical characteristics were investigated. The morphological characteristics showed a significantly reduced lumen diameter and increased cell wall thickness after treated with NaOH at 5%. On the other hand, increased lumen diameter and a decreased cell wall thickness were observed with the decorticated and water retted bast fibers. A slight increase in fiber density was observed for NaOH and benzoate retted bast fibers indicating cell wall densification. Interestingly, the extraction methods used in this study produced bast fibers with high cellulose content and low sugar and starch due to the removal of wax, oil, pectin and hemicelluloses. Regarding the tensile strength, the water retted bast fibers showed highest tensile strength (426.05 MPa), while there was one no difference between decorticated and NaOH retted fibers (386.83 and 393.03 MPa, respectively). © 2012 Elsevier B.V. All rights reserved.
1. Introduction Cellulosic fibers produced from plant stems, such as kenaf, sisal, hemp, flax, jute, and ramie, are a viable reinforcement in composite materials, and there has been tremendous increase in a research interests regarding their applications over the past few years, possibly due to their biodegradability and low cost (Mohan and Kanny, 2012). Currently, natural fibers are used mainly in low performance applications such as construction and automotive industries (Placet et al., 2012). Before fibers can be used as reinforcement in high performance applications, it is a prerequisite to accurately understand their micro-mechanical behavior. This requires application of advanced and sophisticated experimental techniques, as well as a development of theoretical tools, in order to correctly relate the microstructure and complex organization of such fibers to their mechanical properties (Placet et al., 2012). Among these natural fibers kenaf (Hibiscus cannabinus L.) belongs to the Malvaceae family and grows annually in tropical and sub-tropical areas (Kaldor et al., 1990). Kenaf has a long history of cultivation for its fiber especially in United States, India,
∗ Corresponding author at: Department of Biocomposite Technology, Institute of Tropical Forestry and Forest Product (INTROP), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. Tel.: +60 1 32093195; fax: +60 3 89471780. ∗∗ Corresponding author. Tel.: +60 123499784. E-mail addresses: amel [email protected] (B.A. Amel), [email protected] (M.T. Paridah). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.12.015
Bangladesh, Thailand, and to a small extent in Southeast Europe, parts of Africa as well as in Brazil where it is cultivated throughout the year (Shi et al., 2011). There are two distinct types of fiber in kenaf stem, namely bast fiber and inner core which constitutes about 30–40% and 60% of the total dry weight of the stalk, respectively (Abdul Khalil et al., 2010). Kenaf bast fiber is a lignocellulosic fiber and has been used for production of fiber board and particle board, textiles, a fuel and as a reinforcement material for composites (Aleksandra et al., 2007; Abdul Khalil et al., 2009). Ligno-cellulosic fibers have a complicated structure. Kenaf bast fiber is made up from elementary fibers which are glued together by a pectin interface, to form technical fiber bundles. These bundles are separated from one another through partial decomposition of the cell wall, induced by bacteria or mechanical processes. Many studies reported that methods of retting are important in determining fiber properties (Paridah and Khalina, 2009; Kawahara et al., 2005). Retting or so-called as degumming is a process for removing non-cellulosic material attached to the fibers to release the individual cellulosic fibers (Sur, 2005; Zhang et al., 2005). During the retting process phloem-derived fiber bundles are loosened from other stem tissue composed of hemicelluloses, lignin and pectin. Recently, some mechanical methods for separating the kenaf fiber have been described and practically performed (Abdolreza et al., 1997). However, in some developing countries due to cost effective wages, kenaf fibers are separated manually rather than mechanically. Even though, some researchers have also reported that the mechanical decortications is a fast and simple process compared to water and chemical retting, and produces a
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big quantity of fiber with high quality (Liu, 2005; Zhang, 2003; Abdolreza et al., 1997). From SEM analysis it was also reported that 3% NaOH was ineffective concentration for removing the impurities from kenf bast fibers surface, while 6% NaOH was the optimum concentration for the chemical treatment to remove the impurities (Mwaikambo and Ansell, 2002; Edeerozey et al., 2007). Jinshu et al. (2011), Keshk et al. (2006), Song and Obendorf (2006) have reported that high cellulose content can be obtained from kenaf bast fiber using water and NaOH retting processes. It was also reported that the cross-sectional shape of the kenaf fiber varied widely from noncircular shapes such as a kidney bean shape for cotton to the reasonably circular one for wool (Goswani et al., 2004). However, there have been many studies on kenaf and very limited information is available on the effect of extraction methods on the properties of kenaf bast fiber. Therefore, the purpose of the current research is to investigate the effect of different extraction methods, i.e. water retting, chemical retting and mechanical decortications on the characteristics of kenaf bast fibers. These characteristics include changes in fibers morphology such as fiber length, diameter, lumen diameter, and cell wall thickness, density, chemical composition and tensile strength. 2. Materials and methods Kenaf, sp.V36, was collected from Taman Pertanian Universiti (TPU) Malaysia. Stalks were harvested at the age of 3.5 month. A total of five hundred kenaf stems were used in this study. The chemicals (methanol, ethyl alcohol, acetone, acetic acid, sodium chlorite, sodium hydroxide, sodium benzoate, sulfuric acid, barium hydroxide, barium sulphate, anthrone, glucose, fructose, potato starch, 10% potassium iodide, 0.01 N potassium iodate and perchloric acid) used in this study were of analytical grade and obtained from Fisher Scientific Malaysia. The kenaf samples were divided into five groups and treated with different extraction methods. Except the decorticated bast fibers, all other samples were peeled manually to separate the bast and core. The samples were kept in ambient conditions in the room temperature of 25 ◦ C and 50% relative humidity for 3 weeks before extraction process was carried out in Table 1. 2.1. Analysis for fiber surface morphology About 30 g of kenaf bast fiber was sampled from each extraction method used in the study. Hitachi H-7100 transmission electron microscope (TEM) was employed to examine the morphology of kenaf bast fibers, i.e. fiber length, cell diameter, lumen diameter and cell wall thickness. The samples were cut into 1 mm thickness and placed into beam capsules. Then the cross sections were further cut into 1 m thickness using glass knife and ultra microtome. Finally ultra-thin sections (0.1 m) were prepared using ultra microtome, stained with 2% uranyl acetate and viewed under TEM. A total of 200 individual fibers were taken from different extraction methods. Forty individual fibers from each extraction method were examined and the results were expressed as a mean value for each treatment. A Leo 1455 variable pressure scanning electron microscope (SEM) was also used to examine the effect of the extraction methods on the surface morphology of kenaf bast fibers. The acceleration voltage was set to 20 kV and all samples were cut into 2 mm length and 1 mm thickness, stacked onto an aluminum stub; samples were coated with gold, and finally they were viewed under the SEM. 2.2. Fiber density In this study 40 g kenaf bast fibers and three replicates were used from each extraction methods. The densities of kenaf bast fibers
Fig. 1. Fiber bundle diameter.
were measured by using 70 mm fiber bundles length. The densities of kenaf bast fiber bundles were measured by using Archimedes Method (ASTM-D3800-99, 2005). Based on the standard, the method involves immersion of a known weight of fibers into a liquid of lower density than the fiber. Canola oil with a density of about 0.915 g/cm3 was used as a liquid. Prior to testing, the kenaf fibers were conditioned in an oven at 60 ◦ C until the moisture content was reduced to below 5%. 2.3. Chemical analyses From each extraction methods, about 2 kg of kenaf bast fibers was used to investigate the chemical composition. Five samples from each extraction methods were used in the study. The chemical composition (alcohol acetone solubility, holocellulose, ␣-cellulose and lignin) of the kenaf bast fibers was determined according to the standard Technical Association of the Pulp and Paper Industry (TAPPI) T 204 os-76, Wise et al., 1946, T 203 os-76 and T 222 os-76, respectively. The alcohol acetone solubility was determined using alcohol acetone solution to extract the samples. The holocellulose content (␣-cellulose + hemicelluloses) of the kenaf bast fibers was determined by treating the fibers with a mixture of acetic acid and sodium chlorite solutions. The ␣-cellulose content of the fibers was then determined by further treating the holocellulose with 17.5% and 8.3% sodium hydroxide to remove the hemicelluloses. The lignin content of the kenaf bast fibers was determined by treating them with a sulfuric acid solution. The method developed by Yemm and Willis (1954) was applied to determine the sugar content. Also total starch was investigated using Humphreys and Kelly (1961) method. 2.4. Tensile properties The tensile strength test was conducted according to ASTM D885 (1995) using an Instron Universal Testing Machine (Instron: Model 3365) with cell load capacity 5 kN, at a crosshead speed of 1 mm/min. The standard specifies that the gauge should be three times the fiber length (in this case 90 mm – 3 mm × 30 mm), but after it is glued to a cardboard on each end for better gripping, the effective gauge length was 60 mm. In this study, 30 g of kenaf bast fiber bundles in eight replicates was used as representatives for each extraction methods. The fiber diameter was obtained using an optical microscope with camera (image analyzer) at 300× magnification. The fiber bundle diameter was calculated as an average of three readings (Fig. 1). Using the calculated fiber bundle diameter, the tensile strength was calculated. 2.5. Statistical analysis The data were statistically analyzed using Statistical Analysis System (SAS) software, Version 9. Analysis of Variance (ANOVA) was used to examine the effects of extraction methods on some properties of kenaf bast fibers. Least Significant Difference (LSD) method was used for further evaluation of the effect of extraction
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Table 1 Fiber extraction procedures of kenaf using different medium.a Type
Procedure
Crude bast fiber Decorticated bast fiber Water retted bast fiber
Peeling manually (control) – aggregates of fibers still intact in sheet form. Separation of kenaf bast fibers was done using a decorticator machine. 8 kg kenaf bast fibers were soaked in 40 L water (1:5) for 24 days. The water was changed every 12 days after 24 days the stalks were washed with water, air driedb and combed. For each chemical extraction methods about 8 kg of kenaf bast fibers were soaked in 40 L solutionc (1:5) for 24 days, the stalks were washed with water, air driedb , and combed
NaOH retted bast fiber Benzoate retted bast fiber Note: a Mode of fiber production. b 12% moisture content. c 5% (by weight) for each (NaOH and benzoate).
methods. LSD ranks the means and calculates the minimum value to be significantly different with each other at p ≤ 0.05. Means followed by the same letter a, b, c, . . ., etc. are not significantly different.
3. Result and discussion The analysis of variance for the fiber morphology, density, chemical analyses and tensile strength of different extraction methods, i.e. crude, decorticated, water-, NaOH- and benzoate retted from kenaf bast fibers are presented in Table 2. Based on the test results; it is obvious that the extraction methods were significantly (at p ≤ 0.01) influenced the fiber morphology, i.e. fiber length, diameter, lumen diameter and cell wall thickness, density, chemical analyses, i.e. alcohol acetone solubility, ␣-cellulose, lignin, starch and sugar and tensile strength of the kenaf bast fiber. However the effect of extraction methods on the holocellulose was insignificant.
3.1. Effect of extraction methods on the morphology of kenaf bast fibers Table 3 shows the effect of extraction method on the fiber morphology. It was generally found that the fiber length, diameter and lumen diameter decreased as more severe treatments were used. Compared to fibers obtained from crude bast fiber, treatment using 5% NaOH seemed to be the most severe with fiber length reduced from 3228 m to 1757 m similarly with diameter and lumen diameter. Nevertheless, the cell wall thickness increased significantly. The reduction in both length and diameter may be attributed to the reduction in the number of elementary fibers in the fiber bundles after being treated with 5% NaOH. A mild alkali hydrolysis may have degraded the middle lamella thus separating the elementary fibers from the bundles. Similar finding was previously reported by Bos (2004). The cell wall, on the other hand, experienced swelling as a result of reduction in crystalline region (increase in amorphous region) after being treated with alkali. The
Fig. 2. TEM micrographs of the cross sections of kenaf bast fibers: (A) crude bast fiber; (B) decorticated bast fiber; (C) water retted bast fiber; (D) benzoate retted bast fiber and (E) NaOH retted bast fiber (magnification at 4000×).
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Table 2 Summary of ANOVA for the effect of extraction methods on the morphology, density, chemical analyses and tensile strength of kenaf bast fiber bundle. Parameters
DF
p value Fiber morphology
Fiber length Extraction methods
4
Fiber diameter
Chemical composition
Lumen diameter
Cell wall thickness
<0.0001*** <0.0001*** <0.0001*** <0.0001***
Alcohol Acetone
Holo-cellulose
<0.0001*** 0.8752, ns
Cellulose
Lignin
Sugar
Starch
<0.0001***
<0.0001***
<0.0001***
<0.0001***
Density
Tensile strength
<0.0001***
<0.0001***
ns: no significant difference at p ≥ 0.1. *** Significant difference at p ≤ 0.01. Table 3 Effect of extraction method on fiber length, diameter, lumen diameter and cell wall thickness of kenaf bast fibers.a Extraction method
Crude bast fiber Decorticated bast fiber Water retted bast fiber 5%Benzoate retted bast fiber 5% NaOH retted bast fiber a b
Fiber morphology (m)b Length
Diameter
Lumen diameter
Cell wall thickness
3228 a (2) 2315 b (1.5) 3389 a (2) 2225 b (1.2) 1757 c (1)
24.1 a (1.3) 18.2 c (1.9) 17.4 cd (0.58) 21.7 b (1.12) 16.1 d (0.7)
13.4 a (1.03) 8.9 b (0.44) 9.6 b (0.49) 7.7 c (0.4) 3.6 d (0.54)
5.8 c (0.3) 4.0 c (1.98) 5.3 d (0.8) 6.7 b(0.1) 7.3 a (0.2)
Values are average of 20 specimens; ( ) standard deviation. Means followed by the same letters (a, b, c, d) in each column are not significantly different at p ≤ 0.05 according to Least Significant Difference (LSD) method.
increase in cell wall thickness is due to the presence of swollen cellulose in the cell wall. There is no significant difference in the fiber length between crude fiber (3228 m) and water retted (3389 m). This finding is expected since water acts as a medium to better degrade pectinees materials and carbohydrates that bind the fiber together; there are mild chemical reaction initiated by the microbe. Thus, the fibers experienced little degradation therefore resembled those of original crude bast fibers. Both decorticated and benzoate retted fibers are of similar quality. These results are similar to findings by Abdul Khalil et al. (2010), who reported that bast fibers length of 3600 m for crude bast fiber whilst, Ashori et al. (2006) reported an average of 2480 m. The average fiber diameter, lumen diameter and cell wall thickness of the crude kenaf bast fibers was observed 24.1, 13.4 and 5.8 m, respectively. However, there was no significant difference in the diameter between decorticated (18.2 m) and water retted (17.4 m) as well as between water and NaOH (16.1 m) retted but they are significantly different from each other and from benzoate retted (21.7 m). It was interestingly found that decortications process degraded most fiber properties in particular cell wall thickness. Figs. 2 and 3 show the cross section micrographs on different extraction methods of kenaf bast fibers viewing under TEM and SEM. It can be clearly seen that, the whole cross sections of kenaf bast fiber bundles after extraction methods are polygonal to circular. For the crude fibers in their original state, surface is filled with impurities and the fibers are intact while for the decorticated and benzoate retted fibers the lumen shape is similar to that of the crude fibers and their surfaces are clean, Fig. 2. The presence of surface impurities on the surface of kenaf bast fibers was very clear; except for the 5% NaOH retted bast fibers which showed smooth surface compared to other kenaf bast fibers. These results indicate that the extraction methods improve surface characteristics of the kenaf bast fibers as a result of removing natural and artificial impurities, hence producing a smooth surface topography. This result is in accordance with that found by Mohanty et al. (2006) who reported that treating fibers with NaOH removes lignin, pectin, wax substances, and natural oils that cover surface of the fiber cell wall. Morphological examinations carried out by Aziz and Ansell (2004) and Edeerozey et al. (2007) exposed that alkalization
treatments performed on kenaf bast fibers have modified the fibers surface where fine structural changes of the fibers were observed on SEM micrographs. On the other hand, for water retted samples the lumen became larger and more rounded compared to the others. Similar results were reported by Munawar et al. (2006); they found that all the cross-sectional shapes of single fibers provided were polygonal to round. Generally, the bundle shape, single fiber shape, and the lumen diameter observed are different based on the extraction methods. Except for NaOH retted fiber, all other types of fiber exhibited noncircular shapes on the cross section of fiber bundles, very small lumen and densified cells (Fig. 3). 3.2. Effect of extraction methods on the density of kenaf bast fibers The density generally represents all the solid material as well as the pores within the fibers. Table 4 depicts the densities of kenaf bast fiber produced from different methods of extraction, i.e. crude, decorticated, water, NaOH and benzoate retted bast fibers. There is no significant difference between all extraction methods. However, a positive slight change in fiber densities was observed in the order: NaOH-retted > benzoate-retted > waterretted > decorticated > crude bast fibers. A positive change in fiber densities normally signifies cell wall densification as a result of Table 4 Effect of extraction methods on the density (g/cm3 ) of kenaf bast fibers. Extraction methods
Density (g/cm3 )a
Change in density (%)b
Crude bast fiber Decorticated bast fiber Water retted bast fiber Benzoate retted bast fiber NaOH retted bast fiber Kenaf bast fiberc A. Untreated B. Treated with 6% NaOH
1.191 a 1.193 a 1.195 a 1.195 a 1.198 a
0 0.17 0.34 0.34 0.59
1.193 1.222
0 2.4
Note: a Means followed by the same letter (a) are not significantly different with each other at p < 0.01 according to Least Significant Difference (LSD) method. b Change over crude bast fiber. c Aziz and Ansell (2004).
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Fig. 3. SEM micrographs of the cross sections of different kenaf bast fibers: (A) crude bast fiber; (B) decorticated bast fiber; (C) water retted bast fiber; (D) benzoate retted bast fiber (E) and NaOH retted bast fiber (magnification at 1000×).
removal of impurities (less dense fats and waxes) by alkali. A positive change would signify cell wall leading to polymerization of the cellulose molecule. Aziz and Ansell (2004) observed a small positive change in fiber density for both hemp and kenaf fibers after 6% NaOH treatment, indicating a cell wall densification of their fibers. Sawpan et al. (2011) and Mwaikambo and Ansell (2006) also found that the density of hemp fibers increased upon alkali treatment. 3.3. Chemical analyses As shown in Table 5 the chemical composition is greatly affected by extraction methods. All extraction methods namely crude, decorticated, water, NaOH, and benzoate retted fibers decreased alcohol acetone solubility whereas increased both ␣-cellulose and lignin. NaOH retted produced higher ␣-cellulose (73.89%) followed by water retted (72.68%), decorticated (70.89%), benzoate retted (68.94%) and crude (66.94%). However, the lignin contents increased from 9.87% to 11.43%, 13.93%, 14.55% and 13.07%. These changes can be attributed to the effects of retting process as well as to those of extracted by hand (crude). The increase in lignin is expected to decrease wax, pectin and part of hemicelluloses. In addition the increase in cellulose observed in this study could be attributed to the increase in lignin present in the primary wall and secondary wall mainly S1 and S2 layers thus increase in the packing order of the crystalline material. Previous study by Villar et al. (2009) and Thi Bach et al. (2003) showed that the kenaf bast fiber
have low lignin content (9.3–13.2%). The higher ␣-cellulose at 5% NaOH and water retted is probably due to removed pectin, wax and part of hemicellulose. These are in line to some findings by Jinshu et al. (2011) and Keshk et al. (2006); they reported that high cellulose content from kenaf bast fiber can be obtained when using water and NaOH retting process. Moreover, the total sugar percentage for crude, decorticated, water retted, NaOH retted and benzoate retted bast fibers were 0.0038, 0.0035, 0.0033, 0.0037, and 0.0050%, respectively. The total starch percentage for the respective kenaf bast fibers were 1.4769,
Table 5 Chemical analyses of the kenaf bast fiber. Chemical constituenta
Extraction methods
Crude Alcohol acetone solubility Holocellulose ␣-Cellulose Lignin Sugar Starch
2.26 a
Decorticated 1.55 b
81.55 a 80.15 a 66.94 e 70.89 c 9.87 c 11.43 bc 0.0038 b 0.0035 c 1.4759 a 0.6532 d
Water 0.66 c
80.94 a 72.68 b 13.93 a 0.0033 c 0.6514 d
NaOH 0.48 c
77.06 a 73.89 a 14.55 a 0.0037 b 0.8789 c
Benzoate 0.58 c
79.55 a 68.94 d 13.07 ab 0.0050 a 1.3226 b
a At the same rows, chemical constituent mean with the same lower case letters (a, b, c, d) were not significantly different from each other (p < 0.01).
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Table 6 Effect of extraction methods on the tensile strength of kenaf bast fibers. Extraction methods
Tensile strength (MPa)a
Crude bast fiber Decorticated bast fiber Water retted bast fiber Benzoate retted bast fiber NaOH retted bast fiber
171.2 d 386.8 b 426.1 a 290.7 c 393 ab
a
Means followed by the same letters (a, b, c, d) in each column are not significantly different at p < 0.01 according to Least Significant Difference (LSD) method.
0.6532, 0.6514, 0.8789, and 1.3226%, respectively. These results indicate that the lowest starch and sugar contents were recorded for water retted and decorticated bast fibers. The percentages of all the chemical constituents of kenaf are more or less similar to those of wood materials. The average total sugar content was 0.0038 of dry weight of kenaf bast fibers with different extraction methods which is less than that in bamboo 4.92% (Sudin and Swamy, 2006) and rubber wood 1.23% (Kadir and Sudin, 1989). 3.4. Effect of extraction methods on the tensile strength of kenaf bast fibers Table 6 shows the effect of different extraction methods under normal temperature on the tensile strengths of kenaf bast fibers. The crude bast fiber and benzoate retted showed the lowest tensile strength among the other samples and the values were 171 and 291 MPa, respectively. While water retted bast fibers showed the highest value of tensile strength (426.1 MPa). This higher tensile strength is probably due to the fiber length. Therefore, the tensile strength of 5% NaOH (393 MPa) retted significantly was similar to that of water retted and decorticated (386.8 MPa) kenaf bast fibers. However, the tensile strength of 5% NaOH retted kenaf bast fibers were increased compared to that of kenaf crude bast fibers. This may be attributed to the improvement of cellulose chain packing order. The alkali treatment hydrolyses the amorphous parts of cellulose present in the fibers and thus the material contains more crystalline cellulose. The reaction between cellulose and caustic soda is as follows: (␣-Cellulose)-OH + NaOH → (␣-Cellulose-O− )Na+ + H2 O + impurities These results correlate with the findings by Troedec et al. (2008) and Cao et al. (2007) they reported that 5% NaOH retted kenaf bast fibers increased ␣-cellulose and tensile strength. As stated in the literature (Sawpan et al., 2011; Taha et al., 2007) alkali treatment of natural fibers causes a reduction in the spiral angle of cellulose microfibrils which in turn allowed for the rearrangement of the cellulose chains to a more linear arrangement and consequently improves tensile strength. Habibi et al. (2008), Munawar et al. (2007) and Bledzki and Gassan (1999) have also reported that the tensile strength of kenaf bast fibers depend on density, cellulose content, and morphological properties of the fiber. 4. Conclusions The morphological properties of kenaf bast fibers were found be significantly affected by the extraction methods used in this work. NaOH retted fibers resulted in significant reduction in their lumen diameter and an increase in cell wall thickness, whilst both water retting and decorticated fibers showed a decrease in the lumen diameter and cell wall thickness. There was a small positive change in the fiber density that was observed for NaOH and benzoate retted bast fibers indicating a cell wall densification. Among the different extraction methods used water retting, decorticated, and NaOH
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Effects of soda retting on the tensile strength of kenaf (Hibiscus cannabnius L.) bast fibers, Project Report Kenaf EPU. 21 pp. Placet, V., Trivaudey, F., Cisse, O., Gucheret-Retel, V., Boubakar, M.L., 2012. Diameter dependence of the apparent tensile modulus of hemp fibers: A morphological, structural or ultrastructural effect. Composites: Part A 43, 275–287. Sawpan, M.A., Pickering, K.L., Fernyhough, A., 2011. Effect of various chemical treatments on the fibre structure and tensile properties of industrial hemp fibres. Composites Part A: Appl. Sci. Manuf. 42 (8), 888–895.
B.A. Amel et al. / Industrial Crops and Products 46 (2013) 117–123 Shi, J., Shi, S.Q., Barnes, H.M., Horstemeyer, M., Wang, J., Hassan, E.B.M., 2011. Kenaf bast fibers—part I: hermetical alkali digestion. Int. J. Polym. Sci., Article ID 212047, 8 pages. Song, K.H., Obendorf, S.K., 2006. Chemical and biological retting of kenaf fibers. Textile Res. J. 76 (10), 751–756. Sudin, R., Swamy, N., 2006. Bamboo and wood fibre cement composites for sustainable infrastructure regeneration. J. Mater. Sci. 41 (21), 6917–6924. Sur, D., 2005. Understanding Jute Yarn. Institute of Jute Technology, Anindita Sur, Kolkata, pp. 254. Troedec, M.L., Sedan, D., Peyratout, C., Bonnet, J.P., Smith, A., Guinebretiere, R., Gloaguen, V., Krausz, P., 2008. Influence of various chemical treatments on the composition and structure of hemp fibers. Compositions: Part A 39, 514–522. Taha, I., Steuernagel, L., Ziegmann, G., 2007. Optimization of the alkali treatment process of date palm fibres for polymeric composites. Compos. Interfaces 14, 669–684.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Effect of fiber extraction methods on some properties of kenaf bast fiber B. Ahmed Amel a,∗∗ , M. Tahir Paridah a,∗ , R. Sudin b , U.M.K. Anwar b , Ahmed S. Hussein c a
Laboratory Biocomposite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Darul Ehsan, Malaysia Forest Product Division, Forest Research Institute Malaysia (FRIM), 52109 Kepong Selangor, Daurl Ehsan, Malaysia c Advanced Materials and Nanotechnology Lab., Institute of Advanced Technology, Universiti Putra Malaysia,43400 UPM Serdang, Selangor, Malaysia b
a r t i c l e
i n f o
Article history: Received 6 September 2012 Received in revised form 5 December 2012 Accepted 16 December 2012 Keywords: Kenaf Decortications Density Fiber morphology Chemical constituent Tensile strength
a b s t r a c t The objective of this study was to investigate the effect of different extraction methods on the fiber morphology, density, chemical composition and tensile strength of kenaf bast fiber. Kena fbast fibers were extracted using different methods (crude, decorticated, water retted, 5% sodium hydroxide retted and 5% benzoate retted) and their physicochemical characteristics were investigated. The morphological characteristics showed a significantly reduced lumen diameter and increased cell wall thickness after treated with NaOH at 5%. On the other hand, increased lumen diameter and a decreased cell wall thickness were observed with the decorticated and water retted bast fibers. A slight increase in fiber density was observed for NaOH and benzoate retted bast fibers indicating cell wall densification. Interestingly, the extraction methods used in this study produced bast fibers with high cellulose content and low sugar and starch due to the removal of wax, oil, pectin and hemicelluloses. Regarding the tensile strength, the water retted bast fibers showed highest tensile strength (426.05 MPa), while there was one no difference between decorticated and NaOH retted fibers (386.83 and 393.03 MPa, respectively). © 2012 Elsevier B.V. All rights reserved.
1. Introduction Cellulosic fibers produced from plant stems, such as kenaf, sisal, hemp, flax, jute, and ramie, are a viable reinforcement in composite materials, and there has been tremendous increase in a research interests regarding their applications over the past few years, possibly due to their biodegradability and low cost (Mohan and Kanny, 2012). Currently, natural fibers are used mainly in low performance applications such as construction and automotive industries (Placet et al., 2012). Before fibers can be used as reinforcement in high performance applications, it is a prerequisite to accurately understand their micro-mechanical behavior. This requires application of advanced and sophisticated experimental techniques, as well as a development of theoretical tools, in order to correctly relate the microstructure and complex organization of such fibers to their mechanical properties (Placet et al., 2012). Among these natural fibers kenaf (Hibiscus cannabinus L.) belongs to the Malvaceae family and grows annually in tropical and sub-tropical areas (Kaldor et al., 1990). Kenaf has a long history of cultivation for its fiber especially in United States, India,
∗ Corresponding author at: Department of Biocomposite Technology, Institute of Tropical Forestry and Forest Product (INTROP), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. Tel.: +60 1 32093195; fax: +60 3 89471780. ∗∗ Corresponding author. Tel.: +60 123499784. E-mail addresses: amel [email protected] (B.A. Amel), [email protected] (M.T. Paridah). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.12.015
Bangladesh, Thailand, and to a small extent in Southeast Europe, parts of Africa as well as in Brazil where it is cultivated throughout the year (Shi et al., 2011). There are two distinct types of fiber in kenaf stem, namely bast fiber and inner core which constitutes about 30–40% and 60% of the total dry weight of the stalk, respectively (Abdul Khalil et al., 2010). Kenaf bast fiber is a lignocellulosic fiber and has been used for production of fiber board and particle board, textiles, a fuel and as a reinforcement material for composites (Aleksandra et al., 2007; Abdul Khalil et al., 2009). Ligno-cellulosic fibers have a complicated structure. Kenaf bast fiber is made up from elementary fibers which are glued together by a pectin interface, to form technical fiber bundles. These bundles are separated from one another through partial decomposition of the cell wall, induced by bacteria or mechanical processes. Many studies reported that methods of retting are important in determining fiber properties (Paridah and Khalina, 2009; Kawahara et al., 2005). Retting or so-called as degumming is a process for removing non-cellulosic material attached to the fibers to release the individual cellulosic fibers (Sur, 2005; Zhang et al., 2005). During the retting process phloem-derived fiber bundles are loosened from other stem tissue composed of hemicelluloses, lignin and pectin. Recently, some mechanical methods for separating the kenaf fiber have been described and practically performed (Abdolreza et al., 1997). However, in some developing countries due to cost effective wages, kenaf fibers are separated manually rather than mechanically. Even though, some researchers have also reported that the mechanical decortications is a fast and simple process compared to water and chemical retting, and produces a
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big quantity of fiber with high quality (Liu, 2005; Zhang, 2003; Abdolreza et al., 1997). From SEM analysis it was also reported that 3% NaOH was ineffective concentration for removing the impurities from kenf bast fibers surface, while 6% NaOH was the optimum concentration for the chemical treatment to remove the impurities (Mwaikambo and Ansell, 2002; Edeerozey et al., 2007). Jinshu et al. (2011), Keshk et al. (2006), Song and Obendorf (2006) have reported that high cellulose content can be obtained from kenaf bast fiber using water and NaOH retting processes. It was also reported that the cross-sectional shape of the kenaf fiber varied widely from noncircular shapes such as a kidney bean shape for cotton to the reasonably circular one for wool (Goswani et al., 2004). However, there have been many studies on kenaf and very limited information is available on the effect of extraction methods on the properties of kenaf bast fiber. Therefore, the purpose of the current research is to investigate the effect of different extraction methods, i.e. water retting, chemical retting and mechanical decortications on the characteristics of kenaf bast fibers. These characteristics include changes in fibers morphology such as fiber length, diameter, lumen diameter, and cell wall thickness, density, chemical composition and tensile strength. 2. Materials and methods Kenaf, sp.V36, was collected from Taman Pertanian Universiti (TPU) Malaysia. Stalks were harvested at the age of 3.5 month. A total of five hundred kenaf stems were used in this study. The chemicals (methanol, ethyl alcohol, acetone, acetic acid, sodium chlorite, sodium hydroxide, sodium benzoate, sulfuric acid, barium hydroxide, barium sulphate, anthrone, glucose, fructose, potato starch, 10% potassium iodide, 0.01 N potassium iodate and perchloric acid) used in this study were of analytical grade and obtained from Fisher Scientific Malaysia. The kenaf samples were divided into five groups and treated with different extraction methods. Except the decorticated bast fibers, all other samples were peeled manually to separate the bast and core. The samples were kept in ambient conditions in the room temperature of 25 ◦ C and 50% relative humidity for 3 weeks before extraction process was carried out in Table 1. 2.1. Analysis for fiber surface morphology About 30 g of kenaf bast fiber was sampled from each extraction method used in the study. Hitachi H-7100 transmission electron microscope (TEM) was employed to examine the morphology of kenaf bast fibers, i.e. fiber length, cell diameter, lumen diameter and cell wall thickness. The samples were cut into 1 mm thickness and placed into beam capsules. Then the cross sections were further cut into 1 m thickness using glass knife and ultra microtome. Finally ultra-thin sections (0.1 m) were prepared using ultra microtome, stained with 2% uranyl acetate and viewed under TEM. A total of 200 individual fibers were taken from different extraction methods. Forty individual fibers from each extraction method were examined and the results were expressed as a mean value for each treatment. A Leo 1455 variable pressure scanning electron microscope (SEM) was also used to examine the effect of the extraction methods on the surface morphology of kenaf bast fibers. The acceleration voltage was set to 20 kV and all samples were cut into 2 mm length and 1 mm thickness, stacked onto an aluminum stub; samples were coated with gold, and finally they were viewed under the SEM. 2.2. Fiber density In this study 40 g kenaf bast fibers and three replicates were used from each extraction methods. The densities of kenaf bast fibers
Fig. 1. Fiber bundle diameter.
were measured by using 70 mm fiber bundles length. The densities of kenaf bast fiber bundles were measured by using Archimedes Method (ASTM-D3800-99, 2005). Based on the standard, the method involves immersion of a known weight of fibers into a liquid of lower density than the fiber. Canola oil with a density of about 0.915 g/cm3 was used as a liquid. Prior to testing, the kenaf fibers were conditioned in an oven at 60 ◦ C until the moisture content was reduced to below 5%. 2.3. Chemical analyses From each extraction methods, about 2 kg of kenaf bast fibers was used to investigate the chemical composition. Five samples from each extraction methods were used in the study. The chemical composition (alcohol acetone solubility, holocellulose, ␣-cellulose and lignin) of the kenaf bast fibers was determined according to the standard Technical Association of the Pulp and Paper Industry (TAPPI) T 204 os-76, Wise et al., 1946, T 203 os-76 and T 222 os-76, respectively. The alcohol acetone solubility was determined using alcohol acetone solution to extract the samples. The holocellulose content (␣-cellulose + hemicelluloses) of the kenaf bast fibers was determined by treating the fibers with a mixture of acetic acid and sodium chlorite solutions. The ␣-cellulose content of the fibers was then determined by further treating the holocellulose with 17.5% and 8.3% sodium hydroxide to remove the hemicelluloses. The lignin content of the kenaf bast fibers was determined by treating them with a sulfuric acid solution. The method developed by Yemm and Willis (1954) was applied to determine the sugar content. Also total starch was investigated using Humphreys and Kelly (1961) method. 2.4. Tensile properties The tensile strength test was conducted according to ASTM D885 (1995) using an Instron Universal Testing Machine (Instron: Model 3365) with cell load capacity 5 kN, at a crosshead speed of 1 mm/min. The standard specifies that the gauge should be three times the fiber length (in this case 90 mm – 3 mm × 30 mm), but after it is glued to a cardboard on each end for better gripping, the effective gauge length was 60 mm. In this study, 30 g of kenaf bast fiber bundles in eight replicates was used as representatives for each extraction methods. The fiber diameter was obtained using an optical microscope with camera (image analyzer) at 300× magnification. The fiber bundle diameter was calculated as an average of three readings (Fig. 1). Using the calculated fiber bundle diameter, the tensile strength was calculated. 2.5. Statistical analysis The data were statistically analyzed using Statistical Analysis System (SAS) software, Version 9. Analysis of Variance (ANOVA) was used to examine the effects of extraction methods on some properties of kenaf bast fibers. Least Significant Difference (LSD) method was used for further evaluation of the effect of extraction
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Table 1 Fiber extraction procedures of kenaf using different medium.a Type
Procedure
Crude bast fiber Decorticated bast fiber Water retted bast fiber
Peeling manually (control) – aggregates of fibers still intact in sheet form. Separation of kenaf bast fibers was done using a decorticator machine. 8 kg kenaf bast fibers were soaked in 40 L water (1:5) for 24 days. The water was changed every 12 days after 24 days the stalks were washed with water, air driedb and combed. For each chemical extraction methods about 8 kg of kenaf bast fibers were soaked in 40 L solutionc (1:5) for 24 days, the stalks were washed with water, air driedb , and combed
NaOH retted bast fiber Benzoate retted bast fiber Note: a Mode of fiber production. b 12% moisture content. c 5% (by weight) for each (NaOH and benzoate).
methods. LSD ranks the means and calculates the minimum value to be significantly different with each other at p ≤ 0.05. Means followed by the same letter a, b, c, . . ., etc. are not significantly different.
3. Result and discussion The analysis of variance for the fiber morphology, density, chemical analyses and tensile strength of different extraction methods, i.e. crude, decorticated, water-, NaOH- and benzoate retted from kenaf bast fibers are presented in Table 2. Based on the test results; it is obvious that the extraction methods were significantly (at p ≤ 0.01) influenced the fiber morphology, i.e. fiber length, diameter, lumen diameter and cell wall thickness, density, chemical analyses, i.e. alcohol acetone solubility, ␣-cellulose, lignin, starch and sugar and tensile strength of the kenaf bast fiber. However the effect of extraction methods on the holocellulose was insignificant.
3.1. Effect of extraction methods on the morphology of kenaf bast fibers Table 3 shows the effect of extraction method on the fiber morphology. It was generally found that the fiber length, diameter and lumen diameter decreased as more severe treatments were used. Compared to fibers obtained from crude bast fiber, treatment using 5% NaOH seemed to be the most severe with fiber length reduced from 3228 m to 1757 m similarly with diameter and lumen diameter. Nevertheless, the cell wall thickness increased significantly. The reduction in both length and diameter may be attributed to the reduction in the number of elementary fibers in the fiber bundles after being treated with 5% NaOH. A mild alkali hydrolysis may have degraded the middle lamella thus separating the elementary fibers from the bundles. Similar finding was previously reported by Bos (2004). The cell wall, on the other hand, experienced swelling as a result of reduction in crystalline region (increase in amorphous region) after being treated with alkali. The
Fig. 2. TEM micrographs of the cross sections of kenaf bast fibers: (A) crude bast fiber; (B) decorticated bast fiber; (C) water retted bast fiber; (D) benzoate retted bast fiber and (E) NaOH retted bast fiber (magnification at 4000×).
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Table 2 Summary of ANOVA for the effect of extraction methods on the morphology, density, chemical analyses and tensile strength of kenaf bast fiber bundle. Parameters
DF
p value Fiber morphology
Fiber length Extraction methods
4
Fiber diameter
Chemical composition
Lumen diameter
Cell wall thickness
<0.0001*** <0.0001*** <0.0001*** <0.0001***
Alcohol Acetone
Holo-cellulose
<0.0001*** 0.8752, ns
Cellulose
Lignin
Sugar
Starch
<0.0001***
<0.0001***
<0.0001***
<0.0001***
Density
Tensile strength
<0.0001***
<0.0001***
ns: no significant difference at p ≥ 0.1. *** Significant difference at p ≤ 0.01. Table 3 Effect of extraction method on fiber length, diameter, lumen diameter and cell wall thickness of kenaf bast fibers.a Extraction method
Crude bast fiber Decorticated bast fiber Water retted bast fiber 5%Benzoate retted bast fiber 5% NaOH retted bast fiber a b
Fiber morphology (m)b Length
Diameter
Lumen diameter
Cell wall thickness
3228 a (2) 2315 b (1.5) 3389 a (2) 2225 b (1.2) 1757 c (1)
24.1 a (1.3) 18.2 c (1.9) 17.4 cd (0.58) 21.7 b (1.12) 16.1 d (0.7)
13.4 a (1.03) 8.9 b (0.44) 9.6 b (0.49) 7.7 c (0.4) 3.6 d (0.54)
5.8 c (0.3) 4.0 c (1.98) 5.3 d (0.8) 6.7 b(0.1) 7.3 a (0.2)
Values are average of 20 specimens; ( ) standard deviation. Means followed by the same letters (a, b, c, d) in each column are not significantly different at p ≤ 0.05 according to Least Significant Difference (LSD) method.
increase in cell wall thickness is due to the presence of swollen cellulose in the cell wall. There is no significant difference in the fiber length between crude fiber (3228 m) and water retted (3389 m). This finding is expected since water acts as a medium to better degrade pectinees materials and carbohydrates that bind the fiber together; there are mild chemical reaction initiated by the microbe. Thus, the fibers experienced little degradation therefore resembled those of original crude bast fibers. Both decorticated and benzoate retted fibers are of similar quality. These results are similar to findings by Abdul Khalil et al. (2010), who reported that bast fibers length of 3600 m for crude bast fiber whilst, Ashori et al. (2006) reported an average of 2480 m. The average fiber diameter, lumen diameter and cell wall thickness of the crude kenaf bast fibers was observed 24.1, 13.4 and 5.8 m, respectively. However, there was no significant difference in the diameter between decorticated (18.2 m) and water retted (17.4 m) as well as between water and NaOH (16.1 m) retted but they are significantly different from each other and from benzoate retted (21.7 m). It was interestingly found that decortications process degraded most fiber properties in particular cell wall thickness. Figs. 2 and 3 show the cross section micrographs on different extraction methods of kenaf bast fibers viewing under TEM and SEM. It can be clearly seen that, the whole cross sections of kenaf bast fiber bundles after extraction methods are polygonal to circular. For the crude fibers in their original state, surface is filled with impurities and the fibers are intact while for the decorticated and benzoate retted fibers the lumen shape is similar to that of the crude fibers and their surfaces are clean, Fig. 2. The presence of surface impurities on the surface of kenaf bast fibers was very clear; except for the 5% NaOH retted bast fibers which showed smooth surface compared to other kenaf bast fibers. These results indicate that the extraction methods improve surface characteristics of the kenaf bast fibers as a result of removing natural and artificial impurities, hence producing a smooth surface topography. This result is in accordance with that found by Mohanty et al. (2006) who reported that treating fibers with NaOH removes lignin, pectin, wax substances, and natural oils that cover surface of the fiber cell wall. Morphological examinations carried out by Aziz and Ansell (2004) and Edeerozey et al. (2007) exposed that alkalization
treatments performed on kenaf bast fibers have modified the fibers surface where fine structural changes of the fibers were observed on SEM micrographs. On the other hand, for water retted samples the lumen became larger and more rounded compared to the others. Similar results were reported by Munawar et al. (2006); they found that all the cross-sectional shapes of single fibers provided were polygonal to round. Generally, the bundle shape, single fiber shape, and the lumen diameter observed are different based on the extraction methods. Except for NaOH retted fiber, all other types of fiber exhibited noncircular shapes on the cross section of fiber bundles, very small lumen and densified cells (Fig. 3). 3.2. Effect of extraction methods on the density of kenaf bast fibers The density generally represents all the solid material as well as the pores within the fibers. Table 4 depicts the densities of kenaf bast fiber produced from different methods of extraction, i.e. crude, decorticated, water, NaOH and benzoate retted bast fibers. There is no significant difference between all extraction methods. However, a positive slight change in fiber densities was observed in the order: NaOH-retted > benzoate-retted > waterretted > decorticated > crude bast fibers. A positive change in fiber densities normally signifies cell wall densification as a result of Table 4 Effect of extraction methods on the density (g/cm3 ) of kenaf bast fibers. Extraction methods
Density (g/cm3 )a
Change in density (%)b
Crude bast fiber Decorticated bast fiber Water retted bast fiber Benzoate retted bast fiber NaOH retted bast fiber Kenaf bast fiberc A. Untreated B. Treated with 6% NaOH
1.191 a 1.193 a 1.195 a 1.195 a 1.198 a
0 0.17 0.34 0.34 0.59
1.193 1.222
0 2.4
Note: a Means followed by the same letter (a) are not significantly different with each other at p < 0.01 according to Least Significant Difference (LSD) method. b Change over crude bast fiber. c Aziz and Ansell (2004).
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Fig. 3. SEM micrographs of the cross sections of different kenaf bast fibers: (A) crude bast fiber; (B) decorticated bast fiber; (C) water retted bast fiber; (D) benzoate retted bast fiber (E) and NaOH retted bast fiber (magnification at 1000×).
removal of impurities (less dense fats and waxes) by alkali. A positive change would signify cell wall leading to polymerization of the cellulose molecule. Aziz and Ansell (2004) observed a small positive change in fiber density for both hemp and kenaf fibers after 6% NaOH treatment, indicating a cell wall densification of their fibers. Sawpan et al. (2011) and Mwaikambo and Ansell (2006) also found that the density of hemp fibers increased upon alkali treatment. 3.3. Chemical analyses As shown in Table 5 the chemical composition is greatly affected by extraction methods. All extraction methods namely crude, decorticated, water, NaOH, and benzoate retted fibers decreased alcohol acetone solubility whereas increased both ␣-cellulose and lignin. NaOH retted produced higher ␣-cellulose (73.89%) followed by water retted (72.68%), decorticated (70.89%), benzoate retted (68.94%) and crude (66.94%). However, the lignin contents increased from 9.87% to 11.43%, 13.93%, 14.55% and 13.07%. These changes can be attributed to the effects of retting process as well as to those of extracted by hand (crude). The increase in lignin is expected to decrease wax, pectin and part of hemicelluloses. In addition the increase in cellulose observed in this study could be attributed to the increase in lignin present in the primary wall and secondary wall mainly S1 and S2 layers thus increase in the packing order of the crystalline material. Previous study by Villar et al. (2009) and Thi Bach et al. (2003) showed that the kenaf bast fiber
have low lignin content (9.3–13.2%). The higher ␣-cellulose at 5% NaOH and water retted is probably due to removed pectin, wax and part of hemicellulose. These are in line to some findings by Jinshu et al. (2011) and Keshk et al. (2006); they reported that high cellulose content from kenaf bast fiber can be obtained when using water and NaOH retting process. Moreover, the total sugar percentage for crude, decorticated, water retted, NaOH retted and benzoate retted bast fibers were 0.0038, 0.0035, 0.0033, 0.0037, and 0.0050%, respectively. The total starch percentage for the respective kenaf bast fibers were 1.4769,
Table 5 Chemical analyses of the kenaf bast fiber. Chemical constituenta
Extraction methods
Crude Alcohol acetone solubility Holocellulose ␣-Cellulose Lignin Sugar Starch
2.26 a
Decorticated 1.55 b
81.55 a 80.15 a 66.94 e 70.89 c 9.87 c 11.43 bc 0.0038 b 0.0035 c 1.4759 a 0.6532 d
Water 0.66 c
80.94 a 72.68 b 13.93 a 0.0033 c 0.6514 d
NaOH 0.48 c
77.06 a 73.89 a 14.55 a 0.0037 b 0.8789 c
Benzoate 0.58 c
79.55 a 68.94 d 13.07 ab 0.0050 a 1.3226 b
a At the same rows, chemical constituent mean with the same lower case letters (a, b, c, d) were not significantly different from each other (p < 0.01).
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Table 6 Effect of extraction methods on the tensile strength of kenaf bast fibers. Extraction methods
Tensile strength (MPa)a
Crude bast fiber Decorticated bast fiber Water retted bast fiber Benzoate retted bast fiber NaOH retted bast fiber
171.2 d 386.8 b 426.1 a 290.7 c 393 ab
a
Means followed by the same letters (a, b, c, d) in each column are not significantly different at p < 0.01 according to Least Significant Difference (LSD) method.
0.6532, 0.6514, 0.8789, and 1.3226%, respectively. These results indicate that the lowest starch and sugar contents were recorded for water retted and decorticated bast fibers. The percentages of all the chemical constituents of kenaf are more or less similar to those of wood materials. The average total sugar content was 0.0038 of dry weight of kenaf bast fibers with different extraction methods which is less than that in bamboo 4.92% (Sudin and Swamy, 2006) and rubber wood 1.23% (Kadir and Sudin, 1989). 3.4. Effect of extraction methods on the tensile strength of kenaf bast fibers Table 6 shows the effect of different extraction methods under normal temperature on the tensile strengths of kenaf bast fibers. The crude bast fiber and benzoate retted showed the lowest tensile strength among the other samples and the values were 171 and 291 MPa, respectively. While water retted bast fibers showed the highest value of tensile strength (426.1 MPa). This higher tensile strength is probably due to the fiber length. Therefore, the tensile strength of 5% NaOH (393 MPa) retted significantly was similar to that of water retted and decorticated (386.8 MPa) kenaf bast fibers. However, the tensile strength of 5% NaOH retted kenaf bast fibers were increased compared to that of kenaf crude bast fibers. This may be attributed to the improvement of cellulose chain packing order. The alkali treatment hydrolyses the amorphous parts of cellulose present in the fibers and thus the material contains more crystalline cellulose. The reaction between cellulose and caustic soda is as follows: (␣-Cellulose)-OH + NaOH → (␣-Cellulose-O− )Na+ + H2 O + impurities These results correlate with the findings by Troedec et al. (2008) and Cao et al. (2007) they reported that 5% NaOH retted kenaf bast fibers increased ␣-cellulose and tensile strength. As stated in the literature (Sawpan et al., 2011; Taha et al., 2007) alkali treatment of natural fibers causes a reduction in the spiral angle of cellulose microfibrils which in turn allowed for the rearrangement of the cellulose chains to a more linear arrangement and consequently improves tensile strength. Habibi et al. (2008), Munawar et al. (2007) and Bledzki and Gassan (1999) have also reported that the tensile strength of kenaf bast fibers depend on density, cellulose content, and morphological properties of the fiber. 4. Conclusions The morphological properties of kenaf bast fibers were found be significantly affected by the extraction methods used in this work. NaOH retted fibers resulted in significant reduction in their lumen diameter and an increase in cell wall thickness, whilst both water retting and decorticated fibers showed a decrease in the lumen diameter and cell wall thickness. There was a small positive change in the fiber density that was observed for NaOH and benzoate retted bast fibers indicating a cell wall densification. Among the different extraction methods used water retting, decorticated, and NaOH
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