Comparative mechanical and thermal study of chemically treated and untreated single sugarcane fiber bundle

Industrial Crops and Products 58 (2014) 78–90

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Comparative mechanical and thermal study of chemically treated and untreated single sugarcane fiber bundle Mohammad K. Hossain a,∗ , Mohammad R. Karim a , Mahmudur R. Chowdhury a , Muhammad A. Imam b , Mahesh Hosur b , Shaik Jeelani b , Ramsis Farag c a

Department of Mechanical Engineering, Tuskegee University, Tuskegee, AL 36088, USA Department of Materials Science and Engineering, Tuskegee University, Tuskegee, AL 36088, USA c Department of Polymer and Fiber Engineering, Auburn University, Auburn, AL 36849, USA b

a r t i c l e

i n f o

Article history: Received 25 August 2013 Received in revised form 29 March 2014 Accepted 6 April 2014 Keywords: Natural fiber Sugarcane fiber bundle Alkali treatment Tensile property Correction factor Optical Microscopy (OM) Scanning Electron Microscopy (SEM) Thermogravimetric Analysis (TGA) Differential Scanning Calorimetry (DSC) Fourier Transformed Infrared Spectroscopy (FTIR)

a b s t r a c t Natural fiber as a reinforcing constituent can play a dominant role in the field of fiber reinforced polymer composites (FRPC) due to its availability, eco-friendliness, renewability, CO2 -neutrality, flexibility, low density, and low cost. Sugarcane fiber can be a potential candidate to replace the synthetic FRPC. The objective of this study is to evaluate the effect of chemical treatment on the tensile, thermal, and morphological properties of single sugarcane fiber bundles. Locally grown sugarcane was cut into specific lengths from the internode section and the single fiber bundles were extracted from the rind region. These fiber bundles were then dried in an oven to remove the moisture. Surface modification of fiber bundles was accomplished by performing alkali treatment and neutralized by acetic acid solution. The fiber bundles were then rinsed with water and dried at 80 ◦ C for about 24 h in an oven. Optical Microscopy (OM) was employed to measure the diameter of the single fiber bundle which ranged from 0.260 mm (untreated) to 0.155 mm (treated). Tensile tests were carried out on single fiber bundles according to the ASTM standard. Correction factors were also applied for machine displacement and slippage of the fiber bundle. Data from tensile tests showed that maximum improvement in the tensile strength and modulus for the treated fiber bundles were observed to be 106.83% and 20.46%, respectively, compared to those of untreated ones. Strain to maximum strength was enhanced by 25.92% in the treated fiber bundles compared to that of the untreated one. Untreated fiber bundles were observed in Thermogravimetric Analysis (TGA) studies to start decomposing at around 200 ◦ C compared to about 250 ◦ C for treated ones. Differential Scanning Calorimetry (DSC) provided the amount of moisture content present in treated and untreated fiber bundles which matched with the TGA results. Fourier Transformed Infrared Spectroscopy (FTIR) revealed structural modification after treatment. Scanning Electron Microscopy (SEM) was used to evaluate the fracture morphology of failed samples. Fracture morphology of the treated fiber bundles revealed cleaner and rougher fracture surfaces compared to those of untreated ones. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Demand for substitution of metallic materials with FRPC is increasing due to their high strength and stiffness to weight ratio, better corrosion resistance, low thermal and electrical conductivity, low density, ease of fabrication, design flexibility, low cost (Luyt and Malunka, 2005), renewability (Luz and Gonc¸alves, 2007), and improved fuel efficiency (Joshi et al., 2004). However, manufacturing of synthetic fiber composites not only consumes a large amount

∗ Corresponding author. Tel.: +1 334 727 8128; fax: +1 334 724 4224. E-mail addresses: [email protected], [email protected] (M.K. Hossain). http://dx.doi.org/10.1016/j.indcrop.2014.04.002 0926-6690/© 2014 Elsevier B.V. All rights reserved.

of energy, but also their disposal at the end of the life cycle is very difficult since there is virtually no recycling option. Stringent environmental legislation and consumer awareness have forced industry to develop new technologies based on renewable feedstock that are independent of fossil fuels. Industrial crops grown for fibers have the potential to supply enough renewable biomass for various bio-products including composites. The scope of possible uses of natural fibers is enormous (Doan et al., 2006). Natural fiber reinforced composites are light in weight and possess good thermal and acoustic insulating properties, higher specific properties, and higher resistance to fracture (Ahmed and Vijayarangan, 2008; Munikenche Gowda et al., 1999; Zini et al., 2007). Lignocellulosic fibers (LCFs) derived from various sources such as leaf, bast, fruit, grass or cane contribute to the strength and stiffness of

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biopolymer composites in various applications (Dweib et al., 2004). These LCFs are made up of microfibriles of cellulose embedded in an amorphous matrix of hemicellulose and lignin (Martínez et al., 2009). Many LCFs are generally cultivated in developing regions. Hence, the production of LCFs might be a major source of income to the local economically disadvantaged people (Satyanarayana et al., 2007). Since LCFs are renewable, degradable, recyclable, and neutral with respect to CO2 emissions, these fibers are less polluting and contribute to preventing global warming (Monteiro et al., 2009). Thus, environmental as well as economical and social benefits motivate material scientists and engineers to explore the use of natural fibers in fiber reinforced polymer composites. Bagasse (sugarcane) fiber is one of the most promising natural fibers with its biodegradable and disposable properties. When the residue of bagasse is burnt off, it creates air pollution. Bagasse fiber consists of about 35% cellulose, 36% hemicellulose, 16% lignin, and 4% water content (Sasaki et al., 2003). Chemically, bagasse fiber contains 50% ␣-cellulose, 30% pentosans, and 2.4% ash (Pandey et al., 2000). Due to its lower ash content (Guimarães et al., 2009) compared to other residual crops such as rice straw and wheat straw, the sugarcane fiber has many advantages. About 71% fibers of sugarcane bagasse are suitable for paper making and other 16% fibers can act as an energy source that can be used in boilers or ethanol production (Agnihotri et al., 2010). Bagasse fibers are extensively used for making plastic, furniture (Hoareau et al., 2006), cane wax, fiber board (Rowell and Keany, 1991), insulating board, particle board, wood additives (Khan et al., 2004), and filter mud (Lu et al., 2006). Bagasse fibers mixed with polymers are used in automobile applications (Chen et al., 2004). Recently, talc polypropylene has been replaced by bagasse fiber reinforced polypropylene composites in passenger cars due to its light weight and lower fuel consumption (Luz et al., 2010; Sindhuphak, 2007). Sugarcane rind fibers are also successfully used in textile and geotextile applications (Elsunni and Collier, 1996). Previously, sugarcane bagasse was pretreated with enzymatic polymerization which removes up to 96% and 85% hemicellulose and lignin, respectively, thus creating a cellulose rich product (Rezende et al., 2011). Anatomical and morphological characteristics along with proximate chemical analysis have been conducted for sugarcane bagasse fiber (Agnihotri et al., 2010). Mechanical properties and workability of mortars in replacement of cement with sugarcane bagasse ash (SCBA) has been examined (Chi, 2012). Acharya et al. performed alkali treatment at 0%, 1%, 3%, and 5% NaOH solution and mentioned an improvement of flexural properties when fibers were incorporated with polymer (Acharya et al., 2011). Mechanical as well as thermal characterization was generally performed on sugarcane bagasse fiber that was produced from local sugarcane factories. These fibers were already deformed during sugar extraction due to the milling process (Luz et al., 2008). The milling process is done through the use of massive machinery that cuts, crushes, shreds, and breaks the whole cane stalk and juice is then forcefully squeezed out under enormous pressure. The fibrous structure of the rind of sugarcane has special desirable physical and structural characteristics when the fibers are used in the form of fiber bundles. These desirable fiber characteristics are largely destroyed during the conventional milling process. On the other hand, these fiber bundles are highly suitable for the manufacture of building boards, planks, mats, flexible sheets, rolls, and the like (Creighton et al., 1969). However, there has been no systematic investigation on the microstructures, mechanical and thermal properties, and fracture morphology of the chemically treated and untreated virgin single sugarcane fiber bundles. In our research, single sugarcane fiber bundles were used. These fiber bundles were extracted manually from the raw sugarcane procured from the local market.

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A sugarcane stalk is made up of shorter segments and joints. Each joint consists of two distinctive parts: (1) node and (2) internode (Elsunni, 1993). The cross-section of the internode is composed of a rind (outer layer) and pith (inner layer). The majority of sucrose along with bundles of small fibers is found in the pith (Paturau, 1969). The rind contains numerous longer and finer fiber bundles composed of elemental fibers in discrete elongated units embedded in a matrix of lignin and hemicellulose (Creighton et al., 1969). These elemental fibers are bound together by amorphous matrix of lignin and hemicellulose to form a fiber bundle (Bledzki and Gassan, 1999). The most valuable fiber bundles suitable for industrial applications are found in the rind region. Certain amount of lignin and hemicellulose can be removed through chemical treatments of the fiber bundles. These treatments result in fiber bundles of small diameters compared to those of untreated ones (Collier et al., 1992). A high cellulose content and low micro-fibril angle result in desirable properties in the bast fibers. Hereafter, the word fiber will be used in most of this paper to avoid repeating the word fiber bundle. In our study, chemical treatments were performed on the fibers to produce a higher percentage of cellulose content. This higher cellulose concentration results in enhanced mechanical and thermal properties. Mechanical properties were evaluated to determine ultimate strength, Young’s modulus, and strain to maximum strength. Corrected Young’s modulus and strain to maximum strength were then calculated to account for machine displacement and slippage of the fiber during testing. Thermogravimetric Analysis (TGA) was performed to analyze the degradation temperature of the fiber. Percentage of moisture content was evaluated before and after alkali treatment with Differential Scanning Calorimetry (DSC). After alkali treatment, changes in structural composition were verified through Fourier Transformed Infrared Spectroscopy (FTIR). Scanning Electron Microscopy (SEM) was used to observe the surface morphology and mode of failure of fibers. 2. Experimental 2.1. Materials Fiber bundles were extracted from the sugarcane procured from the local market. For chemical treatments, NaOH (1% and 5% solution) and acetic acid (2% solution) were used and all chemicals were supplied by Sigma–Aldrich. 2.2. Fiber bundle extraction Fiber bundles were extracted by soaking sugarcane in water for about 3–4 h at room temperature and then drawing them out from the rind with the use of a needle. Vilay et al. (2008) extracted fiber bundles by soaking sugarcane in water for two days followed by heating at 80 ◦ C for 3 h. Heating can soften the fiber bundle, which enhances pull out of the fiber bundle. However, this process can also damage fiber properties. Hence, fiber bundles were extracted by soaking at room temperature water in our study. This process of single fiber bundle extraction from the rind has avoided the deformation and stress concentration on the fibers. 2.3. Chemical treatment Extracted single fiber bundles were treated with alkali solution followed by acetic acid solution to increase the relative amount of cellulose by removing some of amorphous cellulose, hemicellulose, lignin, and other amorphous materials from the fiber surface (Kabir et al., 2013). Removal of these undesired substances reduces the moisture absorption capability and enhances the mechanical and thermal properties. Alkalization reduces amorphous

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hydroxyl groups from fibers (Fiber-OH + NaOH → Fiber-ONa + H2 O) and results in the reduction of the hydrophilic nature of fibers. The alkali also removes a portion of the hemicellulose and lignin covering the fibers (Das and Chakrabarty, 2008). The amount removed depends on the concentration of the NaOH and soaking time (Hossain et al., 2011b). The chemical treatment thus allows the relative amount of cellulose and the aspect ratio of the fibers to increase. A larger number of possible reaction sites of cellulose also become available for matrix (binder) adhesion (John and Anandjiwala, 2008). Moreover, the alkali treatment broke the intermolecular/intramolecular hydrogen bonding between the hydroxyl groups of the cellulose and hemicellulose of the fiber. The acetyl treatment replaces hydroxyl groups from fibers with the acetyl group (CH3 CO) (Sreekala et al., 2000; Bledzki and Gassan, 1999; Vilaseca et al., 2007). This results in decrease in the hydrophilic nature of the fibers. It also breaks down the hemicellulose/lignin covering from the fiber surface causing more exposure of the cellulose surface for matrix adhesion and enhancing the aspect ratio of the fiber. When fibers are soaked in a strong caustic solution, various Na–cellulose complexes are formed (Park et al., 2006). Acetic acid efficiently removes sodium that is deposited as sediment on fibers during alkali treatment. Moreover, acetic acid reacts with OH groups on the fiber surfaces, which is important for better adhesion with the matrix. In our study, fibers were treated with 1% and 5% alkali solution. First, fibers were soaked in the alkali solution for 2 h and washed with distilled water. Washed fibers were then soaked in 2% acetic acid solution for 1 h to neutralize the alkali and promote delignification. Finally, fibers were washed with distilled water followed by drying at 80 ◦ C for 24 h. Similar study has been performed by other researchers earlier (Cao et al., 2006).

3. Methodology 3.1. Diameter measurement Tests were performed at room temperature on single fiber bundles. Precise diameter measurement is a major challenge for a single natural fiber bundle. Unlike synthetic fibers, natural fibers are characterized by irregular shapes and textures along with their lengths as well as non-uniform thickness (Kabir et al., 2013). Natural fibers also have inherent defects within themselves (Hu et al., 2010). Moreover, a single fiber bundle consists of a large number of elemental fibers attached together by the matrix of lignin and hemicellulose. Hence, the single fiber bundle’s cross-sectional geometry, in general, is not circular. However, it is generally assumed to be circular in cross-section in calculating the tensile properties, even though the cross-section is not uniform along the length of the fiber (Kabir et al., 2013; Mukhopadhyay et al., 2008). Single natural fiber bundle diameters have been determined by researchers using various microscopic measurements for tensile testing (Kabir et al., 2013; Eichhorn and Young, 2004; Devi et al., 1997). Kabir et al. (2013) measured the diameters of single hemp fiber bundles by using optical microscopy (OM) study. Scanning electron microscope (SEM) was used by Eichhorn and Young (2004) to measure hemp fiber bundle diameters. A stereo microscope was used by Devi et al. (1997) to obtain pineapple fiber bundle diameters. However, it should be recognized that the assumption of circular cross-sections in natural fibers will likely lead to measured values of tensile properties which are slightly different from true values. Single fiber bundles were considered to be cylindrical in shape in our study. Similar assumption has been made by other researchers (De Rosa et al., 2010). Each fiber bundle was placed under an optical microscope (Olympus DP72) at a magnification of 32×.

Fig. 1. Diameter measurement of single sugarcane fiber bundle with optical microscope.

Diameters of each fiber bundle were measured at four locations along the length of the fiber and used to obtain the average diameter of each fiber bundle (Fig. 1). The variation in the diameter along the length of the fiber is found to be small. A total of 36 single sugarcane fiber bundles were randomly selected with 12 from each category: untreated, 1% alkali treated, and 5% alkali treated for the diameter measurement. Twelve samples in each category were deemed to provide statistically significant results as other researchers have worked with ten samples (Kabir et al., 2013; Vilay et al., 2008). An average diameter with standard deviation of each category of samples is then determined. Our fiber bundles had average diameters of 0.26 mm for untreated fiber bundle, 0.18 mm for 1% NaOH treated fiber bundle and 0.16 mm for 5% NaOH treated fiber bundle.

3.2. Specimen preparation and measurement for tensile testing Tensile tests were conducted using a gage-length of 25 mm on an INSTRON testing unit according to the ASTM D 3822-01standard. Randomly selected fiber bundles were cut according to the desired length. The fibers were then glued using multi-purpose Elmer’s Glue All in between two thick U-shaped paper frames maintaining a 25 mm free gage length (Fig. 2). This arrangement ensured a good grip of the jaws of the clamps on the test sample. It also allowed us to maintain proper vertical alignment of the fiber (same axis as that of the tensile testing machine). After attaching the test sample to the jaws of the clamps, the paper was cut at the midpoint of the gauge length just before the start of the tensile test so that the entire load is taken by the fiber alone. The upper end of the fiber bundle was clamped first followed by clamping of the lower end. Clamping pads of the grips were covered with PVC tape in order to prevent fiber damage in the clamping area. We followed the mounting specimen preparation procedures which have been used by other researchers using hemp fibers (Fan, 2010) and glass fibers (Feih et al., 2005). A load cell of 100 N was used and the cross head speed was maintained at 2.5 mm/min in our study. Twelve specimens of each category of sample were tested. Samples that broke near the edge of the clamps were excluded from the analysis. The tensile strength was determined by dividing the applied load by the average

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Table 1 Average diameter and standard deviation for untreated and treated fiber bundles.

Average diameter (mm) Standard deviation

Untreated fiber

1% NaOH treated fiber

5% NaOH treated fiber

0.260 0.041

0.182 0.021

0.155 0.019

3.6. Scanning Electron Microscopy (SEM) evaluation Analysis of the fracture mechanism of fibers was conducted using a JEOL JSM 5800 microscope. SEM samples were positioned on a sample holder with a silver paint, coated with gold by a low vacuum sputtering machine prior to loading in the SEM, to prevent the build-up of charges from the electrons absorbed by the specimen. 4. Results and discussions 4.1. Effect of chemical treatment on diameter changes

Fig. 2. Specimen for tensile test.

cross-sectional area of the fiber using Eq. (1). Strain was measured by the change in fiber length. =

Fmax A

(1)

where , Fmax , and A stand for tensile strength, maximum force, and average cross-sectional area of each fiber, respectively. The cross-sectional area of each fiber was determined by using Eq. (2): A=·

 d 2 2

(2)

where d is the average diameter of each fiber. Young’s modulus was determined from the linear portion of the stress/strain curve. 3.3. Thermogravimetric Analysis (TGA) Thermogravimetric Analysis (TGA) was conducted with a TA Instruments Q 500 setup fitted with nitrogen purge gas. The samples were cut into small pieces to maintain the sample weight within the range of 15–20 mg. Then the samples were kept in a platinum sample pan, weighed and heated to 600 ◦ C starting from room temperature (25 ◦ C), at a heating rate of 10 ◦ C/min.

The relative proportion of cellulose and lignin/hemicellulose varies from one natural fiber to another. Even the properties of the same species usually vary considerably (Mohanty et al., 2000). Hence, natural fibers are nonuniform in properties, dimensionally limited, and microstructurally heterogeneous due to the presence of defects, flaws, and irregularities throughout their three spatial dimensions. Diameters of untreated and treated fibers are provided in Table 1. The diameters are also shown in Fig. 3. It can be seen that the treated fibers are smaller in diameter compared to the untreated fibers. This reduction in fiber diameter is expected because only the amorphous parts of cellulose are attacked by alkali and acid treatments (Kabir et al., 2013). Hemicellulose is amorphous and hydrophilic, soluble in alkali, and easily hydrolyzed in acids (Kabir et al., 2013). Lignin is amorphous and hydrophobic in nature, and soluble in alkali solution and acetic acid (Hossain et al., 2011b and Kabir et al., 2013). The acetic acid is more apt to penetrate the porous LCFs’ surfaces and causes delignification of the fiber surfaces (Sahin and Young, 2008). Lignin and hemicellulose attached on both outer and inner fiber surfaces at the initial extraction are subsequently removed during chemical treatments. In our study, chemical treatments resulted in a smaller and a more

3.4. Differential Scanning Calorimetry (DSC) analysis DSC analysis was carried out for untreated, 1%, and 5% alkali treated fibers. In a typical experiment ∼10–15 mg sample was placed in a small aluminum pan covered with a lid tightly crimped around the edge. It was then transferred into the METTLER Toledo DSC cell and heated ranging from 25 ◦ C to 200 ◦ C at a heating rate of 10 ◦ C/min. 3.5. Fourier Transformed Infrared Spectroscopy (FTIR) study FTIR is performed to observe chemical compositional change after chemical treatment. Five percent fiber was mixed with 95% KBr and passed through a disk of FTIR machine. Each spectrum was recorded by co-adding 32 scans at 4 cm−1 resolution within the range of 4000–600 cm−1 .

Fig. 3. Variation in diameter of fiber bundles.

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Fig. 4. Stress–strain curve for untreated and treated fiber bundles: (A) untreated, (B) 1% alkali treated, and (C) 5% alkali treated.

uniform diameter. Fibers treated with 5% alkali solution revealed better removal of lignin and hemicellulose compared to 1% alkali solution. The chemical reaction that may cause these materials to be removed from the fiber surface is given below (Cao et al., 2006). Bagasse-OH + NaOH% = Bagasse-O− Na+ + H2 O

(1)

As the exposure time of fibers in the sodium hydroxide solution increases, the percentage of cellulose content also increases up to a certain point. However, higher concentrations of NaOH or longer exposure to NaOH weakened fibers and made them more brittle (Hossain et al., 2011b). There are no specific standards for the wt% of alkaline solution to be used in chemical treatments of natural fibers. Hence, most of the studies related to chemical treatments on natural fibers are performed based on trials using various amounts of alkali solutions. Researchers have used alkali solutions ranging from 1 to 10 wt% to treat natural fibers (Verma et al., 2012; Acharya et al., 2011; Hossain et al., 2011b; Kabir et al., 2013). Our study on jute fibers demonstrated that fibers treated with 5 wt% NaOH solutions provide better results in terms of physical, mechanical, and thermal properties compared to those in untreated one (Hossain et al., 2011b). Hence, 1 and 5 wt% NaOH solutions have been used in our study. Our results on sugarcane fiber also indicate that the fibers treated with 5 wt% NaOH solutions are becoming more uniform with the removal of lignin and hemicellulose (Table 1 and Fig. 3).

4.2. Tensile properties Stress–strain curves from the tensile machine for untreated and treated fibers are shown in Fig. 4. Mechanical properties calculated from the stress–strain curves are presented in Table 2. Tensile properties increase with the increase in concentration of the alkali solution due to the destruction of hemicellulose and lignin that were surrounded by and cemented with the cellulose (Guimarães et al., 2009). The fibers become cleaner, rougher, and fibrillated after the alkali treatment which is evident from the SEM micrographs (Fig. 14). It is further observed from SEM micrographs (Fig. 14) that smaller cross sections exhibit comparatively lower density of defects/flaws/irregularities (Mukhopadhyay et al., 2008). These lead to an increase in the mechanical properties of the fiber. An increase in strain indicates the introduction of ductility into the fibers after the alkali treatment. The values of ultimate strength, strain to maximum strength, and elastic modulus are presented in Fig. 5 as a function of the alkali concentration during the chemical treatment. It is obvious from these data that all mechanical properties gradually improve with an increase in the alkali concentration. The maximum improvement in mechanical properties was obtained in the fibers treated with 5% alkali solution. In essence, removal of non-cellulosic materials resulted in a higher percentage of cellulose and more uniform cross-sectional area, which were

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Table 2 Uncorrected tensile properties of untreated and treated fiber bundles.

Untreated fiber 1% NaOH treated fiber 5% NaOH treated fiber

Ultimate strength,  max (MPa)

Gain/loss (%) compared to untreated fiber

Young’s modulus, E (GPa)

Gain/loss (%) compared to untreated fiber

Strain (%) at  max (mm/mm)

169.51 ± 18.65 204.5 ± 30.01 350.6 ± 73.40

– +20.64 +106.83

5.18 ± 0.63 5.13 ± 0.82 6.24 ± 1.03

– −0.97 +20.46

6.25 ± 0.01 7.80 ± 0.02 7.87 ± 0.01

the main contributors to higher mechanical properties of the sugarcane fibers. A similar finding for different natural fibers has been observed earlier (Hossain et al., 2011b; Saha et al., 2010; Hong et al., 2008).

1. ˛i =

4.3. Correction of E-modulus and strain to maximum strength

2.

During tensile test, no extensometer was used and displacement was recorded through the movement of clamps. Usually, there is an additional displacement from slippage at the clamp and test setup compliance that adds Lnon fiber to the measured data. Uncorrected strain to maximum strength is summation of the following two constituents (Joffe et al., 2003):

3. Strain correction − (a)

Uncorrected strain to maximum strength = Ltotal Lfiber Lnon fiber = + Test length Test length Test length

(3)

Following equations were used to correct Young’s modulus and strain to maximum strength value (Defoirdt et al., 2010):

(b)

LTotal L0 − F E0 Ai

(4)

Ltotal 1 L0 ε · L0 = · = F  · Ai E Ai

(5)

LNonfiber ˛1 · Ai ·  = L0 L0

LFiber LTotal LNonfiber (Corrected) = − L0 L0 L0

(6) (7)

Here, ˛i is a factor of machine displacement that measures slippage and machine compliance, L0 is the original fiber gage length, E0 is the extrapolated Young’s modulus, E is Young’s modulus for each fiber, F is the force on the fiber, ε is the strain,  is the stress and Ai is the average cross-sectional area of each fiber bundle (Kabir et al., 2013).

Fig. 5. Ultimate strength, strain to failure, and elastic modulus for untreated and treated fiber bundles.

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Table 3 Corrected and uncorrected Young’s modulus and strain to maximum strength data for untreated and treated fiber bundles.

Untreated fiber 1% NaOH treated fiber 5% NaOH treated fiber

Uncorrected Young’s modulus

Uncorrected strain to maximum strength (%)

Corrected Young’s modulus

Corrected strain to maximum strength (%)

Machine displacement, ˛i

5.18 ± 0.63 5.13 ± 0.82 6.24 ± 1.03

6.25 ± 0.01 7.80 ± 0.02 7.87 ± 0.01

5.80 ± 0.58 6.87 ± 1.87 8.07 ± 1.50

5.77 ± 0.01 6.74 ± 0.01 7.04 ± 0.003

0.070 0.071 0.072

From the above equations, corrected stress–strain data were evaluated and plotted in Fig. 6. Three representative curves each from untreated, 1%, and 5% treated fibers are shown in this figure. A comparison of corrected and uncorrected data is presented in Table 3. It can be seen that after correction, Young’s modulus increases, whereas the strain to maximum strength decreases (Figs. 7 and 8). It is assumed that there was a little effect on the data for test setup compliance. In essence, machine displacement ˛i , mainly depends on the slippage. Fig. 9 illustrates relation between alpha and alkali concentration. Thicker materials have larger contact area compared to the thinner one. Hence, slippage is expected to be

higher for thinner materials than thicker ones. In our experiment, diameter of fiber bundles decreased with increase in percent alkali concentration. Therefore, it is logical to increase in alpha value with increase in percent alkali treatment.

Fig. 6. Stress–strain curve for corrected untreated and treated fiber bundles.

Fig. 8. Strain to maximum strength for corrected and uncorrected data.

Fig. 7. Young’s modulus for corrected and uncorrected data.

Fig. 9. Alpha value for corrected and uncorrected data.

4.4. Thermal stability Thermogravimetric Analysis (TGA) was used to measure the thermal stability, moisture absorption, and decomposition of untreated and treated single fiber bundles. The degradation kinetics was evaluated using results from the TGA study. These are

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Fig. 10. Thermogravimetric analysis.

presented in Figs. 10 and 11. Initially between 25 and 150 ◦ C all treated and untreated fibers lost a small amount of weight due to the evaporation of small amount of moisture present in the fibers. Untreated fibers started to decompose at around 200 ◦ C, while treated fibers started to decompose at around 250 ◦ C. The maximum decomposition temperature was determined from the point where the slopes of the Tg curves begin to change drastically. The higher resistance of treated fibers to thermal decomposition is due to the removal of lignin, hemicellulose, and waxy substances (Mwaikambo and Ansell, 2002) from the fiber after chemical treatment. The thermal degradation of natural fibers is usually accomplished in three stages (Hossain et al., 2011a): (1) Moisture loss: This stage is due to the vaporization of the moisture present in the fibers. Chemically treated fibers had less moisture, which is a desired trait for fiber reinforced polymer composites. (2) Degradation of cellulose and hemicellulose: This decomposition stage is due to the degradation of cellulosic and hemicellulose substances. Cellulosic materials decompose quickly in the range of 220–350 ◦ C. (3) Degradation of non-cellulose: This stage (after degradation temperature) is attributed to the degradation of non-cellulosic substances such as lignin (Van de Velde and Baetens, 2001; Wong et al., 2004). Lignin decomposes slower than the cellulose and hemicellulose over a broader temperature ranging from 200 to 500 ◦ C (Brebu and Vasile, 2010; Hajaligol et al., 2001). Figs. 10 and 11 show that fibers treated with 5% alkali solution have the highest decomposition temperature compared to other samples. Thermal degradation of

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Fig. 12. Differential scanning calorimetry.

treated and untreated fibers is observed to correlate to mechanical properties presented earlier. 4.5. Differential Scanning Calorimetry (DSC) result DSC analysis was performed to make relative estimates of moisture and inorganic volatile content of untreated and treated fiber bundles. Fig. 12 shows DSC traces for the treated and untreated fibers along with pure powder cellulose. From DSC traces, it is evident that treated fibers absorb more heat than untreated ones. Hence, treated fibers are more prone to endothermic reaction. A broad endothermic peak was observed between 60 and 125 ◦ C. This peak is correlated to a weight loss of 7–10% as observed by TGA. All natural fibers are hydrophilic in nature and subject to a dehydration process in which water is released. However, the amount of absorbed water by the fibers is dependent on the ambient humidity. On the other hand, chemical treatments on the fiber surfaces lead to a reduced amount of fiber companions including moisture, waxy substances, hemicellulose, and lignin. Waxy substances and hemicellulose usually decompose before cellulose. Hence, untreated fibers are more prone to exothermic reaction. Exothermic reaction happens due to the evaporation of a higher amount of moisture content. It indicates that the treated fibers have lower moisture content than the untreated ones. The 5% alkali treated fiber has the lowest moister content. The treated fiber has lower moisture content than pure cellulose, presented in Fig. 12. The lower moisture content characteristic of alkali treated fibers observed by the DSC analysis is matched well with the TGA results. 4.6. Fourier Transformed Infrared Spectroscopy (FTIR)

Fig. 11. Thermographic analysis for derivatives of weight (% min) vs temperature.

FTIR was conducted to analyze structural composition and their change before and after the treatment (Bilba and Ouensanga, 1996; Mothé and de Miranda, 2009). Fibers treated with 5% alkali solution demonstrated the best result in terms of morphological, mechanical, and thermal properties. Hence, FTIR was performed only on the untreated and 5% alkali treated fibers. Fig. 13 illustrates FTIR spectra of treated and untreated fibers. The peak of 3200–3600 cm−1 indicates axial deformation of the OH group. The peak found ˜ at 3352 cm−1 denotes hydrogen bonded OH stretching (Ganán et al., 2008). The OH absorbance decreases with the increase in alkali concentration from 0.016 to 0.012 for untreated to 5% alkali treated fibers. A similar finding has been observed earlier (Fan et al., 2012). The C H stretching was observed at 2950 cm−1 for

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1457 cm−1 , 1370 cm−1 and 1030 cm−1 is for CH3 asymmetric, CH symmetric stretching and CH aromatic stretching in lignin, respectively (Ray et al., 2002). These peak values also get reduced with alkali treatment. The absorbance peak at 1315 cm−1 is for CH and C O stretching in acetyl group in hemicellulose (Irfan et al., 2011). The absorbance peak of 896 cm−1 was for glucosidic linkage (Liu et al., 2007). These results indicate that a significant amount of hemicellulose and lignin has indeed been removed through our chemical treatment process. 4.7. Surface and fracture morphology evaluation

Fig. 13. Absorbance vs wave numbers.

both cellulose and hemicellulose, as C H is a common element for both materials. Hence, after chemical treatment, the C H absorbance pick decreases. The peak at 1709 cm−1 is for C O band with carboxyl and ester group in hemicellulose and the absorbance peak is observed to decrease in treated fibers. These have also been observed by Haque et al. (2009). The absorbance peak at

It is important to study the fracture morphology to know the mode of failure. Fracture creates new surfaces within the body due to continuous loading. The fracture process may be a combination of three steps: (1) the initiation of a crack, (2) stable propagation under rising or constant load, and (3) unstable propagation leading to failure. The first two steps are dependent on the structure of the material and the final one is always a form of unstable propagation. Tensile fractures can be segregated into brittle and ductile fractures. The principal difference between brittle and ductile fracture is the plastic deformation that takes place in ductile materials before fracture occurs, whereas brittle materials show no or little plastic deformation. Brittle fracture is a rapid run of cracks leading to a sharp break whereas cracks propagate slowly followed by plastic deformation in ductile fracture. Chemically treated and untreated fibers were investigated using SEM micrographs in our study (Fig. 14). The untreated fiber appears to be consisting of waxy substances, hemicellulose, cellulose, and lignin (Fig. 14(A)). Progressively clean, rough, and regular surfaces are observed in the 1% treated (14(B)) and 5% treated (14(C)) fibers,

Fig. 14. SEM surface studies of untreated and treated fiber bundles: (A) untreated, (B) 1% alkali treated and (C) 5% alkali treated.

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Fig. 15. Fracture surface of untreated fiber bundle.

Fig. 16. Fracture surface of 1% alkali treated fiber bundle.

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Fig. 17. Fracture surface of 5% alkali treated fiber bundle.

respectively, compared to those of untreated fibers in Fig. 14(A). It can thus be inferred that after chemical treatment with 5% NaOH much of the waxy substances, hemicellulose, and lignin are removed. The relative amount of cellulose is increased and the mechanical and thermal properties are enhanced due to cellulose being the principal load-carrying constituent of the treated fibers. Moreover, alkali treatment lowered the cementing ability of the fibers. As more cementing materials are removed from the raw single fiber bundles, the surface becomes cleaner and rougher, and defibrillated. This results in a relatively thinner and uniform circular surface. The fibrillar structures are more visible in Fig. 14(C) which is consistent with the literature (Ray and Sarkar, 2001; Shaha et al., 2011). The presence of hemicellulose and other waxy substances reduces the strength of the fiber because of its brittle and amorphous nature. This can be visualized from SEM micrographs of the untreated fiber fracture surface shown at four different magnifications in Fig. 15. It is evident from the SEM micrographs that the fracture surface is smooth at the fracture zone. In Fig. 16(A)–(D), the 1% alkali treated fiber fracture surfaces are shown at four different magnifications. These micrographs reveal the onset of ductile failure as these surfaces are relatively rough compared to the untreated one. This rough fracture surface is an indication of the higher energy absorption ability of the fiber during loading, which, in turn, enhances mechanical properties. In Fig. 17(A)–(D), the fracture surfaces of 5% alkali treated fibers are shown at four different magnifications. These micrographs reveal even rougher fracture surfaces. Moreover, some fiber pull out at the fracture surface is also observed in one of the micrographs indicating ductile

failure (Fig. 17(A)). This phenomenon enhances the fiber fracture toughness (area under the stress–strain curve) significantly, as demonstrated in Fig. 4(C). Five percent alkali treated fibers demonstrated a progressively more ductile fracture leading to an introduction of little plasticity due to the presence of relatively more cellulose in the fiber. 5. Conclusion Single fiber bundles extracted from the rind of the sugarcane were treated with 1% and 5% alkali solution followed by an acetic acid wash. Untreated- and treated-fibers were characterized through tensile test, TGA, DSC, and FTIR spectroscopy analysis. The surface and fracture morphology were evaluated by the SEM study. From the above analysis, some of the important conclusions are given below: • Removal of non-cellulosic materials including hemicellulose and lignin from interfibriller regions resulted in the increase of the cellulose as well as more uniform fiber bundles of smaller diameters. • The ultimate strength, Young’s modulus, and strain to maximum strength increased with alkali concentration in the chemical treatment process. Fibers treated with 5% alkali solution yielded the best mechanical properties. Also, after correction factor analysis, a consistent and better data were obtained. • TGA indicated that treated fibers have a higher thermal degradation temperature than untreated ones.

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• The lower moisture content characteristic of alkali treated fibers observed by the DSC analysis is matched well with that of the TGA data. • FTIR spectroscopy data also indicate that there is a reduction of hemicellulose and lignin with chemical treatment. • SEM micrographs revealed that the alkali treated fibers have a finer fiber bundle with rougher surface compared to those of the untreated ones. Fibers treated with 5% alkali solution demonstrated the best surface morphology. • SEM study revealed brittle failure in untreated fibers whereas treated-fibers exhibited a combination of ductile and brittle failure.

Acknowledgments The authors acknowledge the experimental support from the Tuskegee University Center for Advanced Materials and Auburn University for this research.

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